TH ALL r3
TH ALL r3
TH ALL r3
Percentage Impedance = 100
Current Test
Current Rated
Voltage Rated
Voltage Test
Since the power factor during these measurements could be very low (less than 0.1),
watt meters suitable for such low power factors should be considered. Further, the three
62 Practical HV and MV Testing of Electrical Equipment
watt meter method is preferred when compared to two watt meter method (to avoid a
large multiplication constant).
5.9 No load losses and current measurement
The no load test not only establishes the no load losses but also indicates the soundness of
insulation after HV tests. Hence normally no load losses are taken before and after the
HV tests to ensure that the windings did not suffer any damage due to HV tests.
No load test is conducted by feeding the voltage to the LV winding at the rated
frequency. The core loss consists of eddy current losses and hysteresis losses. The eddy
current value is dependent on the rms value of supply voltage while hysteresis loss
depends on the average value of voltage. Two voltmeters are used with a bridge rectifier
to indicate the average value and a dynamometer type to indicate rms value. The actual
losses P is given by
2
KP
1
P
m
P
P
+
=
Where, P
m
is the measured no load loss
P
1
being the fraction of hysteresis loss to the total iron loss
(0.5 for grain oriented steel and 0.7 for non-grain oriented steel)
P
2
being the fraction of eddy current loss to the total iron loss
(0.5 for grain oriented steel and 0.3 for non-grain oriented steel)
And, K =
2
Voltage Average 11 . 1
voltage RMS
5.10 Insulation resistance tests
These tests are carried out between phases-to-ground, neutral-to-ground, primary-to-
secondary with 500 V/ 1000 V/ 2000 V/ 5000 V meggers depending upon the voltage
ratings. The insulation resistance values shall be in hundreds of megaohms to ensure
proper insulation. These tests are conducted before and after high voltage tests to ensure
integrity of the insulation after HV tests.
Though there is no standard value for these insulation resistance values, based on
experience and temperature conditions some standard acceptable values are applied to
verify the soundness of the insulation. If the test results give reduced values, it is
preferable to take up some improvement methods like drying out, etc, before the
transformer is accepted. Table 5.1 gives typical acceptable values.
Testing of transformers 63
Safe IR values in Mega ohms at applicable
ambient temperatures
Rated Voltage kV
30 C 40 C 50 C 60 C
66 kV and above 600 300 150 75
22 / 33 kV 500 250 125 65
6.6/11 kV 400 200 100 50
Below 6.6 kV 200 100 50 25
Table 5.1
IR values for transformers
5.11 Dielectric tests
The following dielectric tests are conducted on the transformers.
Applied voltage or Dry power frequency withstand voltage
Induced potential test
Normally, the above dielectric tests should be conducted after the lightning impulse and
switching impulse tests, if they are applicable (for EHV windings) or if the customers
specifications demand these impulse tests. Otherwise they can be conducted as a routine
test.
The power frequency voltage is normally applied for one minute, where its magnitude
is almost 2 times the standard voltage and depending upon the grounding method, applied
to the neutral. The line terminals of the windings under test are connected together and
test voltage is applied to these terminals with the other windings and tank connected to
the ground. The application of test voltage is for one minute.
The power frequency withstand voltage values applicable are given in Table 5.2 and are
based on the system voltage. Standard 1 values refer to effectively (solidly) earthed
applications and standard 2 values are for non-effectively earthed systems.
64 Practical HV and MV Testing of Electrical Equipment
Power Frequency withstand Voltage KV rms Operating
Voltage (KV)
Highest System
Voltage (KV)
Std 1 Std 2
3.3 3.6 16 16
6.6 7.2 22 22
11 12 28 28
15.75 17.5 38 38
22 24 40 50
33 36 70 70
66 72.5 140 140
110 123 230 185
132 145 275 230
220 245 460 395
Table 5.2
Dry power frequency voltages
The induced potential voltage test is basically to check the inter turn insulation and the
main insulation between the windings and ground. The test voltage is twice the rated
voltage of the winding with uniformly insulated windings. For graded insulation windings
(generally adopted for 66 kV and above) the test voltage is about 1.5 times the nameplate
voltage. For higher voltages it is usual to raise each V terminal in turn by applying single
phase voltage to the LV winding. The neutral terminal may be raised to a higher potential
to get at least twice the normal voltage per turn of the tested winding. The duration is 60
seconds for upto twice the rated frequency. However in order to avoid core saturation, the
test frequency is chosen at higher value of around 150 to 240 Hz with the time of
application reduced suitably as below.
frequency Test
frequency Rated K
seconds in duration Test
=
The value of K may be 100 or 120 depending on whether it is a 50 Hz or 60 Hz rated
transformer (with a minimum duration of 15 seconds).
5.12 RIV Corona measurements
For transformers rated above 132 kV, the RIV corona voltage measurements are taken by
applying the potential for one-hour. A rating of 1.7 times the normal voltage is applied for
2 minutes and then reduced to 1.5 times and maintained for one hour. Radio Interference
Voltages (RIV) are measured 5 minutes after the voltage is reduced to 1.5 times. The
readings are taken at 5 minute intervals during this one hour. The RIV readings at any
moment in time and at any terminal shall not exceed 100 V with readings not differing
by more that 20 V. If the values/differences are exceeding these values, the tests should
be repeated until the transformer can match these figures.
Testing of transformers 65
5.13 Partial discharge measurements
For voltage ratings 220 kV and above, the partial discharge measurements are also taken
during this one-hour test. The partial discharge test is basically to check the possible
discharges in cavities of the solid insulation and in gas bubbles in the liquid insulation or
along the dielectric surfaces. Partial discharge can result due to the following conditions.
Improper insulation drying/process
High stress areas caused by sharp edges in the conductors
This test requires special circuits to measure partial discharges while applying a higher
voltage for a considerable duration. Typically the transformer phase and neutral is applied
1.3 times the rated phase to neutral voltage value for 5 minutes and raised to 1.5 times the
rated phase to neutral voltage value for 5 seconds and again continuing with 1.3 times the
voltage for 30 minutes. During this entire sequence the partial discharge should not
exceed 300 pC at 1.3 times voltage and should be within 500 pC during the short 5
seconds while applying 1.5 times the voltage. These tests are normally carried out for
power plant and EHV transformers rated 220 kV and above. In practice however
customers require this test at much lower voltages and the new standards revision
currently being debated is expected to reflect that.
5.14 Impulse tests
The lightning impulse voltage magnitudes are shown in Table 5.3 and normally
conducted on EHV transformers as routine tests. The duration of the impulse is 1.2/50
sec. One application of a reduced voltage (50 to 70% of the table values) is done after
which two lightning impulses of the applicable values are applied to the terminal of the
transformer.
66 Practical HV and MV Testing of Electrical Equipment
Lightning Impulse Test
Voltage
kV peak
Operating Voltage
(kV)
Highest System Voltage
(kV)
Std 1 Std 2
3.3 3.6 45 45
6.6 7.2 60 60
11 12 75 75
15.75 17.5 95 95
22 24 125 125
33 36 170 170
66 72.5 325 325
110 123 550 450
132 145 650 550
220 245 1050 900
Table 5.3
Typical lightning impulse test voltage levels for transformer windings
Note:
Std 1: Non-effectively earthed systems (Resistance/Reactance grounding)
Std 2: Effectively earthed system (Solid grounding)
As a special test, chopped wave tests are often prescribed, aimed to simulate spark gaps
and external flashovers across the porcelains. Dependent on the applicable standard, the
value of chopped waves is 100 to 110% of the full wave values. The wave shape is
similar except that the voltage is collapsed to zero after 28 seconds. The standard
sequence for chopped impulse application is
One reduced full impulse
One 100% full impulse
One reduced chopped impulse
Two 100% chopped impulses
Two 100% full impulses
The switching impulse test is similar to the lightning impulse test with one reduced full
wave (75%) and two full waves of the rated impulse magnitude.
5.15 Tests on OLTC
The tests on OLTC normally consist of checking the proper operation of motors, the
sequence of tap changing, manual controls, etc.
Testing of transformers 67
5.16 Type tests
The following are type tests which are optional and carried out on units if the client
specifies the same. Normally these are conducted at additional cost.
Temperature rise test
Lightning impulse test (for below 132 kV)
Switching impulse test (for below 132 kV)
Partial Discharge test (for below 300 kV)
The temperature rise test basically comprises of allowing a full current load to be
passed through the windings until the thermometer readings reach steady state values.
The source is normally a low voltage, high current one. After the steady state temperature
is reached the transformer will start cooling thereby changing the winding resistance
value. The change in resistance value is taken to find the thermal constant of the
transformer windings and to interpolate the rise in winding temperature.
The normal duration of a temperature rise test may be about 10 hours and increasing to
one day for large capacity transformers. Though this is a type test, the temperature rise
within the agreed limits will give a clear condition of the transformer under service
conditions.
Different cooling modes are normally tested separately. On large or important
transformers a test at up to 1.5 times continuous maximum rating is often specified and is
then carried out for a period of some 210 hours to prove compliance with AS/IEC. This
is done subject to a maximum hot-spot temperature of 1201400C and the performance
checked by analyzing the oil for dissolved gases (DGA) afterwards.
5.17 Special tests
Special tests are normally carried out only if required for checking performance. The
following special tests are carried out if specified in the contract.
Measurement of zero sequence impedance
Short circuit test
Sound level measurements
Measurement of harmonics at no load
Measurement of auxiliary power by fans and pumps
5.17.1 Measurement of zero sequence impedance
This test is carried out for star connected transformers with earthed neutral to determine
the fault current value during phase-to-earth faults. The type of core (whether 5 limb or 3
limb) also has an effect on the value, since the reluctance paths are different in the two
types. A five limb construction may have above 90% to 100% value of positive sequence
impedance as zero sequence impedance, while a 3 limb construction value could be 80 to
90% of the positive sequence impedance.
The three terminals of the star winding are connected and a voltage is applied between
these terminals to neutral with the Delta winding left floating. Zero sequence impedance
value in ohms is equal to three times V/I where V is the single phase voltage applied and
I being the resultant current.
68 Practical HV and MV Testing of Electrical Equipment
5.17.2 Short circuit test
The short circuit test is normally a destructive test and to be carried out on an identically
designed transformer. The transformer should pass all the routine tests before being taken
up for the short circuit test. The symmetrical short circuit is calculated using the measured
impedance value plus the system impedance.
This test requires three shots on each phase at each tap, which means 9 shots are
required for three phase transformers with shots at normal tap, minimum tap and
maximum tap. The transformer is supposed to have passed the test if,
No faults result internally during the tests
Dielectric tests are successfully repeated
No visual defects on windings, supports and tank structure are seen
No traces of electric discharges noticed
Reactance measurements after the tests are within 2% of the actual value after
each shot for category 1 and within 1% for category 3 and the category 2
transformer reactance within a variation between 1% and 2%. (Refer 6.5 for
categories per IEC)
5.17.3 Other special tests
The noise level is an environmental issue and is necessary where transformer noise may
become objectionable. The loss measurements for auxiliary pumps and fans depend upon
the cooling method used for the transformers.
5.18 Tests on bushings
Though the transformer bushings are tested at the sub vendors works some of the tests
may be repeated to check integrity. Normally bushing tests are repeated for EHV
bushings that are condenser types.
Bushings are a critical part of the electrical system that transform and switch AC
voltages ranging from a few hundred volts to several thousand volts. Bushings not only
handle high electrical stress, they could be subjected to mechanical stresses, affiliated
with connectors and bus support, as well. Power factor test or Tan test is basically
carried out to check the deterioration and contamination of bushings. The voltage is
applied in steps upto the rated voltage and capacitance and tan delta values are recorded
for each voltage (using a Schering Bridge). Increase in capacitance and tan delta values
over a period of time indicates the deterioration of the bushing.
The following are the important factors measured to decide the condition of a bushing.
C
1
Capacitance (of bushing) this is the capacitance between the high-voltage
conductor and the voltage tap or test tap.
C
2
(Tap capacitance of a capacitance graded bushing) this is the capacitance
between the voltage tap and mounting flange (ground).
Modern condenser bushings are usually equipped with test taps. Bushings rated 115 kV
and above usually have voltage taps. Bushings rated below 115 kV have test taps. The
availability of either a voltage tap or a test tap allows for the testing of the main insulation
C
1.
The test tap is normally designed to withstand only about 500 volts while a voltage tap
may have a normal rating of 2.5 to 5 kV. Before applying a test voltage to the tap, the
maximum safe test voltage must be known and observed. Any excessive voltage may
puncture the insulation and render the tap useless. If absolutely no information is
available on the tap test voltage, 500 volts is the maximum test voltage recommended.
Testing of transformers 69
5.18.1 Main insulation (C
1
) test connections
Ground points of the test set and the apparatus of the bushing under test are
interconnected by ground wire.
The HV lead from the test set to be connected to the center conductor of the
bushing. If the bushing under test is in a transformer, all the bushings of the
same winding shall be jumpered. The bushings of other windings should be
grouped and connected to ground. The bare connector on the HV lead should
extend away from the bushing under test to avoid contact with the bushing
porcelain. The HV lead may be supported by another bushing or an individual
wearing rubber gloves suitable for the voltage rating. The LV lead from the
test set to be connected to the test tap.
The tap housing may contain a small amount of oil or compound. Care must
be taken when removing the screw cap to catch the oil. The oil is to be
replaced after testing is completed.
5.18.2 Test procedure
Power factor testing is extremely sensitive to weather conditions. Tests should
be conducted in favorable conditions whenever possible.
The main insulation test is normally performed at 10 kV in the UST test
mode. If 10 kV exceeds the rating of the bushing, test at or slightly below the
voltage rating.
Actual test voltage, current, Watts, power factor and capacitance as well as
ambient temperature, relative humidity, etc shall be recorded. The power
factor readings shall be corrected to 20C.
5.18.3 Test results and inference
General guidelines on PF values recorded are as below.
Between nameplate pf and up to twice nameplate pf Bushing is acceptable
> Twice nameplate pf and < 3 times Nameplate pf Monitor bushing closely
Above 3 times nameplate pf Replace the bushing
General guidelines for capacitance data are as below.
Nameplate capacitance 5% Bushing acceptable
Nameplate capacitance 5% to 10% Monitor bushing closely
Nameplate capacitance 10% or greater Replace bushing
Changes in C
1
testing are usually contamination issues caused by moisture ingress, oil
contamination or breakdown and short-circuited condenser layers.
The C
2
tests are similar to the above but the test voltage is to be limited as earlier
indicated.
5.18.4 Hot collar test
For bushings not equipped with a test tap or a voltage tap, the only possibility is to
conduct the hot collar test. The test provides a measurement of the losses in the section
directly beneath the collar and is especially effective in detecting conditions such as voids
in compound filled bushings or moisture penetration since the insulation can be
subjected to a higher voltage gradient than can be obtained with normal bushing tests.
70 Practical HV and MV Testing of Electrical Equipment
This method is also useful in detecting faults within condenser layers in condenser-type
bushings and in checking the oil level of oil-filled bushings after a pattern of readings for
a normal bushing has been established.
5.18.5 Test connections
Ground points of the test set and the apparatus of the bushing under test
should be interconnected by ground wire.
The collar should be installed just under the top petticoat of the bushing under
test and should be drawn tight around the bushing for good contact.
The HV lead from the test set should be connected to the collar. The high
voltage cable should extend away from the bushing at 90 and should not rest
against the porcelain.
The center conductor of the bushing should be grounded.
5.18.6 Test procedure
The collar should be energized at 10 kV. If 10 kV exceeds the rating of the
bushing, slightly below the rating of the bushing should be applied.
Actual test voltage, current, and Watts are recorded. Power and dissipation
factor data is not recorded. Current and Watts should be corrected to a
standard test voltage such as 2.5 kV or 10 kV as necessary.
Ambient temperature and relative humidity at the time of the test should be
recorded.
5.18.7 Test results
General guidelines for evaluating the hot collar data are as follows:
Watts-loss values less than 100 mw bushing acceptable
Watts-loss values of 100 mw or more bushing unacceptable (contamination)
Current values within 10% of similar bushings bushing acceptable
Current values less than 10% of similar bushings bushing unacceptable (low
level of liquid or compound)
If Watt-loss values are in the unacceptable range, cleaning may be necessary on the
exposed insulation surface of the bushing. Effects of surface leakage can be substantially
minimized by cleaning and drying the porcelain surface and applying a very thin coat of
Dow Corning #4 insulating grease (or equal) to the entire porcelain surface.
5.18.8 Other tests on bushings
The RIV test is done basically to determine the corona discharges in bushings at the rated
operating voltage (which lowers its performance and life). Oil type bushings are normally
tested for moisture content similar to other transformers.
The other tests include power frequency voltage withstand test, switching impulse tests,
partial discharge test, etc., to test the integrity of the bushings.
6
CT testing
6.1 Instrument transformers
Voltage transformers and current transformers continuously measure the voltage and
current of an electrical system and are responsible to give feedback signals to the relays to
enable them to detect abnormal conditions. The signals are eventually converted to analog
or digital display. With the power generated at high voltages using high capacity
generators, the values of actual currents in modern distribution systems vary from a few
amperes in households, small industrial/commercial houses, etc to thousands of amperes
in power intensive plants, national grids, etc. These also depend on accurate operating
voltages. Similarly the voltages in electrical systems vary from a few hundreds of volts to
many kilo volts. There is also a possibility of change in actual parameters with
addition/deletion of power capacity. Hence it is impossible to have monitoring relays and
meters custom designed and manufactured for each and every distribution system to
match the innumerable voltages and currents likely to be present. Accordingly
international standards have been evolved defining the outputs from the voltage
transformers and current transformers. These ensure that a minimum number and type(s)
of relays and meters are in all types of distribution systems so that their selection and cost
are within manageable ranges and follow uniformity.
The main tasks of instrument transformers are:
To transform currents or voltages from a high value to a value easy to handle
for the relays and the instruments.
To insulate the relays, metering and instruments from the primary high
voltage system.
To provide possibilities of standardizing the relays and instruments etc. to a
few rated currents and voltages.
Instrument transformers are special versions of standard power/distribution
transformers for measurement of current and voltages and are not for transferring power.
Their ratings are in volt-amperes with values not more than 100 VA, in general with the
modern low load relays and electronic meters demanding less than 5 VA, compared to
MVA ratings of transformers.
72 Practical HV and MV Testing of Electrical Equipment
The theory of operation of instrument transformers is similar to the standard
transformer one of the high efficient devices in electrical distribution systems. Basically
a transformer comprises of two windings viz., primary and secondary coupled through a
common magnetic core. When the primary winding is connected to a source and the
secondary circuit is left open, the transformer acts as an inductor with minimum current
being drawn from the source. At the same time a voltage will be produced in the
secondary open circuit winding due to the magnetic coupling. When a load is connected
across the secondary terminals, the current will start flowing in the secondary, which will
be decided by the load impedance and the open circuit secondary voltage. A
proportionate current is drawn in the primary winding depending upon the turns ratio
between primary and secondary. This principle of transformer operation is used in
transfer of voltage and current in a circuit to the required values (for the purpose of
standardization).
A current transformer has its primary winding directly connected in series with the
main circuit carrying the full operating current of the system. An equivalent current is
produced in its secondary, which is made to flow through the coil of a relay or a meter to
get the equivalent measure of the main system current. Standard secondary currents are
invariably 1 ampere and 5 amperes universally and accordingly they are also referred as 1
amp CT and 5 amps CT.
