High Voltage Testing
High Voltage Testing
High Voltage Testing
Withstand Voltage
Creepage Distance
TESTS OF INSULATORS
Type Test
To Check The Design Features and Quality
Type tests are done on samples when new
designs or design changes are introduced
Routine Test
To Check The Quality Of The Individual Test Piece.
To ensure the reliability of the individual test objects
High Voltage Tests Include
(i) Power frequency tests
(ii) Impulse tests
TESTS OF INSULATORS
POWER FREQUENCY TESTS
(a)
Dry and wet flashover tests:
The test object is sprayed for at least one minute before the
voltage application, and the spray is continued during the
voltage application.
If more than two flashovers occur out of five flash over test in
each set, then the insulator is deemed to have failed the test.
(b) Dry and wet withstand tests(one minute)
The test piece should withstand the specified voltage which is applied
under dry or wet conditions.
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Pollution Testing:
(i) Dust, micro-organisms, bird secretions, flies, etc., (ii) Industrial pollution
like smoke, petroleum vapours, dust, and other deposits, (iii) Coastal
pollution in which corrosive and hygroscopic salt layers are deposited on
the insulator surfaces, (iv) desert pollution in which sand storms cause
deposition of sand and dust layers, (v) ice and fog deposits at high altitudes
and in polar countries.
Pollution causes corrosion ,deterioration of the material, partial discharges
and radio interference.
Salt fog test is done.
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TESTING OF BUSHINGS
Power frequency tests
(a ) Power Factor-Voltage Test:
Set up as in service or immersed in oil.
Conductor to HV and tank to earth.
Voltage is applied up to the line value in increasing steps and then
reduced.
The capacitance and power factor are recorded in each step.
(b) Internal or Partial discharge Test:
To find the deterioration or failure due to internal discharges caused in
the composite insulation of the bushing.
Conducted using partial discharge arrangements
Performance is observed from voltage Vs discharge magnitude.
It is a routine test.
(c ) Momentary Withstand Test at Power frequency
Based on IS:2009
The bushing has to withstand the applied test voltage without
flashover or puncture for 30 sec.
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TESTING OF BUSHINGS
(d) One Minute withstand Test at Power Frequency
Most common & routine test
Test is carried in dry &wet for one minute.
In wet test, rain arrangement is mounted as in service.
Properly designed bushing should withstand without
flashover for one minute.
(e) Visible Discharge Test at Power Frequency
Conducted based on IS:2009
Conducted to determine radio interference during service
Conducted in dark room
Should not be any visible discharges other than arcing
horns/ guard rings.
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TESTING OF BUSHINGS
Impulse voltage tests:
Full wave Withstand Test
The bushing is tested for either polarity voltages
Five consecutive full wave is applied
If two of them flashed over, then 10 additional
applications are done.
If the test object has withstood the subsequent
applications of standard impulse voltage then it is
passed the test.
TESTING OF BUSHINGS
Thermal Tests
This is a type test for low rating and routine test for
high ratings.
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ii.
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Direct tests
i.
ii.
Synthetic Tests
Disadvantages:
1. Can be tested only in rated voltage and capacity of the network
2. Inconvenience and expensive installation of control and measuring
equipment is required in the field.
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A SC generator
Master CB
Resistors
Reactors and
Measuring devices
Switch
Reactor
Test device
CT
Short Circuit
Testing Gen
Circuit
Capacitance
5
Ic
Vc
Cc
T
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Iv
C0
Cv
5
Ic
Vc
Cc
T
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Iv
C0
Cv
TESTING OF CABLES
Different tests on cables are
i. Mechanical tests like bending test, fire resistance
and corrosion tests
ii. Thermal duty tests
iii. Dielectric power factor tests
iv. Power frequency withstand voltage tests
v. Impulse withstand voltage tests
vi.Partial discharge test
vii.Life expectancy tests
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TESTING OF CABLES
Dielectric power factor tests:
Done using HV Schering Bridge
The p.f is measured at 0.5, 1.0, 1.66 and 2.0 times the
rated phase-to-ground voltage of the cable
Max. value of p.f and difference in p.f b/w rated voltage and
1.66 times of rated voltage is specified.
The difference between the rated voltage and 2.0 times of
rated voltage is also specified
A choke is used in series with the cable to form a resonant
circuit.
This improves the power factor and rises the test voltage
b/w the cable core and the sheath to the required value
when a HV and high capacity source is used.
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TESTING OF CABLES
High voltage testing on Cables:
Power frequency HV A.C, DC and impulse voltages are applied to
test the withstanding capability
Continuity is checked with high voltage at the time of
manufacturing
Routine test:
Cable should withstand 2.5 times of the rated voltage for 10
mins without damage in insulation
Type test:
Done on samples with HVDC & impulses
DC Test:1.8 times of the rated voltage (-ve) applied for 30 mins.
Impulse Test: 1/50S wave applied. Cable should withstand 5
consecutive impulses without any damage
After impulse test, power frequency & power factor test is
conducted to ensure that no failure occurred during impulse test.
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TESTING OF CABLES
Partial Discharge test:
Discharge measurement:
i.
H
V
D.D
F
C
Equivalent Circuit of Cable for discharges