All current transformers used in electrical circuits are basically similar in construction
to standard transformers in that they consist of magnetically coupled primary and
secondary windings, wound on a common iron core. The main difference of current
transformers is that the primary winding is connected in series with the source line unlike
voltage transformers. Hence the current transformers primary should be capable of
withstanding the systems short-circuit currents.
The basis of all transformers is that:
AMPERE TURNS on the Primary = AMPERE TURNS on the secondary
e.g. 100 amps 1 turn = 1 amp 100 turns
Current transformers are used for bringing down the primary current. Hence the number
of turns on the CT secondary will always be more. The turns requirements for 1 amp
secondary CT is 5 times the corresponding 5 amp CT and that is one main reason for the
increased price of 1 amp CTs.
A voltage transformer is basically an open circuited transformer whose primary
winding is connected across the main electrical system voltage being monitored. A
convenient proportionate voltage is generated in the secondary for monitoring. The most
common voltage produced by voltage transformers is 100 volts to 120 volts (as per local
country standards) for primary voltages from 380 volts up to 800 kilo volts or more.
Voltage transformers are covered in the next chapter.
6.2 Current transformer types
One method of indicating the type of the CT is based on the construction method adopted.
The constructions basically differ on how the primary winding is formed. There are
basically three types of current transformers:
Wound primary type
Bar primary type
Window type
CT testing 73
The Current transformers are also named based on the application in which they are
used. Viz.,
Metering CT
Protection CT
Core Balance CT.
The wound primary and bar primary construction are adopted for metering and
protection current transformers. However the window type CT is used only for protection.
Wound primary type CT is shown in Figure 6.1, where both the primary and secondary
windings are wound over a common magnetic core.
Figure 6.1
Wound primary current transformers
The wound primary type CTs are used for smaller currents only because of their
following limitations.
Thermal limitations on the maximum currents
Special structural requirements at high magnetic forces
Hence wound type CTs are generally used for primary currents up to about 100
amperes. For currents greater than 100 amps, the bar primary type is used. Here the core
is still toroidal type. The bar primary type has its primary in the form of a busbar so that
the above limitations are overcome. Typical construction is shown in Figure 6.2.
Figure 6.2
Bar primary current transformer
74 Practical HV and MV Testing of Electrical Equipment
If the secondary winding is evenly distributed around the complete iron core as shown
in Figure 6.3, its leakage reactance is eliminated.
Figure 6.3
Bar primary CT with evenly distributed secondary winding
Typical symbol used to depict current transformers is as follows. Now the IEC symbols
show the same as a simple circle. Same symbol is used for both the types.
Figure 6.4
Typical CT symbol
The core balance CT or the window CT is used to measure the vector sum of currents
through a three-phase balanced load or a three-phase and neutral unbalanced load. Under
normal conditions the vectorial sum of these respective currents shall be zero. The
secondary windings of the CT will have zero voltage under this condition. Any earth
leakage will create an unbalance in the vectorial sum which generates a voltage to be
generated in the secondary winding terminals which drives a current to the relay
connected to it.
The three-phase or three-phase neutral cable cores or the busbars or the windings (in
case of motors) are passed through the central opening of the CT with the magnetic core
around the same. This chapter mainly covers testing requirements of the other two types
of current transformers.
CT testing 75
6.3 Burden and accuracy classes
The burden of an instrument transformer is the apparent power in volt amperes (VA)
absorbed at a specified power factor and at the rated secondary current. The standard
transformer capacity is referred in volt amperes basically meaning the total VA to which
the transformer can be loaded. The standard transformers have VA values running to
mega values. The burden of a current transformer is same as the load that can be
connected to the secondary side without making it to increase beyond the rated secondary
current under operating conditions.
CT secondary is connected to the coil of an instrument like ammeter, kW meter, kWH
meter, etc or of a relay. These coils also have resistances and the resistances limit the
maximum current that can flow in the respective coil. The currents in turn are dependent
on the voltage appearing across the coil. Accordingly the loads are represented in terms of
volt amperes and the VA is referred to as the burden imposed by the particular coil on the
current transformer. When many instruments are connected the total load or the burden to
be met by the CT is the sum of all the VA ratings of the coils connected. It is to be noted
that all the coils are connected in series, to ensure that the same secondary current flows
in all the coils. As the number of coils increases, the total resistance increases making less
current flowing in the CT secondary. This will defeat the whole purpose for which a CT
is used. Hence it is necessary that the burden of a CT is well assessed depending on the
number and type of coils adopted. In olden days the relays were mostly electro-
mechanical type with (mostly) analog meters. These have high VA ratings and hence the
burden of the earlier CTs used to be 15 to 30 VA in most of the applications. The advent
of numerical relays and electronic meters has made the burden of these components in the
order of decimal values. Hence it is quite common to come across CTs with 5 VA or
10 VA in modern installations. The standard values of rated output up to 30 VA are 2.5,
5, 10, 15 and 30 VA. Values above 30 VA may be selected to suit the application.
As seen earlier, the error in a CT should be chosen to be within required limits based on
the application. The errors are mainly introduced because of the following reasons.
The primary current contains two components:
An exciting current which magnetizes the core and supplies the eddy current
and hysteresis losses etc.
A remaining primary current component which is available for transformation
to secondary current in the inverse ratio of turns.
The exciting current is not getting transformed and is therefore the cause of transformer
errors. The more the exciting current, the lesser would be the accuracy.
The amount of exciting current drawn by a C.T. depends upon the core material and the
amount of flux that must be developed in the core to satisfy the output requirements of
the C.T. That is, to develop sufficient driving voltage required to push the secondary
current through its connected load or burden. Hence the selection of core materials plays
an important role to get higher accuracy.
The error which a CT introduces into the measurement of a current and which arises
from the fact that the actual transformation ratio is not equal to the rated transformation
ratio is called the Ratio error or current error per IEC. The current error expressed in per
cent is given by the following formula:
Current error % = (K
n
I
s
I
p
) 100 / I
p
Where
K
n
is the rated transformation ratio
76 Practical HV and MV Testing of Electrical Equipment
I
p
is the actual primary current and
I
s
is the actual secondary current when I
p
is flowing during measurement.
For an ideal transformer the primary and secondary current vectors shall be in phase
with zero displacement error. However in practice this is not achievable and an error is
introduced in the phase displacement between the primary and secondary currents. The
phase displacement is referred to as positive when the secondary current leads the
primary current and is expressed in minutes or centi-radians.
The metering CT requirements are defined by various accuracy classes and the basic
meanings of these specifications should be understood to ensure that the test results are
verified before accepting a current transformer.
Phase displacement at percentage
of rated current shown below
Percentage current
(ratio)
error at percentage of
rated
current shown below
Minutes Radians
Accuracy
Class
5 20 100 120 5 20 100 120 5 20 100 120
0.1 0.4 0.2 0.1 0.1 15 8 5 5 0.45 0.24 0.15 0.15
0.2 0.75 0.35 0.2 0.2 30 15 10 10 0.9 0.45 0.3 0.3
0.5 1.5 0.75 0.5 0.5 90 45 30 30 2.7 1.35 0.9 0.9
1.0 3.0 1.5 1.0 1.0 180 90 60 60 5.4 2.7 1.8 1.8
Table 6.1
Acceptable errors for metering CTs
It is to be noted that the metering CT is expected to have the least error from 100 to
120% of its rated current. The current displacements are more at lower ranges and less at
rated current.
The protection CT is defined as below:
Rating: XXX/ 5 amps
Burden: 15 VA
Accuracy Class 5P10
Accuracy Limit Factor: 10 or 20
CT testing 77
Typical tolerance values for the accuracy classes for protection CTs are shown in Table
6.2.
Phase displacement at rated
primary current
Accuracy
class
Current error at
rated primary
current
%
Minutes Centiradians
Composite error at
rated accuracy
limited primary
current
%
5P
10P
1
3
60
N.A.
1.8
N.A.
5
10
Table 6.2
Acceptable errors for protection CTs
6.4 Other technical parameters
The other major technical parameters of the CT are as below. These values should be
proved under various tests to be done on the current transformers.
Rated short-time thermal current (I
th
): The r.m.s. value of the primary current which a
CT will withstand for one second without suffering harmful effects when the secondary
winding being short-circuited
Rated dynamic current (I
dyn
) the peak value of the primary current which a
transformer will withstand, without being damaged electrically or mechanically by the
resulting electromagnetic forces with the secondary winding being short-circuited. The
value of the rated dynamic current should normally be 2.5 times the rated short-time
thermal current and it shall be indicated on the rating plate when it is different from this
value.
Rated continuous thermal current (I
c th
) the value of the current which can be
permitted to flow continuously in the primary winding, the secondary winding being
connected to the rated burden, without exceeding the specified temperature rise. The
standard value of rated continuous thermal current is the rated primary current. When a
rated continuous thermal current greater than rated primary current is specified, the
preferred values should be 120 % to 150 % and 200 % of rated primary current.
Limits of temperature rise this is defined as per the following table when the primary
is carrying the rated continuous thermal current with a unity PF rated burden at 40C
ambient temperature.
78 Practical HV and MV Testing of Electrical Equipment
Class of insulation Maximum temperature rise K
All classes immersed in oil 60
All classes immersed in oil and
hermetically sealed
65
All classes immersed in bituminous
compound
50
Classes not immersed in oil or
bituminous compound :
Y 45
A 60
E 75
B 85
F 110
H 135
Table 6.3
Acceptable temperature rise for CTs
Exciting current the r.m.s. value of the current taken by the secondary winding of a
current transformer, when a sinusoidal voltage of rated frequency is applied to the
secondary terminals, the primary and any other windings being open-circuited,
Composite Error: Under steady-state conditions, the r.m.s. value of the difference
between:
The instantaneous values of the primary current, and
The instantaneous values of the actual secondary current multiplied by the
rated transformation ratio, the positive signs of the primary and secondary
currents corresponding to the convention for terminal markings.
The composite error c is generally expressed as a percentage of the r.m.s. values of the
primary current. The composite error should be greater than 10 %, in order to protect the
apparatus supplied by the instrument transformer against the high currents produced in
the event of system fault.
Rated instrument limit primary current (IPL) for metering CTs the value of the
minimum primary current at which the composite error of the measuring current
transformer is equal to or greater than 10 % with rated burden on the secondary.
Instrument security factor (FS) the ratio of rated instrument limit primary current to
the rated primary current
Most of the CT parameters are defined at an ambient temperature not exceeding 40C
and an altitude less than 1000 meters above sea level. According to the lowest
temperature of operation the CTs are categorized as 5/40, 25/40 and 40/40 where the
minus values indicate the minimum allowable operating temperature. The system
maximum voltage of a CT is indicated by U
m
in peak value which decides its rated
operating voltage.
CT testing 79
6.5 Polarity
This is normally marked on the CTs and for proper operation the field connection shall
follow these markings based on the voltage end (phase or neutral) to which the CT is
connected. Impulse tests are done taking into account these polarity markings.
Polarity in a CT is similar to the identification of +ve and ve terminals of a battery.
Polarity is very important when connecting relays as this will determine correct operation
or not depending on the types of relays. The terminals of CT are marked by P1 and P2 on
the primary and S1 and S2 on the secondary as per the following figure.
Figure 6.5
Typical CT polarity marking
Standards indicate that at the instant when current is flowing from P1 to P2 in primary,
the current in secondary must flow from S1 to S2 through the external circuit. This
marking is very important for ensuring that the relays are properly connected to get true
current readings during operation. A reversal of connection will lead to a malfunction.
6.6 Magnetization curve
This curve describes the CTs performance in the best way. When a primary current
flows in a CT, its secondary terminals get a voltage which in turn is responsible to drive a
voltage across the secondary. If the current is slowly increased from zero to the rated
value, the open circuit voltage across the CT also increases proportionately. A graph can
be drawn to show how the open circuit voltage of a CT changes with the change in
primary current. It is a graph of the amount of magnetizing current required to generate a
particular open-circuit voltage at the terminals of the unit. Due to the non-linearity of the
core iron, it follows the BH loop characteristic. The graph comprises of three regions.
Viz.,
Initial region
Unsaturated region
Saturated region
These are shown in the following typical graph.
80 Practical HV and MV Testing of Electrical Equipment
Figure 6.6
Typical CT magnetizing curve
The transition from the unsaturated to the saturated region of the open circuit excitation
characteristic is a rather gradual process in most core materials. This transition
characteristic makes a CT not to produce equivalent primary current beyond a certain
point. This transition is defined by knee-point voltage in a CT, which decides its fairly
accurate working range.
It is generally defined as the voltage at which a further 10% increase in volts at the
secondary side of the CT requires more than 50% increase in excitation current. For most
applications, it means that current transformers can be considered as approximately linear
up to this point.
6.7 Metering and protection CT requirements
The metering CTs are used to read the actual primary currents for
Tariff metering
Reference purposes
The tariff metering requires that the actual current consumption is recorded so that there
is no dispute between the consumer and the power supplier. The consumer is definitely
not interested in paying for higher measurements of consumption due to errors in
measurements. Similarly the supplier wants to ensure that he is not under charging and
losing money. Hence the tariff metering CT demands a very low error in the order of
0.2%
Reference CTs are some times provided to cross check the consumption recorded by the
utility companies and hence they also require equivalent accuracy (as the tariff metering
CT). However the majority of metering CTs are just for reading the primary current to get
a broad idea about the performance of the equipment for which the current is measured.
Normally these requirements do not demand very high accuracy because an error of even
10% may not defeat the purpose for which the CT is used. Hence reference CTs that are
mostly used in switchgears, motor control centers, local control stations, etc. do not
require a very high accuracy like tariff metering CTs. An error of 2 to 5% is generally
accepted for these applications.
The accuracy factor may also depend on the actual primary current with lower values
introducing higher error percentages in the readings. Hence a metering CT requires that
CT testing 81
the secondary current is almost a true reflection of primary current at least for a load
range of 50 to 100%.
Also, it is normally the case that the load current is not expected to go beyond the
primary rated current of the CT. Under fault conditions this is not the case and fault
currents may some times be hundred times the primary current. Secondary currents also
increase proportionately affecting the connected instruments. Hence the metering CT
requires that the secondary current is restricted beyond some value, which is normally
achieved by core saturation. This (core saturation) offers a high reactance beyond a
particular current thereby restricting the secondary current to within 120% of its rated
value. After core saturation any increase in primary current does not have any effect on
the secondary current to protect the instruments in overload and fault conditions. Hence
it is common to have metering CTs with a very sharp knee-point voltage. A special
nickel-alloy metal having a very low magnetizing current is used in order to achieve the
accuracy.
Figure 6.7 shows the magnetization curve of a metering CT.
Figure 6.7
Typical metering CT magnetization curve
The protection CT on the other hand is not expected to give an accurate feedback of the
current under normal operating ranges. However they should be able to offer the actual
current to the relays under fault conditions. Saturation of core at fault currents may not
give the true condition to the relay and hence the relay may not trip when needed (which
in turn may affect the costly main equipment for which it is used). Accordingly the
protection CT requires core saturation happening at much higher values compared to the
metering CT.
In a similar way, higher accuracy is not a major requirement for protection current
transformers. The protection CTs are expected to ensure that the error in measurement at
the worst condition does not differ by more than 10 to 20% compared to 0.2%, 0.5%, 2%,
etc required by metering CTs. Further, protective relays are not normally expected to give
tripping instructions under normal conditions. On the other hand these are concerned with
a wide range of currents from acceptable fault settings to maximum fault currents which
are many times the normal rating. Larger errors may be permitted and it is important that
saturation is avoided (wherever possible) to ensure positive operation of the relays mainly
when the currents are many times the normal currents.
Figure 6.8 shows the typical characteristic required for a protection CT.
82 Practical HV and MV Testing of Electrical Equipment
Figure 6.8
Typical protection CT magnetization curve
6.8 Major tests on a CT
IEC 60044 Part 1 specifies the testing requirements for current transformers. The major
tests recommended as per the standard are as below.
6.8.1 Type tests
The following are the type tests and these are not normally done on all the manufactured
current transformers.
Short-time current tests.
Temperature rise test
Lightning impulse test
Switching impulse test (U
m
300 kV)
Wet test for outdoor type transformers
Determination of errors
Radio interference voltage measurement (RIV) for U
m
123 kV
All the dielectric type tests should be carried out on the same transformer, unless
otherwise specified. After transformers have been subjected to the dielectric type tests,
they shall be subjected to all the routine tests as below.
6.8. 2 Routine tests
The following tests apply to individual current transformer before each is accepted for
dispatch.
Verification of terminal markings/polarity
Power-frequency withstand test on primary winding
Partial discharge measurement (U
m
7.2 kV)
Power-frequency withstand test on secondary windings
Power-frequency withstand tests, between sections
Inter-turn over voltage test
Determination of errors
CT testing 83
The order of the tests is not standardized, but determination of errors should be
performed after all the other tests.
6.8. 3 Special tests
The following are special tests that are to be performed based on an agreement between
the manufacturer and the purchaser:
Chopped lightning impulse test
Measurement of capacitance and dielectric dissipation factor (U
m
72.5 kV)
Multiple chopped impulse test on primary winding
Static withstand load tests (U
m
72.5 kV)
Measurement of transmitted over voltages (U
m
72.5 kV)
6.9 Test procedures
6.9.1 Short time current I
th
withstand test
For this test the CT shall initially be at a temperature between 10C and 40C and the test
is made with the secondary winding(s) short-circuited. A current I for a time t is
circulated such that I
2
t is not less than the square of the rated thermal current I
th
2
and
provided the time t has a value between 0.5 and 5 seconds.
The dynamic test shall be made with the secondary winding(s) short-circuited, and with
a primary current peak value not less than the rated dynamic current I
dyn
for at least one
peak. The dynamic test may be combined with the thermal test, provided the first major
peak current of that test is not less than the rated dynamic current. The transformer shall
be deemed to have passed these tests if, after cooling to ambient temperature, it does not
show any visual damage, it retains its earlier recorded accuracies, etc.
6.9.2 Temperature rise test
Done similar to the power transformer temperature rise test with the test conducted at an
ambient of 10 to 30C with the CT mounted in a manner representative of the service
condition. If practicable this is done by measuring the increase in resistance.
6.9.3 Impulse tests
The test is done by applying the applicable voltage between the primary terminal and
earth with frame, core and secondary terminals connected to ground. The applicable test
voltages are as per the following tables.
84 Practical HV and MV Testing of Electrical Equipment
Highest system
Voltage U
m
(kV peak)
Rated short-duration power-
frequency withstand voltage
kV (r.m.s)
Rated lightning impulse
withstand voltage
kV (peak)
0.72 3 --
1.2 6 --
3.6 10 20/40
7.2 20 40/60
12 28 60/75
17.5 38 75/95
24 50 95/125
36 70 145/170
52 95 250
72.5 140 325
123 185/230 450/550
145 230/275 550/650
170 275/325 650/750
245 395/460 950/1050
Table 6.4
Rated insulation levels for CTs with Um< 300kV
Note: Choose the highest value for exposed installations)
Highest system Voltage U
m
(kV peak)
Rated switching impulse
withstand voltage
(kV peak)
Rated Lighting impulse
withstand voltage
(kV peak)
300 750/850 950/1050
362 850/950 1050/1175
420 1050/1050 1300/1425
525 1050/1175 1425/1550
765 1425/1550 1950/2100
Table 6.5
Rated insulation levels for CTs with Um 300kV
Note: Choose the highest value for exposed installations
CT testing 85
Rated lightning impulse withstand voltage
(peak) kV
Rated power frequency withstand voltage
(r.m.s.) kV
950 395
1050 460
1175 510
1300 570
1425 630
1550 680
1950 880
2100 975
Table 6.6
PF withstand voltages for CTs with U
m
300 kV
For windings with U
m
< 300 kV, the lightning impulse voltage test is done on both
positive and negative polarities by applying 15 consecutive impulses on each polarity. In
case of CTs having U
m
300kV and above the test is done by applying three consecutive
impulses on each polarity. The CT passes the test if there are no disruptive discharges and
no flashovers along the external insulation.
Switching impulse test voltages are applied on positive polarity only and fifteen
consecutive switching impulses as appropriate corrected to atmospheric conditions is
applied. For outdoor transformers, the test is done in wet condition. A maximum of two
flashovers is allowable across the external insulation under this test.
In regard to wet PF tests for windings with U
m
< 300 kV the test is performed with the
applicable voltage while for U
m
300 kV, it is the switching impulse voltage on the
positive polarity.
6.9.4 RIV test
The test is done with ambient temperature limited between 10 to 30C and a humidity
level of 45 to 75%.
The test voltage shall be applied between one of the terminals of the primary winding of
the test object and earth. The frame, case (if any), core (if intended to be earthed) and all
terminals of the secondary winding(s) shall be connected to earth. The measuring circuit
is provided in IEC. The measuring circuit should preferably be tuned to a frequency in the
range of 0.5 MHz to 2 MHz, the measuring frequency being recorded. The results shall be
expressed in micro volts. The impedance between the test conductor and earth (Zs + (R
1
+
R
2
)) shall be 300 40 with a phase angle not exceeding 20. A capacitor C
s
may also
be used in place of the filter Z
s
and a capacitance of 1 000 pF is generally adequate.
The filter Z shall have high impedance at the measuring frequency in order to decouple
the power frequency source from the measuring circuit. A suitable value for this
impedance has been found to be 10 000 to 20 000 at the measuring frequency.
The radio interference background level (radio interference caused by external field and
by the high-voltage transformer) shall be at least 6 dB (preferably 10 dB) below the
specified radio interference level.
86 Practical HV and MV Testing of Electrical Equipment
A pre-stress voltage of 1.5 Um/3 shall be applied and maintained for 30 s. The voltage
shall then be decreased to 1.1 Um/3 in about 10 s and maintained at this value for 30 s
before measuring the radio interference voltage.
The CT is considered to have passed the test if the radio interference level at 1.1 Um/3
does not exceed the limit prescribed per IEC. Some times by agreement between
manufacturer and purchaser, this test may be replaced by partial discharge test. In such
case the allowable PD value is 300 pC at 1.1 U
m
/ 3.
6.9.5 Partial discharge test
Procedure A: The partial discharge test voltages are reached while decreasing the voltage
after the power-frequency withstand test.
Procedure B: The partial discharge test is performed after the power-frequency withstand
test.
The applied voltage is raised to 80% of the power-frequency withstand voltage,
maintained for not less than 60 s, then reduced without interruption to the specified partial
discharge test voltages.
If not otherwise specified, the choice of the procedure is left to the manufacturer. The
test method used shall be indicated in the test report.
6.9.6 PF voltage tests
The applicable test voltage shall be applied for 60 seconds in sequence between the short-
circuited terminals of each winding section, or each secondary winding and the earth. The
frame, core (if there is a special earth terminal), and the terminals of all the other
windings or sections shall be connected together and to earth when one winding/section is
tested.
6.9.7 Inter-turn over-voltage test
Procedure A: with the secondary windings open-circuited (or connected to a high
impedance device which reads peak voltage), a substantially sinusoidal current at a
frequency between 40 Hz and 60 Hz and r.m.s. value equal to the rated primary current
shall be applied for 60 sec to the primary winding. The applied current shall be limited if
the test voltage of 4.5 kV peak is obtained before reaching the rated current.
Procedure B: with the primary winding open-circuited, the prescribed test voltage (at
some suitable frequency) shall be applied for 60 seconds to the terminals of each
secondary winding, ensuring that the r.m.s. value of the secondary current is not
exceeding the rated secondary current. The value of the test frequency shall not be greater
than 400 Hz. At this frequency, if the voltage value achieved at the rated secondary
current is lower than 4.5 kV peak, the obtained voltage is to be regarded as the test
voltage.
The procedure to be adopted is based on agreement between manufacturer and the
purchaser.
6.9.8 Chopped impulse test on primary winding
The test shall be carried out with negative polarity only, and be combined with the
negative polarity lightning impulse test. The voltage shall be a standard lightning
impulse, chopped between 2 s and 5 s. The chopping circuit shall be so arranged that
the amplitude of overswing of opposite polarity of the actual test impulse shall be limited
CT testing 87
to approximately 30% of the peak value. The test voltage of the full impulses shall have
the appropriate values based on the highest system voltage and the specified insulation
level.
The sequence of impulse applications shall be as following:
For windings having Um < 300 kV:
One full impulse
Two chopped impulses
Fourteen full impulses.
For windings having Um 300 kV:
One full impulse
Two chopped impulses
Two full impulses
Differences in wave shape of full wave applications before and after the chopped
impulses are an indication of an internal fault. Flashovers during chopped impulses along
self-restoring external insulation shall be disregarded in the evaluation of the behavior of
the insulation.
6.9.9 Capacitance and dielectric dissipation factor
The measurement of capacitance and dielectric dissipation factor shall be made after the
power-frequency withstand test on the primary windings. The test voltage shall be applied
between the short-circuited primary winding terminals and earth. Generally the short-
circuited secondary winding(s), any screen, and the insulated metal casing shall be
connected to the measuring bridge. If the current transformer has a special device
(terminal) suitable for this measurement, the other low-voltage terminals shall be short
circuited and connected together with the metal casing to the earth or the screen of the
measuring bridge.
6.9.10 Transmitted over voltages measurement
A low-voltage impulse (U
1
) shall be applied between one of the primary terminals and
earth.
For single-phase current transformers for GIS metal-enclosed substations, the impulse
shall be applied through a 50 coaxial cable adapter with the enclosure of the GIS
section connected to earth as to be done in service. The terminal(s) of the secondary
winding(s) intended to be earthed shall be connected to the frame and to earth.
The transmitted voltage (U2) shall be measured at the open secondary terminals
through a 50 coaxial cable terminated with the 50 input impedance of an
oscilloscope having a bandwidth of 100 MHz or higher which reads the peak value. If the
current transformer comprises more than one secondary winding, the measurement shall
be successively performed on each of the windings. In the case of secondary windings
with intermediate tappings, the measurement shall be performed only on the tapping
corresponding to the full winding. The over voltages transmitted to the secondary
winding (U
s
) for the specified over voltages (U
p
) applied to the primary winding shall be
calculated as follows:
U
s
= (U
2
/
U
1
) U
p
88 Practical HV and MV Testing of Electrical Equipment
In the case of oscillations on the crest, a mean curve should be drawn, and the
maximum amplitude of this curve is considered as the peak value U
1
for the calculation of
the transmitted over voltage.
The current transformer is considered to have passed the test if the value of the
transmitted over-voltage does not exceed the limits given in the IEC table.
6.9.11 Ratio verification test
The CT ratio is typically represented as 100/5 amps, 200/1 amps where 100, 200 are the
primary currents and 1 and 5 the corresponding secondary currents. This ratio shall be
inverse to the turns ratio as per transformer fundamentals.
The ratio verification is similar to the turns ratio test done on a transformer. The
primary current is passed through the primary and the secondary currents are measured to
ensure that the secondary current follows a proportionate change in line with primary
current variations. The ratio test is more relevant for a metering CT where the secondary
currents follow the primary current with minimum error in the 50 to 100% range.
Generally, present day current transformers exhibit a good characteristic even at around
20% rated primary current.
The errors in the protection CT are permitted in the lower range and again readings
should preferably taken higher than the rated current. The main issue would be the
withstand time and hence fast reading instruments which apply the current for a few
seconds and automatically display the secondary current are to be used for the same.
6.9.12 Accuracy class verification
Type tests to prove compliance with accuracy classes in the case of CTs of classes 0.1 to
1, should be made at each value of current given as per Table 6.1 at 25 % and at 100 % of
rated burden (subject to 1 VA minimum). Transformers having extended current ratings
greater than 120 % shall be tested at the rated extended primary current instead of at 120
% of rated current. Transformers of class 3 and class 5 shall be tested for compliance with
the two values of current given in table at 50 % and at 100 % of rated burden (subject to 1
VA minimum).
6.9.13 Polarity test
Figure 6.9 shows the simple testing arrangement for verifying the CT polarity markings at
the time of commissioning electrical systems. The factory test is similar in principle
except for the large power source.
CT testing 89
Figure 6.9
Testing of a C.T polarity
Connect the battery negative terminal to the current transformer P2 primary terminal.
This arrangement will cause a current to flow from P1 to P2 when +ve terminal is
connected to P1 till the primary gets saturated due to the DC Voltage. If the polarities are
correct, a momentary current will flow from S1 to S2.
A center zero galvanometer is connected across the secondary of the current
transformer. Touch or flick the +ve battery connection to the current transformer primary
terminal P1. If the polarity of the current transformer is correct the galvanometer should
flick in the +ve direction.
6.9.14 Test for CT magnetizing curve
It is necessary to test the characteristics of a CT before it is put into operation, since the
results produced by the relays and meters depend on how well the CT behaves under
normal and fault conditions. Figure 6.10 shows a simple test connection diagram that is
adopted to find the magnetic curve of a CT.
90 Practical HV and MV Testing of Electrical Equipment
Figure 6.10
Circuit to test magnetization curve
In the above circuit the current is passed through the secondary from zero to the full
rated current across S1 and S2. Hence a milli-ammeter is used to measure the currents,
and the corresponding voltages across S1 and S2 are measured. This basically indicates
the voltage generated at the secondary terminals corresponding to the currents flowing in
the winding.
The readings shall be taken until the effect of increase in the current does not generate a
proportionate in the voltage. The curve is to be drawn and the exact knee point is decided
where the current increase of 50% causes less than 10% change in the excitation voltage.
6.9.15 Short circuit test
This is normally a destructive test and hence not done as a routine test. The short circuit
ratings are generally defined to match the switchgear ratings in which they are used.
6.10 Safety precautions
Current transformers generally work at a low flux density. Hence the core is made of very
good metal to give small magnetizing current. On open-circuit mode, secondary
impedance becomes infinite and the core saturates. This induces a very high voltage in
the primary up to approximately system volts and the corresponding volts in the
secondary will depend on the number of turns, multiplying up by the ratio (i.e. volts/turn
no. of turns). Since CT normally has much more turns in secondary compared to the
primary, the voltage generated on the open circuited CT will be much more than the
system volts, leading to Flashovers.
HENCE AS A SAFETY PRECAUTION, NEVER OPEN-CIRCUIT A CURRENT
TRANSFORMER ON LOAD!!!
The general safety procedures to be followed while handling HV or MV equipment
shall be applicable while testing the CT.
7
VT testing
7.1 Types of voltage transformers
As seen in the previous chapter, the operating principle of the voltage transformer (VT) or
potential transformer (PT) is the same as that of power transformers and the secondary
voltage is standardized to 110 volts or 120 volts AC in almost all countries. The term
potential transformer is generally used to distinguish itself from a standard transformer.
There are basically two types of voltage transformers used in electrical systems.
Electro-magnetic type (commonly referred to as a VT)
Capacitor type (referred to as a CVT).
The electro magnetic type basically is a step down transformer whose primary (HV)
and secondary (LV) windings are connected as below.
Figure 7.1
Electro-magnetic type VT
The number of turns in a winding is directly proportional to the open circuit voltage
being measured or produced across it. The Figure 7.1 is basically a single-phase VT. In
92 Practical HV and MV Testing of Electrical Equipment
the three-phase system it is necessary to use three VTs at one per phase and they being
connected in star or delta depending on the method of connection of the main power
source being monitored. This construction method of electro magnetic transformers is
used in high voltage circuits up to 110 kV/132 kV.
For still higher voltages, it is common to adopt the second type namely the capacitor
voltage transformer (CVT). Figure below gives the basic connection adopted in this type.
Here the primary portion consists of capacitors connected in series to split the primary
voltage to convenient values.
Figure 7.2
Capacitor voltage transformer
The potential transformer is similar to a power transformer and differs only in so far as
a different emphasis is placed on cooling, insulating and mechanical aspects. The primary
winding has a large number of turns and is connected across the line voltage either phase-
to-phase or phase-to-neutral.
The secondary has lesser turns but the volts per turn on both primary and secondary
remain the same.
The capacitor V.T. is more commonly used on high voltage outdoor substations beyond
132 kV. The capacitors also allow the injection of high frequency signals onto the power
line conductors to provide end-to-end communications between substations (for distance
relays, telemetry/supervisory and voice communications). Hence in the HV national grid
networks of utilities, the CVTs are most commonly used for both protection and
communication purposes.
Similar to current transformers, potential transformers are used for both metering and
protection purposes. It is possible to have one common primary winding and two or more
secondary windings in one unit. The voltage transformers having this kind of arrangement
VT testing 93
are referred to as two-core or three-core VT/PT depending on the number of secondary
windings.
7.2 Basic technical terms
Similar to the current transformers, the VTs have some basic technical characteristics to
meet the service requirements. The major terms which define the performance of the VT
and which need to be tested and verified are given below:
Voltage error (Ratio error): This is the error which a voltage transformer introduces
into the measurement of a voltage and which arises when the actual transformation ratio
is not equal to the rated or anticipated transformation ratio. The voltage error is normally
expressed in per cent and is given by the formula:
Voltage error % = (K
n
U
s
U
p
)/U
p
100%
Where
K
n
is the rated transformation ratio.
U
p
is the actual primary voltage
U
s
is the measured / actual secondary voltage when rated primary voltage is applied.
Phase displacement: Under ideal conditions the secondary voltage is supposed to be in
phase with the primary sinusoidal voltage, which is not the case in real conditions. The
difference in phase between the primary voltage and the secondary voltage vectors is
referred as the phase displacement. The phase displacement is said to be positive when
the secondary voltage vector leads the primary voltage vector. It is usually expressed in
minutes or centiradians.
The voltage transformers should be capable of producing secondary voltages, which are
proportionate to the primary voltages over the full range of input voltage expected in a
system. Voltage transformers for protection are required to maintain reasonably good
accuracy over a large range of voltage from 0173% of normal. However the close
accuracy is more relevant for metering purposes, while for protection purposes the margin
of accuracy can be comparatively less. The standard accuracy classes for protective
voltage transformers are 3P and 6P, and the same limits of voltage error and phase
displacement will normally apply at both 5% of rated voltage and at the voltage
corresponding to the rated voltage factor. At 2% of rated voltage, the error limits will be
twice as high as those at 5% of rated voltage. Permissible errors vary depending on the
burden and purpose of use and typical values for the standard accuracy classes as per IEC
are as follows:
Phase Displacement Accuracy
Class
Percentage Voltage
(Ratio) Error Minutes Centiradians
0.1 0.1 5 0.15
0.2 0.2 10 0.3
0.5 0.5 20 0.6
1.0 1.0 40 1.2
3.0 3.0 N.A. N.A.
Table 7.1
Acceptable errors for measuring voltage transformers
94 Practical HV and MV Testing of Electrical Equipment
Phase Displacement Class Percentage Voltage
(Ratio) Error Minutes Centiradians
3P 3.0 120 3.5
6P 6.0 240 7.0
Table 7.2
Acceptable errors for protection voltage transformers
Accuracy is not a major cost-deciding factor for a voltage transformer due to the high
efficiency of the transformers which normally ensures that there is no major voltage
drop in the secondary leads. Thus it is common to select voltage transformers based on
the loads (choosing appropriate rated burden). The question of accuracy of VTs used in
protection circuits can be ignored and is generally neglected in practice.
Burden: It is usually expressed as the apparent power in voltamperes, absorbed by the
voltage transformer at a specified powerfactor and at the rated secondary voltage. The
capacity of a voltage transformer is normally represented in VA rating, which indicates
the maximum load that can be connected across its secondary.
Output burdens of 500 VA per phase are common for potential transformers compared
to very low burden ratings of current transformers. The recommended standard values as
per IEC are 10 VA, 25 VA, 50 VA, 100 VA, 200 VA and 500 VA.
Rated primary and secondary voltage: Rated Primary voltage is the continuous voltage
for which the primary is designed to provide the rated secondary voltage across the
secondary open circuit terminals. The values of rated primary voltage of three-phase
transformers (between lines in a three-phase system) and of single-phase transformers for
use in a single-phase system should be equal to the rated system voltage for which they
are used. Normally three-phase transformers use single-phase transformers for each phase
and then their terminals are interconnected externally as needed (star or delta). The
standard values of rated primary voltage of a single-phase transformer connected between
one line of a three-phase system and earth or between a system neutral point and earth
should be 1/3 times one of the values of rated system voltage.
The rated secondary voltages based on the current practice of a group of European
countries are
100 V and 110 V
200 V for extended secondary circuits
Based on the current practice in the United States and Canada, these are
120 V for distribution systems
115 V for transmission systems
230 V for extended secondary circuits.
Standard values of rated voltage factor: The voltage factor is determined by the
maximum operating voltage which, in turn, is dependent on the system and the voltage
transformer primary winding earthing conditions.
VT testing 95
Rated voltage
factor
Rated time Method of connecting the primary winding and system
earthing conditions
1.2 Continuous Between phases in any network. Between transformer star-
point and earth in any network
1.2 Continuous
1.5 30 seconds
Between phase and earth in an effectively earthed neutral
system
1.2 Continuous
1.9 30 seconds
Between phase and earth in a non-effectively earthed
neutral system with automatic earth-fault tripping
1.2 Continuous
1.9 8 hours
Between phase and earth in an isolated neutral system
without automatic earth-fault tripping or in a resonant
earthed system without automatic earth-fault tripping
Table 7.3
Rated voltage factors of VTs
Limits of temperature rise: The temperature rise limits are the same as the ones earlier
referred for current transformers (depending on the insulation type). The temperature rise
of a voltage transformer is defined at the specified voltage, at rated frequency and at rated
burden, or at the highest rated burden if there are several rated burdens, at any power
factor between 0.8 lagging and unity.
Insulation levels for primary windings: The rated insulation level of a primary winding
of an inductive voltage transformer should be based on its highest voltage for equipment
Um. For windings having Um = 0.72 kV or 1.2 kV, the rated insulation level is
determined by the rated power-frequency withstand voltage, according to the Table 7.7.
For windings having Um = 3.6 kV and greater but less than 300 kV, the rated insulation
level is determined by the rated lightning impulse and power-frequency withstand
voltages and should be chosen in accordance with the same table.
For windings having Um 300 kV, the rated insulation level is determined by the rated
switching and lightning impulse withstand voltages and should be chosen in accordance
with another table.
7.3 Connection of voltage transformers
Electro-magnetic voltage transformers may be connected inter-phase or between phase
and earth. However capacitor voltage transformers can only be connected phase-to-earth.
Voltage transformers are commonly used in 3-phase groups, generally in star-star
configuration. The secondary voltages provide a complete replica of the primary voltages
as shown in the next picture below and any voltage (phase-to-phase or phase-to-earth)
may be selected for monitoring at the secondary.
96 Practical HV and MV Testing of Electrical Equipment
Figure 7.1
Voltage Transformers connected in star-star configuration
Figure 7.2
Vector diagram for VTs in star-star configuration
VT testing 97
7.4 Tests on voltage transformers
Following are the tests recommended per IEC 60044 Part 2
7.4.1 Type tests
Temperature-rise test
Short-circuit withstand capability test
Lightning impulse test
Switching impulse test
Wet test for outdoor type transformers
Determination of errors
Measurement of the radio interference voltage (RIV) (U
m
123 kV)
All the dielectric type tests should be carried out on the same transformer, unless
otherwise specified. After transformers have been subjected to the dielectric type tests,
they should be subjected to all routine tests.
7.4.2 Routine tests
The following tests apply to each individual transformer:
Verification of terminal markings
Power-frequency withstand tests on primary windings
Partial discharge measurement (U
m
7.2 kV)
Power-frequency withstand tests on secondary windings
Power-frequency withstand tests between sections (see 9.3);
Determination of errors
The order of the tests is not standardized but determination of errors should be
performed after the other tests. Repeated power-frequency tests on primary windings
should be performed at 80 % of the specified test voltage.
7.4.3 Special tests
The following tests are performed upon agreement between manufacturer and purchaser:
Chopped impulse test on primary winding
Measurement of capacitance and dielectric dissipation factor(U
m
72.5 kV)
Static withstand load tests (U
m
72.5 kV)
Transmitted over voltage measurement (U
m
72.5 kV)
7.5 Test procedures
The test procedures are mostly similar to the current transformers but are reproduced to
have a review on the same.
7.5.1 Temperature rise test
Done similar to the power transformer temperature rise test with the test conducted at an
ambient of 10 to 30C with the VT mounted in a manner representative of the service
condition. If practicable this is done by measuring the increase in resistance.
98 Practical HV and MV Testing of Electrical Equipment
The voltage to be applied to the transformer for the temperature rise test should be one
of the following (as applicable).
All voltage transformers irrespective of voltage factor and time rating should
be tested at 1.2 times the rated primary voltage. If a thermal limiting output is
specified, the transformer should be tested at rated primary voltage, at a
burden corresponding to the thermal limiting output at a unity power factor
without loading the residual voltage winding. If a thermal limiting output is
specified for one or more secondary windings, the transformer should be
tested separately with each of these windings connected, one at a time, to a
burden corresponding to the relevant thermal limiting output at a unity power
factor. The test should be continued until the temperature of the transformer
has reached a steady state.
Transformers having a voltage factor of 1.5 or 1.9 for 30 seconds should be
tested at their respective voltage factor for 30 s starting after the application of
1.2 times rated voltage for a time sufficient to reach stable thermal conditions;
the temperature rise should not increase by more than 10K the value
specified in the standard table (given in the previous earlier chapter).
Alternatively, such transformers may be tested at their respective voltage
factor for 30 seconds starting from the cold; the winding temperature rise
should not exceed 10K.
Transformers having a voltage factor of 1.9 for 8 hours should be tested at 1.9
times the rated voltage for 8 hours starting after the application of 1.2 times
rated voltage for a time sufficient to reach stable thermal conditions; the
temperature rise should not exceed 10K the values specified in the table.
Class of insulation Maximum temperature rise K
All classes immersed in oil 60
All classes immersed in oil and
hermetically sealed
65
All classes immersed in bituminous
compound
50
Classes not immersed in oil or
bituminous compound :
Y 45
A 60
E 75
B 85
F 110
H 135
Table 7.4
Acceptable temperature rise for VT
VT testing 99
7.5.2 Short-circuit withstand capability test
For this test, the voltage transformer should initially be at a temperature between 10C
and 30C. The voltage transformer is energized from the primary side and the secondary
terminals are shorted.
One short circuit should be applied for the duration of 1 second. During the short
circuit, the r.m.s. value of the applied voltage at the transformer terminals should not be
less than its rated voltage. In the case of transformers provided with more than one
secondary winding, or section, or with tappings, the test connection should be agreed
between manufacturer and purchaser.
The transformer is accepted to have passed this test if, after cooling to ambient
temperature,
It is not visibly damaged
Its errors do not differ from those recorded before the tests by more than half
the limits of error in its accuracy class
It withstands the dielectric tests with the test voltage reduced to 90 %
The insulation next to the surface of the primary and the secondary windings
does not show significant deterioration (e.g. carbonization)
7.5.3 Impulse test on primary winding
Typical insulation test voltages are as given in Tables 7.5 to 7.7. The test voltage should
be applied between each line terminal of the primary winding and earth. The earthed
terminal of the primary winding or the non-tested line terminal in the case of an unearthed
voltage transformer, at least one terminal of each secondary winding, the frame, case (if
any) and core (if intended to be earthed) should be earthed during the test. The reference
impulse voltage should be between 50% and 75% of the rated impulse withstand voltage.
The peak value and the waveshape of the impulse should be recorded. For failure
detection the record of current(s) to earth or of voltages appearing across the secondary
winding(s), should be performed in addition to the voltage record.
7.5.4 Lightning impulse test
For Windings having Um < 300 kV the test should be performed with both positive and
negative polarities. Fifteen consecutive impulses of each polarity, not corrected for
atmospheric conditions, should be applied. The transformer passes the test if for each
polarity
No disruptive discharge occurs in the non-self-restoring internal insulation
No flashovers occur along the non-self-restoring external insulation
No more than two flashovers occur across the self-restoring external
insulation
No other evidence of insulation failure is detected (e.g. variations in the wave
shape of the recorded quantities)
For unearthed voltage transformers, approximately half the number of impulses should
be applied to each line terminal in turn with the other line terminal connected to earth.
For windings having Um 300 kV the test should be performed with both positive and
negative polarities. Three consecutive impulses of each polarity, not corrected for
atmospheric conditions, should be applied.
100 Practical HV and MV Testing of Electrical Equipment
The transformer passes the test if:
No disruptive discharge occurs
No other evidence of insulation failure is detected (e.g. variations in the wave
shape of the recorded quantities).
Highest system
Voltage U
m
(kV peak)
Rated short-duration power-frequency
withstand voltage
kV (r.m.s)
Rated lightning impulse
withstand voltage
kV (peak)
0.72 3 --
1.2 6 --
3.6 10 20/40
7.2 20 40/60
12 28 60/75
17.5 38 75/95
24 50 95/125
36 70 145/170
52 95 250
72.5 140 325
123 185/230 450/550
145 230/275 550/650
170 275/325 650/750
245 395/460 950/1050
Table 7.5
Rated insulation levels for VTs with Um< 300kV
(Note: Choose the highest value for exposed installations)
Highest system Voltage U
m
(kV peak)
Rated switching impulse
withstand voltage
(kV peak)
Rated Lighting impulse
withstand voltage (kV peak)
300 750/850 950/1050
362 850/950 1050/1175
420 1050/1050 1300/1425
525 1050/1175 1425/1550
765 1425/1550 1950/2100
Table 7.6
Rated insulation levels for VTs with Um 300kV
(Note: Choose the highest value for exposed installations)
VT testing 101
Rated lightning impulse
withstand voltage (peak) kV
Rated power frequency withstand
voltage (r.m.s.) kV
950 395
1050 460
1175 510
1300 570
1425 630
1550 680
1950 880
2100 975
Table 7.7
PF withstand voltages for VTs with U
m
300 kV
7.5.5 Switching impulse test
The test voltage should have appropriate values, depending on the highest voltage for
equipment and the specified insulation level. The test should be performed with positive
polarity. Fifteen consecutive impulses, corrected for atmospheric conditions, should be
applied. For outdoor-type transformers the test should be performed under wet conditions.
The transformer passes the test if:
No disruptive discharge occurs in the non-self-restoring internal insulation
No flashovers occur along the non-self-restoring external insulation
No more than two flashovers occur across the self-restoring external
insulation
No other evidence of insulation failure is detected (e.g. variations in the wave
shape of the recorded quantities)
7.5.6 Wet test for outdoor type transformers
For windings having Um < 300 kV, the test should be performed with power-frequency
voltage of the appropriate value depending on the highest voltage for equipment applying
corrections for atmospheric conditions. For windings having Um 300 kV, the test
should be performed with switching impulse voltage of positive polarity of the
appropriate value, depending on the highest voltage for equipment and the rated
insulation level.
7.5.7 RIV test
The test is done with ambient temperature limited between 10 to 30C and a humidity
level of 45 to 75%.
The test voltage should be applied between one of the terminals of the primary winding
of the test object and earth. The frame, case (if any), core (if intended to be earthed) and
all terminals of the secondary winding(s) should be connected to earth. The measuring
circuit is provided in IEC. The measuring circuit should preferably be tuned to a
frequency in the range of 0.5 MHz to 2 MHz, the measuring frequency being recorded.
102 Practical HV and MV Testing of Electrical Equipment
The results should be expressed in micro volts. The impedance between the test
conductor and earth (Z
s
+ (R
1
+ R
2
)) should be 300 40 with a phase angle not
exceeding 20. A capacitor C
s
may also be used in place of the filter Z
s
and a capacitance
of 1 000 pF is generally adequate.
The filter Z should have a high impedance at the measuring frequency in order to
decouple the power frequency source from the measuring circuit. A suitable value for this
impedance has been found to be 10 000 to 20 000 at the measuring frequency.
The radio interference background level (radio interference caused by external field and
by the high-voltage transformer) should be at least 6 dB (preferably 10 dB) below the
specified radio interference level.
A pre-stress voltage of 1.5 Um/3 should be applied and maintained for 30 seconds.
The voltage should then be decreased to 1.1 Um /3 in about 10 seconds and maintained
at this value for 30 s before measuring the radio interference voltage.
The VT is considered to have passed the test if the radio interference level at 1,1 Um/
3 does not exceed the limit prescribed by IEC. Some times by agreement between the
manufacturer and purchaser, this test may be replaced by the partial discharge test. In
such a case the allowable PD value is 300 pC at 1.1 U
m
/ 3.
7.5.8 Power frequency withstand test
This is normally a routine test and similar to those done on power transformers. For
separate source withstand test, the duration should be 60 seconds. For the induced voltage
withstand test, the frequency of the test voltage may be increased above the rated value to
prevent saturation of the core. The duration of the test should be 60 seconds.
If, however, the test frequency exceeds twice the rated frequency, the duration of the
test may be reduced from 60 seconds as below:
Duration of test (in seconds) = (twice the rated frequency/test frequency) 60
with a minimum of 15 seconds.
For windings having Um < 300 kV test values are as per table based on the systems
highest voltage.
The applicable test voltage should be applied for 60 seconds in sequence between the
short-circuited terminals of each winding section, or each secondary winding and the
earth. The frame, core (if there is a special earth terminal), and the terminals of all the
other windings or sections should be connected together and to earth when one
winding/section is tested.
7.5.9 Partial discharge test
Procedure A: The partial discharge test voltages are reached while decreasing the voltage
after the induced voltage withstand test.
Procedure B: The partial discharge test is performed after the induced voltage withstand
test.
The applied voltage is raised to 80% of the induced voltage, maintained for not less
than 60 seconds, and then reduced without interruption to the specified partial discharge
test voltages.
If not otherwise specified, the choice of the procedure is left to the manufacturer. The
test method used should be indicated in the test report.
For unearthed transformers two tests are done by applying voltages alternately to each
of the HV terminals with the other terminal connected to ground (along with the other
windings and frame).
VT testing 103
7.5.10 Chopped impulse test on primary winding
The test should be carried out with negative polarity only, and be combined with the
negative polarity lightning impulse test. The voltage should be a standard lightning
impulse, chopped between 2 s and 5 s. The chopping circuit should be so arranged that
the amplitude of over swing of opposite polarity of the actual test impulse should be
limited to approximately 30% of the peak value. The test voltage of the full impulses
should have the appropriate values based on the highest system voltage and the specified
insulation level.
The sequence of impulse applications should be as following:
For windings having Um < 300 kV:
One full impulse
Two chopped impulses
Fourteen full impulses
For unearthed transformers, two chopped impulses and approximately half the number
of full impulses should be applied to each terminal.
For windings having Um 300 kV:
One full impulse
Two chopped impulses
Two full impulses
Differences in wave shape of full wave applications before and after the chopped
impulses are an indication of an internal fault. Flashovers during chopped impulses along
self-restoring external insulation should be disregarded in the evaluation of the behavior
of the insulation.
7.5.11 Capacitance and dielectric dissipation factor
The measurement of capacitance and dielectric dissipation factor should be made after the
power-frequency withstand test on the primary windings. The test voltage should be
applied between the short-circuited primary winding terminals and earth. Generally the
short-circuited secondary winding(s), any screen, and the insulated metal casing should
be connected to the measuring bridge. If the current transformer has a special device
(terminal) suitable for this measurement, the other low-voltage terminals should be short
circuited and connected together with the metal casing to the earth or the screen of the
measuring bridge.
7.5.12 Transmitted over voltages measurement
A low-voltage impulse (U1) should be applied between one of the primary terminals and
earth.
For single-phase current transformers for GIS metal-enclosed substations, the impulse
should be applied through a 50 coaxial cable adapter with the enclosure of the GIS
section connected to earth as to be done in service. The terminal(s) of the secondary
winding(s) intended to be earthed should be connected to the frame and to earth.
The transmitted voltage (U2) should be measured at the open secondary terminals
through a 50 coaxial cable terminated with the 50 input impedance of an
oscilloscope having a bandwidth of 100 MHz or higher which reads the peak value. If the
current transformer comprises of more than one secondary winding, the measurement
should be successively performed on each of the windings. In the case of secondary
104 Practical HV and MV Testing of Electrical Equipment
windings with intermediate tappings, the measurement should be performed only on the
tapping corresponding to the full winding. The over voltages transmitted to the secondary
winding (U
s
) for the specified over voltages (U
p
) applied to the primary winding should
be calculated as follows:
U
s
= (U
2
/
U
1
) U
p
In the case of oscillations on the crest, a mean curve should be drawn, and the
maximum amplitude of this curve is considered as the peak value U
1
for the calculation of
the transmitted over voltage.
The voltage transformer is considered to have passed the test if the value of the
transmitted over voltage does not exceed the limits given per IEC table.
7.5.13 Ratio and accuracy class verification test
These are both type tests and routine tests. The requirement is that the voltage error and
phase displacement at rated frequency should not exceed the values (given earlier) at any
voltage between 80% and 120% of rated voltage and with burdens between 25% and
100% of rated burden at a power factor of 0.8 lagging. To prove compliance with this,
type tests should be made at 2%, 5% and at 100% of rated voltage and at rated voltage
multiplied by the rated voltage factor, at 25% and at 100% of rated burden at a power-
factor of 0.8 lagging.
The routine tests for accuracy are in principle the same as the type tests, but routine
tests at a reduced number of voltages and/or burdens are permissible, provided it has been
shown by type tests on a similar transformer that such a reduced number of tests is
sufficient to prove its characteristics.
For measuring voltage transformers of accuracy class 0.1 and 0.2 and having a rated
burden lower than 10 VA an extended range of burden can be specified. The voltage error
and phase displacement should not exceed the values given in the table, when the
secondary burden is any value from 0 VA to 100 % of the rated burden, at a power factor
equal to 1. This requirement is mostly requested for certified accuracy of energy
measurements.
The measurement errors should be determined at the terminals of the voltage
transformer and should include the effects of any fuses or resistors as an integral part of
the VT.
8
Ducter testing
8.1 Need for the instrument
A ducter is basically a low resistance meter (unlike mega ohm meter) used in insulation
resistance tester. Ohms Law dictates that for a specified energy source, operating on V
AC or V DC, the amount of current drawn will be dependent upon the resistance of the
circuit or the component. The satisfactory operation of the circuit or the component
depends on the controlled flow of current within the design parameters for the given piece
of equipment.
In this age of electronics, increased demands are placed on all aspects of electrical
circuitry. In the present demanding industrial electronic environments, the engineer is
now required to make measurements which show repeatability within a few micro-ohms
or less to prove the reliability of the equipment.
The electrical system comprises of many interconnections and joints that introduce
considerable resistance in various electrical circuits. These may be a few micro ohms at
each point but their summation may introduce long term damage to existing equipment
and will also waste considerable energy as heat. Any restrictions in current flow will
prevent a machine from generating its full power and may also allow insufficient current
to flow to activate protective devices in the case of a fault. Hence it is necessary at an
early stage to test and establish resistance values and then continuously monitor any
upward changes to identify unexpected changes in measured values. The trending of this
data helps to forecast possible failure conditions. Excessive changes in measured values
would need corrective actions to prevent a major failure.
A low resistance measurement is typically a measurement below 1 ohm. At this level it
is important to use test equipment that will minimize errors introduced by the test lead
resistance and/or contact resistance between the probe and the material being tested. Also,
at this level, standing voltages across the item being measured (e.g. thermal emfs at
junctions between different metals) may cause errors and need to be identified.
8.2 Description of instrument
The original DUCTER low resistance ohmmeter was developed by Evershed &
Vignoles in 1908 and employed the cross coils meter movement that was already used in
106 Practical HV and MV Testing of Electrical Equipment
insulation resistance testers. This initial design evolved into field units in the 1920s that
required a leveling procedure at the time of the test due to the sensitivity of the coil.
These early models did not travel well and were sensitive to shock and vibration.
To allow a measurement to compensate the errors, a four terminal measurement method
had been developed with a reversible test current along with a suitable Kelvin Bridge
meter which enables measuring very low resistance values. Subsequent demands required
ranges up to kilo ohms which used a Wheatstone Bridge.
The low range on many resistance ohmmeters resolves 0.1 micro-ohms. This level of
measurement is required to perform a number of low range resistance tests.
8.3 Working principle
8.3.1 Kelvin bridge
The Kelvin Bridge (also known as the Thomson Bridge) is used for precision
measurements below the typical range of the Wheatstone Bridge. Sir William Thomson
(Lord Kelvin) devised this in 1854. The classic arrangement has six resistors in a
rectangle, bisected by a galvanometer (see Figure 8.1), which includes an unknown
resistance. A comparatively large current is passed through this unknown resistance and
also through a known resistance of a comparatively low value. The galvanometer
compares the voltage drops across these two resistances with the double-ratio circuit
comprised of the other four resistors.
Figure 8.1
Kelvin Bridge
The two pairs of ratio resistors (A/B, a/b) are in parallel to each other and connected
across with the galvanometer. One pair (a/b) is in series with the unknown (X) and the
reference standard (R). The latter is an adjustable low-resistance, usually a manganin bar
with a sliding contact.
This arrangement introduces a total resistance of A+X+a and B+R+b in parallel with
the galvanometer. A connecting link (Y), sometimes called the yoke, shunts the ratio pair
(a/b) that are otherwise in series with the unknown and the standard. This has minimal
effect on the accuracy of the measurement so long as the two pairs of parallel ratio
Ducter testing 107
resistors are kept exactly equal (A to a, B to b). Lead and contact resistances are included
in the value of the ratio pairs, and any effects can be nullified by keeping the resistance of
the yoke extremely low. Keeping the yoke resistance low also accommodates the large
test currents often used in Kelvin Bridges without causing unwanted heating effects.
When potential is balanced across the two parallel circuits, the unknown is equivalent
to the parallel ratio multiplied by the adjusted reference value.
X = A/B R
For very low measurements, the Kelvin Bridge has the advantage of nullifying
extraneous resistances from leads and contacts by employing the system of double-ratio
arms. The resistances of the connecting leads are in series with the high-resistance ratio
arms and not with the reference or tested resistors.
8.3.2 Wheatstone bridge
A pioneering method for measuring resistance was devised in 1833 by S. H. Christie and
made public by Sir Charles Wheatstone. The arrangement is a square pattern having four
resistors with a galvanometer connected across one diagonal and a battery across the
other (see Figure 8.2). Two of the resistors are of known appropriate values and comprise
the ratio arm (A + B). A third has a known value which can be varied in small increments
over a wide range, and is thus designated the rheostat arm (R). The fourth is the resistance
being measured, the unknown arm (X).
Figure 8.2
Wheatstone Bridge
The bridge is considered balanced when the rheostat arm has been adjusted so that the
current is divided in such a way that there is no voltage drop across the galvanometer and
it ceases to deflect (is nulled). The resistance being measured can then be calculated from
108 Practical HV and MV Testing of Electrical Equipment
the knowledge of the values of the ratio resistors and the adjusted value of the rheostat
arm. The basic formula is:
X = B/A R
Where
B and A are the ratio resistors
R is the rheostat resistance used.
The Wheatstone Bridge can be constructed to a variety of ranges and is generally used
for all but the highest and lowest measurements. It is suited to a range of about 1 to
100,000 ohms.
8.3.3 Four wire instrument
There are basically three ways of measuring resistance of an element viz., 2-wire, 3-wire
and 4-wire instruments.
Two wire testing is the simplest method and is used to make a general assessment of a
circuit element or a conductor in a circuit. The two-wire lead configuration is the most
familiar one, used in multi-meters. It is generally used when the probes contact resistance,
series lead resistance or parallel leakage resistances, do not degrade the quality of the
measurement beyond an acceptable point.
The disadvantage with this method is that the measured value will include the test lead
wire resistance and contact probe resistance values. This resistance may be equal to some
tens of milli-ohms to the actual resistance, thereby introducing considerable errors when
the resistance value is low. In most instances this may make little difference to the
measured value, but when the measurement is below 1.000 ohm the two-wire method can
easily introduce an error. This error could be several percentage of the actual resistance
value. The lead resistance may be zeroed out, but that leaves the uncertainty of the
contact resistances, which can change with each measurement. Contact resistance values
may be in the 35 milli-ohm range at each probe and can vary with the temperature of the
material under investigation.
The two-wire test method is best used for readings above 10.00 ohms up to about 10.0
megohms.
Three-wire testing is used for very high resistance and is typically used for
measurements above 10.0 megohms. This is nothing but the insulation resistance tester
where a third test lead is used as a guard, and allows for resistances in parallel with the
test circuit to be eliminated from the measurement. This parallel resistance is usually
considerably lower than the insulation resistance being measured. In fact it may, in severe
cases, effectively short out the insulation resistance such that a meaningful measurement
cannot be carried out without the use of a guarding circuit.
Four-wire measurement is the concept used by a Ducter. This is more suitable for
accurately measuring resistances starting from milli ohms up to about 10 ohms. Out of the
four wires two are called voltage leads and the other two are called current leads. A
typical arrangement is as shown in Figure 8.3. The four-wire measurement compensates
for usual errors that are introduced by the probe lead wire and the contact resistance
values in the final reading, thus ensuring more accurate measurements.
Ducter testing 109
A DC instrument should be used when trying to measure the pure resistance of a circuit
or device. An AC instrument is used for applications such as ground bed testing or
impedance testing.
Figure 8.3
Four wire measurement
Current is injected into the item under test via leads C1 and C2. The current that flows
will be dependent upon the total resistance of this loop and the power available to push
the current through that resistance. Since this current is measured, and the measured value
is used in subsequent calculations, the loop resistance, including the contact resistance of
the C1 and C2 contacts and the lead resistance of C1 and C2, does not have an effect on
the final result.
From Ohms Law, if we pass a current through a resistance we will generate a voltage
across the resistance. This voltage is detected by the P1 and P2 probes. The voltmeter to
which these probes are connected internally has a high impedance, which prevents current
flowing in this potential loop. Since no current flows, the contact resistance of the P1 and
P2 contacts produces no voltage and thus has no effect on the potential difference
(voltage) detected by the probes. Furthermore, since no current flows through the P leads
their resistance has no effect.
110 Practical HV and MV Testing of Electrical Equipment
Figure 8.4
Measurement principle
A high current output is one of the qualifying characteristics of a true low resistance
ohmmeter. Generic multi-meters do not supply enough current to give a reliable
indication of the current-carrying capabilities of joints, welds, bonds and the like under
real operating conditions. At the same time, little voltage is required, as measurements are
typically being made at the extreme low end of the resistance spectrum. Only the voltage
drop across the measured resistance is critical, and it is measured at the milli-volt level.
8.4 Milli-ohmmeter vs micro-ohmmeter
As the name implies, a milli-ohmmeter is less sensitive than a micro-ohmmeter, with
measurement capability in milli-ohms rather than micro-ohms (minimum resolution of
0.01 milli-ohm). This type of instrument is normally used for general circuit and
component verification. Milli-ohmmeters also tend to be less expensive than micro-
ohmmeters, making them a good choice if measurement sensitivity and resolution are not
critical. The maximum test current is typically less than two amperes and as low as 0.2
amperes.
In contrast, the micro-ohmmeter uses 10-amp maximum test current which provides a
comfortable and suitable test current through the test sample to make the measurements.
The best 10-amp micro-ohmmeters offer measurements from 0.1 micro-ohm to 2000
ohms with a best resolution of 0.1 micro-ohm at the low end of the range and accuracy of
0.2%, 0.2 micro-ohms. On some instruments, different measurement modes may be
selected which address different types of testing conditions. Measurement modes could
include manual, automatic or continuous testing, or a high power test for large windings.
Ducter testing 111
The following is a selected list of key DC resistance measurement applications for 10-
amp micro-ohmmeters.
Switch and contact breaker resistance
Busbar and cable joints
Small transformer and motor winding resistance
Aircraft frame bonds and static control circuits
Welded joint integrity
Intercell strap connections on battery systems
Resistive components (quality control)
Rail and pipe bonds
Metal alloy welds and fuse resistance
Graphite electrodes and other composites
Wire and cable resistance
Transmitter aerial and lightning conductor bonding
There are different ways in which the leads are provided as shown in Figure 8.4.
Figure 8.5
Different means of measurement
8.5 Breaker contact resistance measurement
According to IEC62271-100, testing the contact resistance of high voltage AC circuit
breakers calls for a test current with any convenient value between 50 A and the rated
normal current. ANSI C37.09 specifies that the test current should be a minimum of
100 A. Most electrical utilities prefer to test at higher currents, as they believe this is
more representative of working conditions. Field portable instruments are available that
can supply anywhere from 100 A up to 600 A (subject to the load resistance and supply
voltage). The best instruments have measurement resolution to 0.1 micro-ohm and offer
variable test current to address a wider range of applications.
By testing at 10 Amp and then at a higher current, the operator can get a better
understanding of the maintenance requirements for the circuit breaker. In addition to
112 Practical HV and MV Testing of Electrical Equipment
circuit breakers, electrical utilities and testing companies use higher current micro-
ohmmeters on other high voltage apparatus, including:
Cables
Cable joints
Welds
Busbars
Switchgear in general
It is some times a practice to initially perform a 10 amp test and then see improved
resistance readings with test currents beyond 100 amps as per standards. However it is
necessary to realize that high current meters are intended to be used at high currents.
Their accuracy may reduce considerably at low currents, particularly when measuring
small resistances.
Mechanical wear and tear on circuit breaker contacts reduces the area of the contact
surfaces. This reduction combined with sparking and/or arcing during operations increase
the resistance across the working connections. This condition will produce heat that can
reduce the effectiveness of the circuit breaker. Periodic measurements will show the rate
of increase of the contact resistance value. When these values are compared to the
original manufacturers specification, a decision can be made to continue or repair. By
tracking the trend of the readings, the operator gets an idea of when the circuit breaker
should be pulled for service before damage is done.
8.6 Transformer resistance measurement
Some of the transformer ohmmeters include dual meters with independent range controls
such that the high voltage/primary (high resistance) and low voltage/secondary (low
resistance) windings of a transformer can be measured at the same time.
The transformer ohm meter is a multi current device with measurement resolution to 1
micro-ohm and is used both in factory tests and for field operating verification. Operation
of the transformer ohmmeter is sometimes enhanced by connecting the test current
through both windings with opposite polarity, thus providing the fastest test time (the
mutual inductance between the windings is minimized by this way). This current
connection operation is used on wye-to-wye, wye-to-delta and delta-to-delta transformers.
The ability to measure primary and secondary windings at the same time also speeds up
the testing time.
The power supply is often designed to deliver the energy to saturate the winding and
then provide a stable level of test current. The test set should also be able to test the
windings and contact resistance on tap-changers with make-before-break contacts and
voltage regulators. Tap-changers are the most vulnerable part of the transformer and face
more failures and outages than any other component. Frequent testing is required to
ensure proper and reliable operation. A transformer ohmmeter is used to:
Verify factory test readings.
Help locate the presence of defects in transformers, such as loose connections.
Check the make-before-break operation of on-load tap-changers.
Perform heat runs to determine the internal temperature changes, via the
winding resistance, that occur under rated current conditions.
Ducter testing 113
8.7 Precautions during measurements
The temperature of the device will have a strong influence on the measured values. For
example, the resistance measured for a hot motor will be different from a measurement
done in cold conditions. As the motor warms up, the resistance readings will go up. The
resistance of copper windings responds to changes in temperature based on the basic
nature of copper as a material. Different materials will have different temperature
coefficients. As a result, the temperature correction equation will vary depending on the
material being tested.
As a general safety measure, normal testing should always be performed on de-
energized samples. Special training and equipment are required to perform tests on
energized circuits. Internal fused input circuits are designed into a few instruments that
will protect the instrument if inadvertently connected to an energized test sample. The
low input impedance of the current supply internal to general instruments becomes a
willing current sink when connected across a live circuit.
Safety is the responsibility of the field test engineer or technician, whoever will be in
contact with the sample being tested. The majority of field tests are performed on de-
energized circuits. When testing magnetic components, a state of winding saturation may
occur. The operator should connect a short circuit across the winding to neutralize the
energy stored in the winding and then make a voltage test to verify the neutral state of the
sample.
Battery strap testing represents a special condition, as the batteries must remain
connected. The operator is required to use insulated gloves, facemask and a body apron
for protection when performing these tests. This is one of the few times when electrical
resistance tests are performed in the field on energized systems. Special probes, rated for
600 V operation, are available with the newer instruments to perform these tests.
When planning a test on circuit breakers, the operator must be aware of IEC62271-100
and ANSI C37.09 for test current requirements. When testing large oil circuit breakers,
the best instrument is one that ramps up current slowly and steadily, holds it for a period
of time to complete measurements and then ramps down in a similar fashion. This method
reduces magnetizing, which would otherwise be created by the sudden switching ON and
OFF of the test current. This may also result in inaccurate CT measurements when the
system is returned to normal AC operation.
Care should be taken when making a measurement across a CT as high DC currents
may saturate the CT, leading to potential faults. Also, any ripple on the test current may
cause circuit breakers to trip. Careful positioning of the current probes should prevent this
happening, and the ripple present on the current waveform may be minimized by
separating the test leads.
When connections have higher than normal resistance measurements, one should not
resort to retightening the bolts, as this will over stress the soft lead connection. Over
tightening does not cure the problem. The proper procedure is to disassemble the straps,
clean, grease and then reconnect with the bolts tightened to the suppliers torque level.
All the three phase resistances should be balanced within a narrow tolerance of 10 to
20%.
A common error in the field is to use a low resistance ohmmeter to sample the
resistance of a ground bed. This application is incorrect, as the ground bed test method
requires an instrument that toggles the test signal at a known frequency and current level.
A low resistance ohmmeter used in this application will provide an erroneous reading as
the ground current will have an undue influence on the measurement. A proper ground
tester performs in essentially the same way as a low resistance ohmmeter, that is, by
114 Practical HV and MV Testing of Electrical Equipment
injecting a current into the test sample and measuring the voltage drop across it.
However, the earth typically carries numerous currents originating from other sources,
such as the utility. These will interfere with the DC measurement being performed by a
low resistance ohmmeter. The genuine ground tester, however, operates with a definitive
alternating square wave of a frequency distinct from utility harmonics. In this manner, it
is able to perform a discrete measurement, free of noise influence.
9
Tests on other major equipment
9.1 Other major equipment
This chapter briefly covers the tests normally done on other HV/MV equipment not
covered in earlier chapters. These are
HV/MV switchgears
Outdoor circuit breakers
MV motors and generators
HV disconnectors
MV Capacitors
9.2 HV/MV switchgears and breakers
MV switchgears are generally designed for indoor use and basically comprise of the
following compartments on one vertical section. (A switchgear panel line up may consist
of many such vertical sections.)
Draw out breaker trolley or contactor assembly
Metering/relay auxiliary compartment
Cable compartment
Busbar chamber
It is necessary to have tests on integrated assembly of all the units. IEC recommends the
following tests on switchgears.
9.2.1 Routine tests
Power frequency voltage withstand tests
Dielectric tests on auxiliary circuits
Measurement of resistance in the main circuit
Mechanical operation tests
Tests on auxiliary electrical, hydraulic and pneumatic devices
116 Practical HV and MV Testing of Electrical Equipment
Verification of correct wiring
Partial discharge tests
9.2.2 Type tests
The type tests should be carried out on a maximum of four test specimens unless
otherwise specified in the relevant IEC standards and/or mutually discussed with the
supplier.
Dielectric tests on main, auxiliary and control circuits
Radio interference voltage (R.I.V.) test
Measurement of resistance of the main current path
Temperature rise tests
Short-time withstand current and peak withstand current tests
Making and breaking tests
Tests to verify the degrees of protection of enclosures
Tightness tests (where applicable)
Mechanical tests
Humidity tests
Thermal stability tests
Test under arcing conditions during internal faults
Ageing tests
Most of the tests are related to their names. Some of the main tests which generally use
higher than the rated voltages are as below.
9.2.3 Lightning impulse voltage tests
Switchgear and outdoor circuit breakers should be subjected to lightning impulse voltage
tests. While panels are tested for their ability to withstand these voltages in dry
conditions, the outdoor equipments are also tested in wet conditions. The tests should be
performed with voltages of both polarities using the standard lightning impulse 1.2/50
seconds according to IEC. The applicable voltages are based on the maximum system
voltages for which the switchgears and equipment are designed and are given in the table
below.
9.2.4 Power-frequency voltage tests
These are similar to the PF tests on transformers and bushings covered earlier. All
switchgears and breakers should be subjected to short-duration power-frequency voltage
withstand tests in accordance with IEC guidelines. For each test condition the test voltage
should be raised to the appropriate test value and maintained for one minute. The tests
should only be performed in dry conditions (for indoor units). Tables 9.1 and 9.2 are not
only for assembled switchgears but also for the stand-alone high voltage circuit breakers.
For outdoor breakers the wet test is conducted at the prescribed wet PF voltage by
maintaining the applicable voltage for 10 seconds.
Tests on other equipment 117
Rated
voltage
kV (r.m.s.)
Rated short-duration power-frequency
withstand voltage
kV (r.m.s.)
Rated lightning impulse
withstand voltage
kV (peak)
Common
value
Across the
Isolating
distance
Common
value
Across the isolating
distance
(1) (2) (3) (4) (5)
3,6 10 12 20 23
40 46
7,2 20 23 40 46
60 70
12 28 32 60 70
15 85
17,5 38 45 75 85
95 110
24 50 60 95 110
125 145
36 70 80 145 165
170 195
52 95 110 250 290
72,5 140 160 325 375
100 150 175 380 440
185 210 450 520
123 185 210 450 520
230 265 550 630
145 230 265 550 630
275 315 650 750
170 275 315 650 750
325 375 750 860
245 360 415 850 950
395 460 950 1050
460 530 1 050 200
Table 9.1
Rated insulation levels for European design ranges
118 Practical HV and MV Testing of Electrical Equipment
Rated short-duration power-frequency
withstand voltage
kV (r.m.s)
Rated lightning impulse
withstand voltage
kV (peak)
Maximum
System voltage
kV (r.m.s.)
Common value Across the isolating
contacts
Common
value
Across
isolating
distance
(1) (2) (2a) (3) (3a) (4) (5)
4.76 19 21 60 70
8.25 26 24 29 27 75 80
35 30 39 33 95 105
15 35 30 39 33 95 105
50 45 55 50 110 125
25.8 50 45 55 50 125 140
70 60 77 66 150 165
38 70 60 77 66 150 165
95 80 105 88 200 220
48.3 120 100 132 110 250 275
72.5 160 140 176 154 350 385
Table 9.2
Test voltages for North American standard equipment
9.2.5 Testing for internal faults
Internal faults inside metal-enclosed switchgears can occur in a number of locations. The
arc energy resulting from an arc developed in air at atmospheric pressure or in another
insulating gas within the enclosure will cause an internal overpressure and local
overheating. This will result in mechanical and thermal stressing of the equipment.
Moreover, the materials involved may produce hot decomposition products, either
gaseous or vaporous, which may be discharged to the outside of the enclosure.
IEC specifies a method of testing switchgear enclosures against the effects of internal
faults, as a type test. It involves setting up deliberate faults within the switchgear
enclosure and then testing for the effect by use of what are called indicators; i.e. large
metal screens covered with black cotton textile material. This test procedure demands for
operator safety and is covered here.
IEC gives allowance for internal overpressure acting on covers, doors, inspection
windows, etc. of the switchgear and also takes into consideration the thermal effects of
the arc or its roots on the enclosure and of ejected hot gases and glowing particles. But it
does not cover the damages to partitions and shutters and hence does not cover all effects
that may constitute a risk (such as toxic gases). The test procedure only simulates
situations when doors and covers are fully closed and correctly secured.
Tests on other equipment 119
The choice of functional units, their numbers, their equipment and their position in the
test area as well as the place of initiation of the arc are to be decided between the
manufacturer and user. The following points should be observed:
Functional units of representative sizes should be tested.
Mounting conditions should be as close as possible to those of normal service.
The test room should at least reflect the actual installation conditions viz., the
floor, the ceiling, two walls perpendicular to each other and simulated cable
access ways. If the switchgear is installed in combination with a special
exhaust channel which normally leads the gas out of the room in actual
working conditions then no room mock-up is necessary.
The functional units should be fully equipped. Mock-ups of internal
components are permitted, provided they have the same volume and external
material as the original items.
The test unit should be earthed at the point provided.
In the case of gas-filled compartments the test should be made with the
original gas at the rated filling pressure. As an alternative and with the
agreement of the manufacturer, the test may be carried out with air, but
equivalent pressure rise should be given due consideration.
The arc should be initiated in a way that is representative of faults which
could occur under service conditions.
The tests on metal-enclosed switchgear should be carried out on three-phase units. The
short-circuit current applied during the test corresponds to the rated short-time withstand
current. It may be lower, if specially required by the manufacturer. The applied voltage of
the test circuit should be equal to the rated voltage of the metal-enclosed switchgear. A
lower voltage may be chosen if the following conditions are met:
The current remains practically sinusoidal
The arc is not extinguished prematurely.
The short-circuit current for which the metal-enclosed switchgear is specified with
respect to arcing should be set within a +5%0% tolerance. This tolerance applies to the
prospective current only if the applied voltage is equal to the rated voltage. The current
should remain constant. If the test plant does not permit this, the test should be extended
until the integral of the AC component of the current equals the value specified within a
tolerance of + 10%0%. In this case, the current should be equal to the specified value at
least during the first three full cycles and should not be less than 50% of the specified
value at the end of the test.
The instant of closing should be chosen so that the prospective value of the peak current
(with a tolerance of + 5%0%) flowing in one of the outer phases is 2.5 times the r.m.s.
value of the AC component defined above so that a major loop also occurs in the other
outer phase. If the voltage is lower than the rated voltage, the peak value of the short-
circuit current for the metal-enclosed switchgear under test should not drop below 90% of
the prospective peak value.
At a rated frequency of 50 Hz or 60 Hz, the frequency at the beginning of the test
should be between 48 Hz and 62 Hz. At other frequencies it should not deviate from the
rated value by more than 10%.
The duration of the arc is chosen in relation to the probable duration of the arc
determined by the protection facilities and does not normally exceed 1 second. For testing
metal-enclosed switchgears with pressure relief devices, an arc duration of 0.1 second is
120 Practical HV and MV Testing of Electrical Equipment
generally sufficient to prove its resistance to internal pressure. This does not apply for
gas-filled compartments. It is generally not possible to calculate the permissible arc
duration for a current which differs from that used in the test. The maximum pressure
during the test will generally not decrease with a shorter arcing time and there is no
universal rule according to which the permissible arc duration may be increased with a
lower test current.
The neutral is only earthed in the case of metal-enclosed switchgear to be operated in a
solidly earthed system. Care should be taken in order that the connections do not alter test
conditions. Generally, inside the enclosure, the arc may be fed from two directions and
the direction should be the one which is likely to result in the highest stress.
The arc should preferably be initiated between the phases by means of a metal wire of
about 0.5 mm diameter or, in the case of segregated phase conductors, between one phase
and earth. If the application of such a wire is not practicable (for arc initiation in a
component), as an alternative it is permissible to initiate the fault by other methods. In
this case, the method chosen should be agreed upon by the manufacturer and the user. In
functional units where live pacts are covered by solid insulating material, the arc should
be initiated between two adjacent phases or, in the case of segregated phase conductors,
between one phase and earth at the following locations:
At gaps in the insulation of insulation-embedded parts
By perforation at insulated joints made on-site when prefabricated insulation
parts are not used. Solid insulation should not be perforated.
The in-feed from the supply circuit should be three-phase to allow the fault to
become three-phase. The point of initiation should be chosen so that the
effects of the resultant arc produce the highest stresses in the functional unit.
In case of doubt it may be necessary to make more than one test on each
functional unit.
Pieces of black cotton cloth so arranged that their cut edges do not point
toward the test unit, serve as indicators of the performance. Care should be
taken to see that they cannot ignite each other. This is achieved by fitting
them in a mounting frame of steel sheet. The indicator dimensions should be
about 150 mm 150 mm.
Indicators should be fitted vertically at the operators side of the enclosed switchgear
and, if applicable, at sides which are readily accessible to personnel. They should be
placed, up to a height of 2 m and at a distance of 30 cm + 5% from the enclosed
switchgear, facing all points where gas is likely to be emitted (e.g. joints, inspection
windows, doors). Care should be taken when positioning the indicators to take into
account the possibility of hot gas escaping in slant directions. Indicators should also be
arranged horizontally at a height of 2 m above the floor and between 30 cm and 80 cm
from the enclosed switchgear. Black cretonne (cotton fabric approximately 150 g/m
2
)
should be used for the indicators.
Indicators should be fitted vertically on all accessible sides of the enclosed switchgear.
They should be placed, up to a height of 2 m and at a distance of 10 cm 5% from the
enclosed switchgear, facing all points where gas is likely to be emitted (e.g. joints,
inspection windows, doors). Care should be taken when positioning the indicators. They
should also be arranged horizontally at a height of 2 m above the floor and between 10
cm and 80 cm from the enclosed switchgear and control-gear. If the test unit is lower than
2 m, indicators should be placed horizontally on the top covers, facing all points where
gas is likely to be emitted and close to the vertical indicators, which in this case, are only
Tests on other equipment 121
required up to the actual height of the equipment. Black cotton-interlining lawn
(approximately 40 g/m
2
) should be used for the indicators.
It is to be observed:
Whether correctly secured doors, covers, etc., do not open
Whether parts which may cause a hazard fly off. This includes large parts or
those with sharp edges, for example inspection windows, pressure relief flaps,
cover plates, etc.
Whether arcing does not cause holes to develop in the freely accessible
external parts of the enclosure as a result of burning or other effects.
Whether the indicators arranged vertically do not ignite. Indicators ignited as
a result of paint or stickers burning are excluded from this assessment.
Whether the indicators arranged horizontally do not ignite. Should they start
to burn during the test, the assessment criterion may be regarded as having
been met, if proof is established of the fact that the ignition was caused by
glowing particles rather than hot gases. Pictures taken by high-speed cameras
should be produced as evidence.
Whether all the earthing connections are still effective.
The following information should be given in the test report:
Rating and description of the test unit with a drawing showing the main
dimensions, details relevant to the mechanical strength, the arrangement of the
pressure relief flaps and the method of fixing the metal-enclosed switchgear to
the floor and to the walls.
Arrangement of the test connections.
Point and method of initiation of the internal fault.
Arrangement and material of indicators with respect to the type of
accessibility.
For the prospective or test current:
RMS value of the AC component during the first three half-cycles
Highest peak value
Average value of the AC component over the actual duration of the
test
Test duration
Assessment of the test results, including a record of the observations.
9.3 MV motors
The following are the major tests conducted on medium voltage motors that are used to
drive mechanical equipment like compressors, blowers, pumps, etc. The tests not only
determine the losses and the efficiency but also cover procedures involved in calculating
the various loss components to arrive at the efficiency figures.
Insulation resistance test
HV test on windings to round for 1 minute
Over speed test
Measurement of losses
Measurement of stator winding resistance across every two terminals
No load test
Locked rotor test and measurement of copper losses
Polarization index for each winding
122 Practical HV and MV Testing of Electrical Equipment
Measurement of shaft voltages
Shaft vibration tests
Bearing housing vibration tests
Temperature rise test
Insulation power factor test
Over load test
Noise level measurements
Some of the tests like noise level measurements and heat run test are type tests which
are conducted on one motor if multiple motors of the same rating are supplied. The stator
coils and insulation also go through high voltage tests before assembly of motor coils.
9.4 MV capacitors
The capacitors are mainly in the form of multiple banks and mounted either indoor or
outdoor. The major tests to be conducted per IEC are as below.
9.4.1 Routine tests
Capacitance measurement.
Measurement of the tangent of the loss angle (tan ) of the capacitor.
Voltage test between terminals.
AC voltage test between terminals and container.
Test of internal discharge device.
Sealing test.
Discharge test on internal fuses.
9.4.2 Type tests
Thermal stability test.
Measurement of the tangent of the loss angle (tan ) of the capacitor at
elevated temperature
AC voltage test between terminals and container
Lightning impulse voltage test between terminals and container.
Short-circuit discharge test.
Test of an external fuse in combination with a capacitor.
Disconnecting test on internal fuses.
9.4.3 Test procedures
Some of the test procedures are given below
Capacitance measurement
The capacitance should be measured at 0.9 to 1.1 times the rated voltage, using a method
that excludes errors due to harmonics. Measurement at another voltage is permitted,
provided that appropriate correction factors are agreed upon between the manufacturer
and the purchaser. The final capacitance measurement should be carried out after the
voltage test.
Tests on other equipment 123
In order to reveal any change in capacitance, for example due to puncture of an
element, or failure of an internal fuse, a preliminary capacitance measurement should be
made, before the other electrical routine tests. This preliminary measurement should be
performed with a reduced voltage not higher than 0.15 times the nominal voltage.
The capacitance should not differ from the rated capacitance by more than:
5 % to +15 % for capacitor units or banks containing one unit per phase.
5 % to +10 % for banks up to 3 Mvar total rating.
0 % to +10 % for banks from 3 Mvar to 30 Mvar total rating.
0 % to +5 % for banks above 30 Mvar total rating.
TAN of the capacitor
The capacitor losses (tan ) should be measured at 0.9 to 1.1 times the rated voltage using
a method that excludes errors due to harmonics. The accuracy of the measuring method
and the correlation with the values measured at rated voltage and frequency should be
given.
High voltage test
The AC test should be carried out with a substantially sinusoidal voltage equal to 2.15
times the nominal system voltage. In the United States of America the value is twice the
nominal voltage. In case of DC test the test voltage should be 4.3 times.
Lightning impulse test
This is a type test and conducted between terminals and the container. The lightning
impulse test is applicable for capacitor units intended for use in banks with insulated
neutral and for connection to overhead lines.
Units having all terminals insulated from the container, and with the containers
connected to ground, should be subjected to fifteen impulses of positive polarity followed
by 15 impulses of negative polarity applied between bushings joined together and the
container. After the change of polarity, it is permissible to apply some impulses of lower
amplitude before the application of the test impulses.
The capacitor is considered to have passed the test if:
No puncture has occurred
Not more than two external flashovers occurred at each polarity
The wave shape has revealed no irregularities or no significant deviation from
recordings at reduced test voltage
The lightning impulse test should be made with a wave of 1.2/50 ms to 5/50
ms having a crest value corresponding to the insulation test requirement.
9.5 Disconnectors
9.5.1 Type tests
Mandatory type tests
Tests to prove satisfactory operation and mechanical endurance
124 Practical HV and MV Testing of Electrical Equipment
Optional type tests
Tests to prove the short-circuit making performance of earthing switches
Tests to prove satisfactory operation under severe ice conditions
Tests to prove satisfactory operation at temperature limits
Tests to verify the proper function of position indicating devices
Tests to prove the bus-transfer current switching capability of disconnectors
Tests to prove the induced current-switching capability of earthing switches
Tests to prove the bus-charging current switching ability of disconnectors
used in metal enclosed switchgear.
9.5.2 Routine tests
Dielectric test on the main circuit
Dielectric test on auxiliary and control circuits
Measurement of the resistance of the main circuit
Tightness test
Design and visual checks
Mechanical operating tests
9.5.3 Procedures
Dielectric tests
Dielectric tests on disconnectors or earthing switches when in the OPEN position should
be carried out
With the minimum isolating distance for the disconnector or
Gap for the earthing switch at which the indicating or signaling device can
signal the position OPEN or
The minimum isolating distance compatible with the locking arrangements
whichever is the smallest.
The disconnector or earthing switch should be considered to have passed the impulse
tests if the following conditions are fulfilled:
The number of disruptive discharges should not exceed two for each series of
15 impulses.
No disruptive discharges on non-self-restoring insulation should occur.
This is verified by at least five impulses without disruptive discharge following that
impulse out of the series of 15 impulses, which caused the last disruptive discharge. If
this impulse is one of the last five out of the series of 15 impulses, additional impulses
should be applied. Some times disruptive discharges may occur and evidence cannot be
given during testing that the disruptive discharges were on self-restoring insulation. In
such cases, after the completion of the dielectric tests the disconnector or earthing switch
should be dismantled and inspected. If punctures of non-self-restoring insulation are
observed, the disconnector or earthing switch should be considered to have failed the test.
10
Field tests
10.1 Need for field tests
The units are normally tested at the manufacturers works and transported to the site once
the test results are satisfactory. Invariably there will be considerable time elapsed between
the factory tests and the readiness of the site where the installation will take place. The
time elapsed may vary from a month to several months depending on many factors. The
installation and provision of necessary connections also takes considerable time, even if
the site is waiting for the equipment. The following factors are unavoidable from the time
the equipment is ready at manufacturers place till it is ready to get charged in the place
of use.
Time for packing and arranging for the transportation after completing all the
commercial formalities.
Transportation time from the manufacturers works to the place of installation
depending upon whether the item is imported or locally available. Even in
case of locally available equipment distances may make the transportation and
delivery time go from a week to few weeks.
Possible damages during transportation, either directly on the equipment or
indirectly due to unknown reasons. Rough handling and improper packing can
also lead to unknown damages.
The delay in readiness of the foundation or the building may result in the
equipment being kept in unfavorable climatic conditions that can affect
internal insulation. Some times environmental conditions at a construction site
or an existing nearby plant can also result in some deterioration to internal
components and oil used in the equipment.
All the above reasons plus many other possible human errors generally delay the
energizing of electrical equipment. Hence it is necessary to ensure that there are no
internal damages that can affect its life and performance. For example, the oil dielectric
strength might have gone down over a period of time that would require filtration before
charging a transformer. If not, an internal flashover or short circuit may make the whole
126 Practical HV and MV Testing of Electrical Equipment
project wait while the transformer is repaired. Similarly in a switchgear panel, some
internal links or shorting may lead to problems.
The above reasons are basically related to the delay in energizing due to unavoidable
reasons. Once the equipment is energized, it is necessary to ensure periodical
maintenance to maintain health. Maintenance may result in replacement of components,
some adjustments in the internal mechanism, rewiring, etc. All these need to be checked
for correctness before the equipment is put back into service.
Some basic tests are prescribed (especially for HV/MV equipment) that are to be
conducted before restoring the service or charging the equipment for the first time. These
tests are called pre-commissioning tests/checks, commissioning tests, maintenance tests,
etc and since most of these tests are done in the field of service, these are referred to as
field tests in this chapter.
Field tests are usually performed by independent contractors, the installation contractor
or the manufacturer himself. The individuals who perform the acceptance tests should
preferably be certified and/or licensed for the equipment under test. The system should be
initially checked for damage, deterioration and component failures using specific
component checks, inspections, and tests defined by the equipment manufacturer. Then
the interconnection of the system components should be checked in a de-energized and
energized state, to verify the proper interconnection and operation of components,
ON/OFF control, system interlocks and protective relaying functions. It is recommended
that all field tests are witnessed by a person who could be the operator of the plant or in
case of shortage of skilled man power, a commissioning engineer who is not associated
professionally with the agency/person performing the tests. Once the above tests are
complete, the system can be energized, operational tests conducted and measurements
recorded. All steps and results of the field tests should be carefully documented for
review and for use in future for comparison. Considerable variation in the results of
present tests compared to earlier tests is indicative of problems like deterioration of
insulation, dirty environmental conditions, etc.
10.2 General safety procedures
The safety procedures given below are from IEEE Standard 510-1983 which stipulates
safety practices to be followed by all personnel dealing with high voltage applications and
measurements, so that any possible accidents due to the presence of electrical hazards
while conducting the tests are avoided.
Safety considerations in electrical testing apply not only to personnel but to the test
equipment and apparatus and or the system under test. These recommended practices
generally cover the practices needed while testing in laboratories, in the field and of
systems incorporating high voltage power supplies, etc. A voltage of approximately 1,000
volts has been assumed as a practical minimum for these types of tests. Individual
judgment is necessary to decide if the requirements of these recommended practices are
applicable in cases where lower voltages or special risks are involved.
10.2.1 Basic precautions
All ungrounded terminals of the test equipment or apparatus under test should be treated
as energized and hence any contact with enclosures and internal parts always avoided.
The test set grounding connections should be solidly connected to the equipment being
tested. As a minimum, the current capacity of the ground leads should exceed that
necessary to carry the maximum possible ground current. The effect of ground potential
rise due to the resistance and reactance of the earth connection should be considered.
Field tests 127
Precautions should be taken to prevent accidental contact of live terminals by
personnel, either by shielding the live terminals or by providing barriers around the area.
The circuit should include instrumentation for measuring and/or indicating the test
voltages.
Appropriate master isolation switch or an observer should be provided to ensure
immediate de-energizing of test circuits in case of unforeseen problems occurring. In the
case of DC tests, provisions for discharging and grounding charged terminals and
supporting the insulation should also be considered.
High-voltage and high-power tests should be performed and supervised by qualified
personnel only.
10.2. 2 Test area safety practices
Appropriate warning signs like DANGER HIGH VOLTAGE should be posted on or
near the entrance in case of indoor equipment or on the barrier at all possible entry points.
Automatic grounding devices should be provided to apply a visible ground on the high-
voltage circuits once they are de-energized after the test. This may not be practically
feasible for most HV/MV equipment. In such cases the operator should attach a ground to
the high-voltage terminal using a suitably insulated handle. In the case of several
capacitors connected in series, it is not always sufficient to ground only the high-voltage
terminal. The exposed intermediate terminals should also be grounded. This applies in
particular to impulse generators where the capacitors should be short-circuited and
grounded before and while working on the generator.
Safe grounding of instrumentation should take precedence over proper signal grounding
unless other special precautions have been taken to ensure personnel safety.
10.2.3 Control and measurement circuits
Leads should not be run from a test area unless they are contained in a grounded metallic
sheath and terminated in a grounded metallic enclosure and other precautions have been
taken to ensure personnel safety. Control wiring, meter connections and cables running to
oscilloscopes fall into this category. Meters and other instruments with accessible
terminals should normally be placed in a metal compartment with a viewing window.
Temporary measuring circuits should be located completely within the test area and
viewed through the fence. Alternatively, the meters may be located outside the fence,
provided the meters and leads, external to the area are enclosed in grounded metallic
enclosures.
Temporary control circuits should be treated the same as measuring circuits and housed
in a grounded box with all controls accessible to the operator at ground potential.
10.2.4 Grounding and shorting
The routing and connections of temporary wiring should be such that they are secure
against accidental interruptions that may become hazardous to personnel or equipment.
Devices which rely on a solid or solid/liquid dielectric for insulation should preferably
be grounded and short-circuited when not in use.
Any capacitive object which is not in use but may be in the influence of a DC electric
field should have its exposed high-voltage terminal grounded. Failure to observe this
precaution may result in a voltage induced in the capacitive object by the field.
Capacitive objects having a solid dielectric should be short-circuited after DC proof
testing. If not, it may result in a buildup of voltage on the object due to dielectric
128 Practical HV and MV Testing of Electrical Equipment
absorption in the insulation. The short circuit should remain on the object until the
dielectric absorption has dissipated or until the object has been reconnected to a circuit. It
is good practice for all capacitive devices to remain short-circuited when not in use.
Any open circuited capacitive device should be short-circuited and grounded before
being contacted by personnel.
10.2.5 Spacing
All objects at ground potential must be placed away from all exposed high voltage points
at a minimum distance of one inch (25.4 mm) for every 7,500 volts, e.g. 50 kV requires a
spacing of at least 6.7 inches (171 mm).
A creepage distance of at least one inch (25.4 mm) for every 7,500 volts for insulators
placed in contact with high voltage points.
10.2.6 High-power testing
High-power testing involves a special type of high-voltage measurement in that the level
of current is very high. Careful consideration should be given to safety precautions for
high-power testing for this very reason. The explosive nature of the test specimen also
brings about special concern relating to safety in the laboratory.
Protective eye and face equipment should be worn by all personnel
conducting or observing a high-power test where there is a reasonable
probability that eye or face injury can be prevented by such equipment.
Typical eye and face hazards present in high-power test areas include intense
light (including ultraviolet), sparks, and molten metal.
Safety glasses containing absorptive lenses should be worn by all personnel observing a
high-power test even if electric arcing is not expected. Lenses should be impact-resistant
and have shade numbers consistent with the ambient illumination level of the work area
but yet capable of providing protection against hazardous radiation due to any inadvertent
electric arcing.
Whenever electric arcs are to be directly observed, safety glasses containing filter
lenses should be worn by all personnel observing the electric arc test.
10.2.7 General
All high-voltage generating equipment should have a single obvious control to switch the
equipment off under emergency conditions.
All high-voltage generating equipment should have an indicator which signals that the
high-voltage output is enabled.
All high-voltage generating equipment should have provisions for external connections
(interlock) which, when open, cause the high-voltage source to be switched off. These
connections may be used for external safety interlocks in barriers or for a foot or hand
operated safety switch.
The design of any piece of high-voltage test equipment should include a failure analysis
to determine if the failure of any part of the circuit or the specimen, to which it is
connected, will create a hazardous situation for the operator. The major failure shall be
construed to include the probability of failure of items that would be overstressed in the
event of a major failure. The analysis may be limited to the effect of one major failure at a
time, provided that the major failure is obvious to the operator.
Field tests 129
10.3 Transformers
10.3.1 Visual and mechanical inspection
Inspect for physical damage and record, if any.
Ensure nameplate information meets latest one line diagram and record
discrepancies, if any.
Verify proper operation of all auxiliary devices.
Check and ensure tightness of bolted joints as per manufacturers
recommendations.
Ensure proper level of oil in tank and bushings.
Conduct mechanical tests of auxiliary devices like OLTC, etc.,
10.3.2 Electrical tests
Insulation resistance tests shall be conducted between windings and windings
to ground. Recommended test voltages are
150 600 V Rating 1000 V megger
501 5000 V Rating 2500 V megger
Above 5001 V 5000 V megger
Polarization index value (10 minutes IR to 1 minute IR) should be found and
must exceed 1.5
Turns ratio test on all tap positions.
Measurement of power factor test values for bigger transformers generally
above 10 MVA.
Oil Dielectric test results should comply with the following.
Dielectric breakdown voltage 35 kV minimum below 69 kV, and
30 kV minimum for 69 kV upwards.
Neutralization number 0.025 mg KOH/gm, maximum.
Interfacial tension 35 dynes/cm minimum.
Color 1.0 maximum.
Winding resistance values shall not exceed 1.0% for adjacent windings and
comparable overall.
AC high voltage potential test not exceeding 75% of the factory test
values.
130 Practical HV and MV Testing of Electrical Equipment
10.3.3 Acceptance criteria
Site Acceptance tests Test criteria and Acceptable values
Oil Dielectric breakdown voltage test Normally with disc or spherical electrodes having
2.5 m spacing 30 k minimum acceptable voltage
Insulation Resistance test between
windings and windings to earth
The IR values shall be as below.
Oil filled: 100 Meg Ohm upto 600 V, 1000 Meg
Ohm 5000 V and 5000 Mega Ohm beyond 5000 V.
Dry Type: five times the above figures.
Ratio check at normal tap and other taps Values should be within 0.5% of the calculated
values, same as the factory tests.
Winding Resistance Cross check for conformance with factory tests.
Changes require thorough investigation.
Pressure test if transformer is supplied with
inert gas.
At least 6 pounds pressure for 12 hours minimum
and check for any leaks using soap solution around
seals and gaskets.
Power factor (DDF) test for above 15 kV
windings rated above 10 MVA
Ensure the values are below 0.5%
Oil sample test In an approved laboratory and ensure the test values
are within acceptable figures given earlier.
Accessories test Ensure proper operation of all accessories, relays,
pressure relief device, gauges, etc.
Table 10.1
Transformer field tests acceptance values
10.4 Switchgears
10.4.1 Visual and mechanical inspection
Verify missing parts or damaged parts
Check all components as per approved drawings
Check and ensure tightness of bolted joints as per manufacturers
recommendations
Inspect and ensure proper anchorage and grounding
Checking of breaker alignment
Proper operation of safety shutter
Mechanical ON/OFF Operation verification
Field tests 131
10.4.2 Electrical tests
Contact resistance check by ducter
Insulation resistance of main bus with appropriate tester (1 kV or 2.5 kV or 5
kV)
Insulation resistance test on PT & control power transformers
Insulation resistance test on breakers phase-to-phase, phase-to-ground and
across open contacts
Hi-pot test on vacuum bottles to check integrity across open contacts
Calibration of all relays by primary and secondary injection as appropriate
Electrical ON/OFF operation with auxiliary AC/DC supply.
Tripping checks on set values of the protective relays at minimum voltages
Check for continuity and correct operation of all remote wiring
Insulation resistance check of control wiring
10.4.3 Acceptance criteria
Normal value shall be around 500 micro ohms with breakers in closed
position and shall generally be provided by manufacturers.
Over potential and DC high-pot test values shall be as per tables given in
chapters 2 and 3.
10.5 High voltage disconnectors
10.5.1 Visual and mechanical checks
General inspection and verification of nameplate ratings
Mechanical ON/OFF operation both manual and motor, if provided.
Blade alignment and contact separation verification
Mechanical key interlocks and their functions.
10.5.2 Electrical checks
Insulation resistance test between phase-to-phase and phase-to-ground using a
suitable tester, based on the equipments rating
DC over potential test pole to pole and pole to ground
Contact resistance across each switch blade with ducter.
10.5.3 Test values
Over potential values shall meet the table values as per equipment ratings. Generally tests
are limited to around 75% of he values given in standards to minimize damages.
Contact resistance values shall be limited to around 50 micro ohms and differences of
more than 50% with respect to the adjacent contact values shall be investigated and
corrected.
10.6 MV cables
10.6.1 Visual and mechanical inspection
Inspect exposed parts for mechanical damages, if any
132 Practical HV and MV Testing of Electrical Equipment
Ensure that sizes are proper matching the loads
Inspect for proper supports, shield groundings and proper termination and
bolted connections
Ensure bending radii are meeting the recommended values.
10.6.2 Electrical tests
The first test is the DC high-pot test for each conductor with appropriate test voltages
based on the system voltages and insulation. This shall be done in incremental values to
about 8 steps from zero and record the leakage currents at each incremental voltage. The
test at required voltage shall be for 10 minutes. Take the readings of leakage currents
during the incremental voltage steps (one every minute or 30 seconds) and the same
during the last 10 minutes with the test voltage. The voltages shall then be brought to zero
slowly and the voltage held up in the tested terminals shall be discharged to ground.
Rated Line voltage
Volts
Conductor size
AWG
100% insulation level 133% insulation level
20015000 81000 25 25
50018000 61000 35 35
800115000 21000 55 65
1500125000 11000 80 100
2500135000 1/01000 100 N.A.
Table 10.2
IR test values for MV cables
Insulation resistance test phase to phase and phase to ground with appropriate
instrument
10.6.3 Acceptance criteria
The variation in leakage currents shall be linear and proportionate to the incremental
voltages.
The slope shall be negative.
Maximum leakage current shall preferably be restricted to about
The IR values shall be not less than 250 megohms
10.7 MV bus ducts
10.7.1 Visual and mechanical inspection
Inspect the bus for physical damage, if any and ratings in line with approved
drawings and nameplate data
Bus bar material and hardware as per design data
Proper Bracing, Insulator supports, suspension alignment and grounding
connection
Tightness of bolts in line with the manufacturers recommendations
Field tests 133
10.7.2 Electrical tests
Insulation resistance test phase to phase and phase to ground with appropriate
instrument
DC Hi-pot test on each phase to phase and phase to ground.
10.7.3 Acceptance criteria
Bus tightening values shall be proper with correct torque wrench
IR values in line with the table below
Over potential tests withstood for appropriate voltages based on system rated
voltage
Rated Voltage AC Voltage DC Voltage
5 14.3 20.2
15 27 37.5
25 45 ---
35 60 ---
Table 10.3
Hi-pot test values for busducts
10.8 Instrument transformers
10.8.1 Visual and mechanical inspection
Inspection and verification on physical damages, if any and compliance with
approved drawings
Mechanical clearances and proper operation of disconnecting switches for
potential transformers
Proper grounding and CT shorting links.
10.8.2 Electrical tests
Polarity verification as per connections
Transformer ratio in case of voltage transformers
Insulation resistance test on secondary to ground with 500 V instrument
Optional saturation curve and burden test on secondary side
Transformer ratio
Secondary LV injection tests on VT with primary disconnected
10.8.3 Acceptance criteria
Polarity shall meet the requirements as per connections. If not, correct the
connections.
The ratio shall be within tolerance as per approved test reports.
The IR values shall be around 100 mega ohms.
134 Practical HV and MV Testing of Electrical Equipment
10.9 Rotating machinery
10.9.1 Visual and mechanical inspection
Inspection for physical damage, if any
Nameplate information meeting the requirements and load data
Proper anchoring, mounting and grounding connections
10.9.2 Electrical tests
Dielectric absorption test
Polarization index test
Insulation resistance phase to ground
No load and full load currents measurements
Vibration tests on bearings with portable devices
Over potential test winding to ground based on the system voltage 80% of the
factory test value plus 1000 volts.
10.9.3 Acceptance criteria
Polarization index of less than 3 shall be investigated for correction.
Full load current shall not exceed the nameplate value.
No issues in over-potential and IR tests.
Maximum vibration altitudes shall be less than specified values. Generally
less than 0.001 inch peak to peak for two pole, 0.002 inch for four pole,
0.0025 inch for six pole and 0.003 for higher pole motors.
10.10 Surge arresters
10.10.1 Visual and mechanical inspection
Inspect for physical damages, chipped or broken porcelain
Nameplate information meeting the system requirements
Grounding connections proper
10.10.2 Electrical tests
Sparkover test
RIV test
Power factor test (optional)
Ground continuity test
10.10.3 Acceptance criteria
Sparkover voltage must be between 1.5 to 2.0 times the rating.
No RIV below the rated voltage.
Power factor test values not much differing from test certificates.
Ground grid resistance les than 0.5 ohm.
Field tests 135
10.11 Outdoor bus structures
10.11.1 Visual and mechanical inspection
Arrangements in line with the plans
Verify supports are intact with no cracks, chipped porcelain, etc
Tightness of bus bar bolts by torques wrench
10.11.2 Electrical tests
IR test on each section phase to phase and phase to ground
Over potential test phase to phase and phase to ground
Bus section joints contact resistance measurements
10.11.3 Acceptance criteria
Bolt torque values as per manufacturers recommendation
IR and over potential results are satisfactory
Measured resistance not above 115% of calculates value or the earlier test
results. Investigate and correct if it is more.
10.12 Engine generators
10.12.1 Visual and mechanical inspection
Inspect for physical damages, if any.
Nameplate rating meeting the requirements
Proper anchorage, support and grounding.
10.12.2 Electrical tests
Dielectric absorption test winding to ground and polarization index
measurement
Engine shutdown protection checks
Resistive load bank test at 100% rated capacity not less than 30 minutes at
25%, 50% and 75% loads in steps and for 3 hours at 100% load. Record all
electrical parameters and vibration readings at coupling and bearings.
Over potential test phase to ground.
10.12.3 Acceptance criteria
Polarization index less than 3 requires investigation and correction.
Load test figures shall meet the manufacturers figures.
Vibration amplitudes shall be less than the factory test values.
10.13 Maintenance tests
International Electrical Testing Association Inc (NETA) recommends the following table
to be followed for deciding the periodicity of the above field tests as periodical
maintenance tests. The next table has values that decide the periodicity in months. This is
136 Practical HV and MV Testing of Electrical Equipment
only for guidance and the actual user should justify the periods based on the actual load
conditions and environmental factors.
10.13.1 Multiplication factors
The table called maintenance matrix table gives the multiplication factor to be applied for
the period provided in the next clause based on equipment conditions which may depend
on the usage and environmental factors. These have to be decided by the user and the
table serves as a mere guide. The periodicity also depends upon the criticality of the
equipment for satisfactory running of the total plant. High critical equipment may require
three to four times the periodicity needed for a low critical item. Similarly the poor
condition of the equipment due to local factors may require roughly three times more
maintenance inspection/tests compared to equipment in good condition.
EQUIPMENT CONDITION Equipment Reliability
Requirement POOR AVERAGE GOOD
LOW 1.0 2.0 2.5
MEDIUM 0.5 1.0 1.5
HIGH 0.25 0.50 0.75
Table 10.4
NETA matrix table
10.13.2 Recommended schedule
The values given in the following table shall be multiplied with the factors given in the
above table to arrive at the actual schedule in months for various equipments.
ITEM DESCRIPTION Visual Visual and
Mechanical
Visual,
Electrical and
Mechanical
Switchgear Panels 12 12 24
Small Dry type transformers 2 12 36
Large Dry type transformers 1 12 24
Oil filled transformers 1 12 24
Oil sampling -- -- 12
LV/MV/HV Cables 2 12 36
MV Busducts 2 12 24
MV/HV open switches 1 12 24
MV Vacuum/ SF6 Breakers 1 12 24
HV SF6 Breakers 1 12 12
AC/DC Motors 1 12 24
Field tests 137
AC/DC Generators 1 12 24
MV Motor control centers 2 12 24
Surge Arresters 2 12 24
Capacitors 1 12 12
Dry type reactors 2 12 24
Outdoor Bus structures 1 12 36
Engine Generators 1 2 12
Table 10.5
Inspection and testing frequency (in months)
138 Practical HV and MV Testing of Electrical Equipment
Appendix A
Exercises
1. INTRODUCTION
S.No Questions Participants answers / remarks
1 Briefly indicate the tests you have
witnessed or been involved in for three
kinds of HV and MV equipment in the
last couple of years.
2 Indicate the voltage levels based on which
the LV, MV, HV and EHV equipment are
classified per ANSI and IEC. What had
been your practice?
3 There is a power station with gas turbine
and heat recovery system with capacities
of the generators as below:
140 Practical HV and MV Testing of Electrical Equipment
a) Gas turbine generator :
Capacity: 292 MVA Full load current at
unit PF = 10704 amps
b) Steam turbine generator:
Capacity: 167 MVA Full load current at
unit PF = 6120 amps
The plant needs to evacuate the power at
220 kV using an outdoor switchyard.
Calculate what would be the ratings of the
transformers and list five major kinds of
HV and MV equipment that would be
interfacing with the generated power and
the switchyard.
4 Indicate the four insulating mediums most
commonly used in the present day HV and
MV equipment along with the names of
two equipments for each of the insulating
medium in which they are used.
5 Can vacuum be considered an insulating
medium? Why?
Indicate the ratings up to which the
vacuum technology is preferred.
6 List three main reasons for testing
electrical equipment before it is cleared
for shipment.
Exercises 141
7 List the likely problems you may face if
an MV switchgear which had been
manufactured as per the drawings was
sent into the field without conducting any
tests.
8 List the four categories of tests that are
normally conducted from the time an HV
equipment is ready for testing till it is
energized in the field
.
9 What are the normal temperature and
altitude on which the equipment electrical
standards and the test performance results
are based? List two likely issues that need
to be considered in regard to the tests and
results if the equipment is to be designed
for an ambient temperature of 10
0
C over
the standard temperature and an altitude
of about 5000 metres above sea level. The
equipment may be considered as a
transformer or switchgear.
2. INSULATION TESTING
2.1 Name the two main purposes for which
insulation is used around a conductor
charged at a high voltage.
142 Practical HV and MV Testing of Electrical Equipment
2.2 Name two insulations that are normally
adopted in the extra high voltage
transmission lines running through
transmission towers.
2.3 Name the two most common insulation
materials used in present day HV and MV
cables and indicate the temperature rise
permitted with these insulations for their
current ratings.
2.4 A power conductor at 6000 volts is
separated by an insulating medium from
the ground by 750 mm. Indicate the
minimum dielectric strength required for
the insulation medium to avoid flashover.
2.5 Name the three types of current that start
flowing when a high voltage insulation
tester applies a voltage between a terminal
and the ground.
2.6 Out of the above three currents, indicate
the type of current which is measured by
the tester. Will this current stabilize after
some time or will it start decaying for a
good insulation?
2.7 Indicate the common voltages that are
used in the insulation testers and whether
it is AC or DC
2.8 Indicate the likely trend in the values of
insulation resistances measured on
equipment today compared to the values
Exercises 143
measured one year back without doing
any changes. Indicate the possible factors
that could be responsible for the increase
or decrease in these values.
2.9 Specify at least two reasons why
insulation testing is preferred before the
equipment is energized in the field
2.10 Indicate the recommended test voltages
for equipment rated a) 4.16 kV b) 7.2 kV
and c) 11 kV
2.11 A transformer with vector group Dyn11 is
tested with an insulation tester a) phase to
neutral on secondary b) phase to phase on
primary and c) neutral to ground. Indicate
the likely readings if the transformer is in
good condition.
2.12 Indicate all the normal insulation tests
recommended for the above transformer.
Take primary terminals as A,B,C and
secondary terminals as a,b,c and n.
2.13 Indicate briefly five precautions that are
required while doing insulation testing on
high voltage equipment
144 Practical HV and MV Testing of Electrical Equipment
2.14 The ratio of insulation resistance at the
end of 15 minutes to the IR value at the
end of one minute is B while its
polarization index is A. Indicate which is
correct.
a) A = B
b) A > B
c) A < B
2.15 Indicate what measurement is taken
during a step voltage test, and the number
of steps recommended. Normally, for
what voltage ranges of equipments is this
test applied?
2.16 Indicate to what categories you will
associate the quality of insulation with
polarization readings of a) 2.5 b) 3.7 and
c) 5.6
GOOD
BAD
Needs inspection
Acceptable -
2.17 What is the type of current that is read in a
dielectric absorption test? Indicate the
time period for which the insulation is
tested to find its absorption ratio.
2.18 Calculate the dielectric absorption ratio of
the insulation with the following results.
What is the conclusion based on the value
arrived at?
Test Voltage : 2.5 kV
Current after one minute: 4 amperes
Machine capacitance: 1.5 Farad
Exercises 145
3. HIGH POTENTIAL TESTS
3.1 What is meant by the power factor value
of the insulation? Indicate the expected
power factor value for a good insulation.
3.2 Indicate the principle of AC Hertz test in
deciding the insulation quality.
3.3 Give a brief comparison of AC Hi-pot test
and DC Hi-pot test in regard to the factors
indicated, taking a value of A for AC
tester factor and D for DC tester.
a) Test Voltage Values
b) Size -
c) Test Current Values for the same
insulation-
3.4 Refer to Table 3.1 and indicate the
applicable values for 6600 volts
equipment in a Hi-pot test.
a) Factory AC Proof test value =
b) DC test voltage before
commissioning =
c) DC test Voltage during
maintenance =
3.5 Make a comparison of Tables 3.1 and 3.2,
Do you find them the same? What are
your observations on these two tables?
146 Practical HV and MV Testing of Electrical Equipment
4. OIL TESTING
4.1 Name the three main reasons why oil is
still considered a good medium for use in
transformers.
4.2 Indicate four major parameters of mineral
oil and the recommended minimum or
maximum permissible values for these
parameters.
4.3 Name three main factors that are
responsible for the deterioration in the
quality of oil in a transformer during its
service.
4.4 A transformer operates at a temperature of
70
0
C for about 2 hours, 80
0
C for 1 hour
and at 60
0
C for the rest of the time every
day. Calculate the anticipated life of the
transformer using the table 4.1.
4.5 Five dielectric tests on a transformer
indicate the BDV values as 28.5kV, 32
kV, 29kV, 31 kV and 32 kV. Check
whether the transformer oil can be
considered to have passed the test or not.
Do you recommend the transformer be
continued in service?
4.6 Indicate whether the statements given on
the right are TRUE or FALSE
a) The vacuum pump in the Oil
filtration unit is mainly used for
Exercises 147
removing the sludge.
b) The BDV of the oil will be double
its original value after two filtrations.
C) It is preferable to heat the oil to
around 60
0
C for fast removal of
moisture
4.7 About 60 mg of KOH is used to neutralize
500ml of a new transformer oil sample.
Check whether the oil quality is
acceptable or not assuming a density of
0.9 for the oil.
4.8 Name at least four gases that are produced
when the transformer in service is getting
overheated
4.9 Refer to Table 4.2. Indicate the acceptable
percentage of combustible gas in the
transformer. Give the reason.
4.10 Indicate four common methods that are
used for analyzing the content of
dissolved gases in transformer oil
4.11 Indicate a simple method to check
whether free water is present in
transformer oil.
148 Practical HV and MV Testing of Electrical Equipment
4.12 Indicate whether TRUE or FALSE for the
statements on the right.
a) Oil sample shall be taken with the
oil temperature at a lower temperature
b) Sealed transformer need inert gas
while taking oil sample.
5. TESTING OF TRANSFORMERS
5.1 List the six routine tests that are normally
conducted on a transformer at the
manufacturers works.
5.2 Indicate the allowable tolerances for the
test measurements as per standards,
compared to the guaranteed values for a
transformer.
a) Impedance Voltage:
b) Turns ratio
5.3 Indicate the acceptable turns ratio at all
the five taps for a transformer rated
110kV/36kV with OCTC having two taps
above and two taps below the normal tap
in 2.5% steps.
5.4 A, B, C and a,b,c,n are the primary and
secondary terminals of a transformer with
a vector group Dyn11. Terminals A,a are
interconnected and a 3 phase test voltage
of 400 volts is applied across A-B-C.
Draw the connections and indicate the
Exercises 149
expected readings of voltage between the
various terminals.
5.5 A transformer rated 11kV/ 400 volts,
1000KVA is short circuit tested and it is
noted that the short circuit current at
270Volts across primary causes 900
amperes in the secondary. Calculate the
impedance voltage based on these test
values.
5.6 Calculate the time in seconds for
conducting the induced voltage test at
150Hz for a 50Hz rated transformer.
5.7 Name the type of test that can roughly
indicate the correct performance of a
transformer under fully loaded condition
in the field. Why?
6. CT TESTING
6.1 Indicate the ratio of a CT with ratio of
100/5 amperes
6.2 Indicate whether the statements are TRUE
or FALSE
a) A transposing CT is connected in
parallel to the main current circuit.
b) Bar primary current transformers
are commonly used in the MV
switchgears.
150 Practical HV and MV Testing of Electrical Equipment
6.3 Calculate the current error for a CT which
gives an output of 3.1 amperes when 60
amperes primary current is flowing, with
the normal ratio being 100/5 amps.
6.4 Indicate the common accuracy classes of
current transformers used for metering.
6.5 Calculate the acceptable range of
secondary currents when 20% of the
primary current is circulated in a CT rated
250/5 amperes with 0.5 accuracy class.
6.6 The rated dynamic current of a CT is 450
amperes. What is its rated short time
thermal current?
6.7 What is the recommended CT primary
range for use in a 2000kVA, 11kV/400
volts transformer circuit?
6.8 Identify whether the statements are TRUE
/ FALSE
a) Knee point voltage of a 1 amp CT is
more than a 5 amps CT
b) It is not necessary to define the
knee point voltage for a CT used in
over current relay application
c) Knee point voltage of a metering
CT is more than that of a protection
CT with same secondary rated
currents.
Exercises 151
6.9 List the three main routine tests
recommended on a CT
6.10 List the three main type tests
recommended on a CT
6.11 List the three main special tests
recommended on a CT
7. VT TESTING
7.1 Calculate the voltage error of a 120 volts
secondary VT which produces 72.5 volts
on application of 60% of the rated primary
voltage
7.2 Indicate the rated voltage factors
commonly adopted for the voltage
transformers.
7.3 Identify the applicable values for the
various tests on a voltage transformer
a) Applicable test current for temperature
rise test
b) Number of impulses for lightning
impulse test on VTs with U
m
< 300kV
c) Number of impulses for switching
impulse test on VTs with U
m
300kV
d) Acceptable RIV value
152 Practical HV and MV Testing of Electrical Equipment
8. DUCTER TESTING
8.1 Which bridge instrument is necessarily
used in a standard ducter? Up to what
range of resistances can the same be
effectively used?
8.2 Identify five major electrical areas where
low resistance readings using ducter is
more common.
8.3 What are the recommended test current
values for breaker resistance tests with
respect to ANSI and IEC standards?
TESTS ON OTHER MAJOR EQUIPMENT & FIELD TESTS
This would preferably be an interactive session, with participants identifying tests they feel
can be conducted over and above the tests covered in these chapters, and discussing how
those tests can help in deciding or improving the performance of the various equipment
covered under these chapters.
Appendix B
Answers to Exercises
1. INTRODUCTION
S.No Questions Suggested answers
1 Briefly indicate what the tests you have
witnessed or been involved in for three
kinds of HV and MV equipment in the
last couple of years.
By Participants
2 Indicate the voltage levels based on which
the LV, MV, HV and EHV equipment are
classified per ANSI and IEC. What had
been your practice?
Not a firm answer. But following is a
close break-up
< 1000 V. LV
1000V to 69000V--- MV
69001V to 132000V HV
> 132000V .. EHV
3 There is a power station with gas turbine
and heat recovery system with capacities
of the generators as below:
a) Gas turbine generator :
Capacity: 292 MVA Full load current at
From the full load currents, it can be
noted that both the generators are
rated for 15.75 kV.
Hence these need transformers to step
Practical HV and MV Electrical Equipment Testing
154
unit PF = 10704 amps
b) Steam turbine generator:
Capacity: 167 MVA Full load current at
unit PF = 6120 amps
The plant needs to evacuate the power at
220 kV using an outdoor switchyard.
Calculate what the ratings of the
transformers would be and list five major
HV and MV equipment that would be
interfacing with the generated power and
the switchyard.
up from 15.75kV to 220kV to
evacuate the generated power.
Transformer capacities are generally
chosen close to the generated power
close to the preferred transformer
ratings per standards.
Here the preferred capacities are 300
MVA and 175 MVA respectively.
4 Indicate the four insulating mediums most
commonly used in the present day HV and
MV equipment along with the names of
two equipments for each of the insulating
medium in which they are used.
Air, Oil, Vacuum and SF6
5 Can vacuum be considered an insulating
medium? Why?
Indicate the ratings up to which the
vacuum technology is preferred.
Vacuum is like air at very low
pressure and hence it can be called
low pressure air insulation.
Vacuum technology is generally
limited to MV ratings with majority
limiting to around 33 kV.
6 List the three main reasons for testing
electrical equipment before it is cleared
Available in the manual.
EXERCISES WITH ANSWERS
155
for shipment.
7 List the likely problems you may face if a
MV switchgear which had been
manufactured as per the drawings was
sent into the field without conducting any
test.
- Malfunctioning of control circuits.
- Low IR value
- Possible arcing when rated voltage is
applied.
- Damaged components
8 List the four categories of tests that are
normally conducted from the time HV
equipment is ready for testing till it is
energized in the field
.
- Routine Tests
- Type Tests
- Sample/Acceptance Tests
- Special Tests
- Field Tests
9 What are the normal temperature and
altitude on which the equipment electrical
standards and the test performance results
are based? List two likely issues that need
to be considered in regard to the tests and
results if the equipment is to be designed
for an ambient temperature of 10
0
C over
the standard temperature and an altitude
of about 5000 metres above sea level. The
equipment may be considered as a
transformer or switchgear.
40 deg C and 1000 metres above sea
level.
Variations to these may lead to
- Higher temperature rise at nameplate
current
- Lower Test voltages
2. INSULATION TESTING
2.1 Name the two main purposes for which
insulation is used around a conductor
charged at a high voltage.
- Human safety
- Lower losses to ground
Practical HV and MV Electrical Equipment Testing
156
2.2 Name two insulations that are normally
adopted in the extra high voltage
transmission lines running through
transmission towers.
Porcelain Insulators and air
2.3 Name the two most common insulation
materials used in present day HV and MV
cables and indicate the temperature rise
permitted with these insulations for their
current ratings.
PVC 70 deg C
XLPE 90 deg C
2.4 A power conductor at 6000 volts is
separated by an insulating medium from
the ground by 750 mm. Indicate the
minimum dielectric strength required for
the insulation medium to avoid flashover.
8 volts per mm but allow tolerance to
take care of impurities which may
give a preferred value of 16 volts per
mm.
2.5 Name the three types of current that start
flowing when a high voltage insulation
tester applies a voltage between a terminal
and the ground.
Capacitance current
Dielectric absorption current
Leakage current
2.6 Out of the above three currents, indicate
the type of current which is measured by
the tester. Will this current stabilize after
some time or will it start decaying for a
good insulation?
Leakage current and it will not decay.
2.7 Indicate the common voltages that are
used in the insulation testers and whether
it is AC or DC
DC 500V, 1000V, 2500V, 5000 V
2.8 Indicate the likely trend in the values of
insulation resistances measured on
equipment today compared to the values
measured one year back without doing
any changes. Indicate the possible factors
It will be lower because of the
impurities, environmental factors and
general aging.
EXERCISES WITH ANSWERS
157
that could be responsible for the increase
or decrease in these values.
2.9 Specify at least two reasons why
insulation testing is preferred before the
equipment is energized in the field
Refer to the manual.
2.10 Indicate the recommended test voltages
for equipment rated a) 4.16 kV b) 7.2 kV
and c) 11 kV
a) 2500V b) 5000V and c) 5000 or
10000V
2.11 A transformer with vector group Dyn11 is
tested with an insulation tester a) phase to
neutral on secondary b) phase to phase on
primary and c) neutral to ground. Indicate
the likely readings if the transformer is in
good condition.
b) Zero. a) and c) In thousands of
megaohms.
2.12 Indicate all the normal insulation tests
recommended for the above transformer.
Take primary terminals as A,B,C and
secondary terminals as a,b,c and n.
2.13 Indicate briefly five precautions that are
required while doing insulation testing on
high voltage equipment
Refer to the manual.
2.14 The ratio of insulation resistance at the
end of 15 minutes to the IR value at the
end of one minute is B while its
polarization index is A. Indicate which is
correct.
A < B
Practical HV and MV Electrical Equipment Testing
158
2.15 Indicate what measurement is taken
during a step voltage test and the number
of steps recommended. Normally, for
what voltage ranges of equipments is this
test applied?
Indirectly the resistance of the
insulation, by measuring the current in
five steps of voltage for equipments
2.5 kV and above.
2.16 Indicate to what categories you will
associate the quality of insulation with
polarization readings of a) 2.5 b) 3.7 and
c) 5.6
GOOD
BAD
Needs inspection (c)
Acceptable (a) and (b)
2.17 What is the type of current that is read in a
dielectric absorption test? Indicate the
time period for which the insulation is
tested to find its absorption ratio.
Current absorbed by the capacitance
during voltage application. Charge
with DC for 10 to 30 minutes and take
readings keeping each step voltage for
one minute.
2.18 Calculate the dielectric absorption ratio of
the insulation with the following results.
What is the conclusion based on the value
arrived at?
Test Voltage : 2.5 kV
Current after one minute: 4 amperes
Machine capacitance: 1.5 Farad
1.07 and hence good.
3. HIGH POTENTIAL TESTS
3.1 What is meant by the power factor value
of the insulation? Indicate the expected
power factor value for a good insulation.
Resistive current to Capactive
current ratio. Preferred value is
less than 0.005.
3.2 Indicate the principle of AC Hertz test in
deciding the insulation quality.
High capacitive impedance at low
frequencies
EXERCISES WITH ANSWERS
159
3.3 Give a brief comparison of AC Hi-pot test
and DC Hi-pot test in regard to the factors
indicated, taking a value of A for AC
tester factor and D for DC tester..
a) Test Voltage Values
D>A
b) Size -
A>D
c) Test Current Values for the same
insulation- A> D
3.4 Refer to Table 3.1 and indicate the
applicable values for a 6600 Volts
equipment in a Hi-pot test.
a) Factory AC Proof test value =
b) DC test voltage before
commissioning =
c) DC test Voltage during
maintenance =
3.5 Make a comparison of Tables 3.1 and 3.2,
Do you find them the same? What are
your observations on these two tables?
Debatable. These values indicate the
different guidelines followed in
Electrical Tests and hence need a clear
specification while ordering
equipment.
4. OIL TESTING
4.1 Name the three main reasons why oil is
still considered a good medium for use in
transformers.
Insulation, Cooling and Arc quenching
4.2 Indicate four major parameters of mineral
oil and the recommended minimum or
maximum permissible values for these
parameters.
Practical HV and MV Electrical Equipment Testing
160
4.3 Name three main factors that are
responsible for the deterioration in the
quality of oil in a transformer during its
service.
4.4 A transformer operates at a temperature of
70
0
C for about 2 hours, 80
0
C for 1 hour
and at 60
0
C for the rest of the time every
day. Calculate the anticipated life of the
transformer using the table 4.1.
4.5 Five dielectric tests on a transformer
indicate the BDV values as 28.5kV, 32
kV, 29kV, 31 kV and 32 kV. Check
whether the transformer oil can be
considered to have passed the test or not.
Do you recommend the transformer be
continued in service?
It is preferable to go for filtration to
avoid failure within a short span.
4.6 Indicate whether the statements given on
the right are TRUE or FALSE
a) The vacuum pump in the Oil
filtration unit is mainly used for
removing the sludge. FALSE
b) The BDV of the oil will be double
its original value after two filtrations.
FALSE
C) It is preferable to heat the oil to
around 60
0
C for fast removal of
moisture
FALSE
EXERCISES WITH ANSWERS
161
4.7 About 60 mg of KOH is used to neutralize
500ml of a new transformer oil sample.
Check whether the oil quality is
acceptable or not assuming a density of
0.9 for the oil.
60 mg for 450 grams of oil which
gives a factor of 0.133 and hence is
not acceptable.
4.8 Name at least four gases that are produced
when the transformer in service is getting
overheated
4.9 Refer to Table 4.2. Indicate the acceptable
percentage of combustible gas in the
transformer. Give the reason.
4.10 Indicate four common methods that are
used for analyzing the content of
dissolved gases in transformer oil
4.11 Indicate a simple method to check
whether free water is present in
transformer oil.
Keep oil in refrigerator overnight.
4.12 Indicate whether TRUE or FALSE for the
statements on the right.
a) Oil sample shall be taken with the
oil temperature at a lower temperature
FALSE
b) Sealed transformer needs inert gas
while taking oil sample. TRUE
Practical HV and MV Electrical Equipment Testing
162
5. TESTING OF TRANSFORMERS
5.1 List the six routine tests that are normally
conducted on a transformer at the
manufacturers works.
5.2 Indicate the allowable tolerances for the
test measurements as per standards,
compared to the guaranteed values for a
transformer.
a) Impedance Voltage:
b) Turns ratio
5.3 Indicate the acceptable turns ratio at all
the five taps for a transformer rated
110kV/36kV with OCTC having two taps
above and two taps below the normal tap
in 2.5% steps.
OCTC is provided on the HV winding
which will correspond to voltages of
115.5kV, 112.75kV, 110kV,
107.25kV and 104.5kV. Turns ratio
shall match the voltage ratio at the
respective taps within a tolerance of
0.5%
5.4 A,B,C and a,b,c,n are the primary and
secondary terminals of a transformer with
a vector group Dyn11. Terminals A,a are
interconnected and a 3 phase test voltage
of 400 volts is applied across A-B-C.
Draw the connections and indicate the
expected readings of voltage between the
various terminals.
EXERCISES WITH ANSWERS
163
5.5 A transformer rated 11kV/ 400 volts,
1000KVA is short circuit tested and it is
noted that the short circuit current at
270Volts across primary causes 900
amperes in the secondary. Calculate the
impedance voltage based on these test
values.
Transformer full load current on
secondary is 1433 amperes.
Impedance value will be about 3.91%.
5.6 Calculate the time in seconds for
conducting the induced voltage test at
150Hz for a 50Hz rated transformer.
More than 30 seconds.
5.7 Name the type of test that can roughly
indicate the correct performance of a
transformer under fully loaded condition
in the field. Why?
Temperature rise test where the
maximum current is carried by the
winding till a stable temperature rise.
6. CT TESTING
6.1 Indicate the turns ratio of a CT with ratio
of 100/5 amperes
Turns ratio = 1 to 20
6.2 Indicate whether the statements are TRUE
or FALSE
a) A transposing CT is connected in
parallel to the main current circuit.
TRUE
b) Bar primary current transformers
are commonly used in the MV
switchgears.TRUE
Practical HV and MV Electrical Equipment Testing
164
6.3 Calculate the current error for a CT which
gives an output of 3.1 amperes when 60
amperes primary current is flowing, with
the normal ratio being 100/5 amps.
[20 3.1/60 1] 100 = 3.33%
6.4 Indicate the common accuracy classes of
current transformers used for metering.
0.2 and 0.5
6.5 Calculate the acceptable range of
secondary currents when 20% of the
primary current is circulated in a CT rated
250/5 amperes with 0.5 accuracy class.
Not more than 1 ampere 0.75% on
secondary.
6.6 The rated dynamic current of a CT is 450
amperes. What is its rated short time
thermal current?
450/2.5
6.7 What is the recommended CT primary
range for use in a 2000kVA, 11kV/400
volts transformer circuit?
Rated primary current about 105
amperes. 120 to 150 amperes is the
preferred range.
6.8 Identify whether the statements are TRUE
/ FALSE
a) Knee point voltage of a 1 amp CT is
more than a 5 amps CT FALSE
b) It is not necessary to define the
knee point voltage for a CT used in
over current relay application TRUE
c) Knee point voltage of a metering
CT is more than that of a protection
CT with same secondary rated
currents. FALSE
EXERCISES WITH ANSWERS
165
6.9 List the three main routine tests
recommended on a CT
6.10 List the three main type tests
recommended on a CT
6.11 List the three main special tests
recommended on a CT
7. VT TESTING
7.1 Calculate the voltage error of a 120 volts
secondary VT which produces 72.5 volts
on application of 60% of the rated primary
voltage
+ 0.5 volts.
7.2 Indicate the rated voltage factors
commonly adopted for the voltage
transformers.
1.2 continuous, 1.5 (30 sec) and 1.9
(30 sec and 8 hours)
7.3 Identify the applicable values for the
various tests on a voltage transformer
a) Applicable test Voltage for temperature
rise test
b) Number of impulses for lightning
impulse test on VTs with U
m
< 300kV
c) Number of impulses for switching
impulse test on VTs with U
m
300kV
d) Acceptable RIV value
a) 120% of rated voltage
b) 30 impulses
c) 15 impulses
d) 2500 V.
Practical HV and MV Electrical Equipment Testing
166
8. DUCTER TESTING
8.1 Which bridge instrument is necessarily
used in a standard ducter? Up to what
range of resistances the same can be
effectively used?
Kelvin Bridge. 10 ohms
8.2 Identify five major electrical areas where
low resistance readings using ducter is
more common.
8.3 What are the recommended test current
values for breaker resistance tests with
respect to ANSI and IEC standards?
IEC- 50 amps to rated current.
ANSI- Minimum 100 amperes to rated
current.
TESTS ON OTHER MAJOR EQUIPMENT & FIELD TESTS
This would preferably be an interactive session, with participants identifying tests they feel
can be conducted over and above the tests covered in these chapters, and discussing how
those tests can help in deciding or improving the performance of the various equipment
covered under these chapters.
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Technical Questions
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2. What is the purpose of insulation testing?
3. What does Hi-Pot test mean?
4. How is the dielectric test on oil performed?
5. How does one test for internal faults on switchgear?
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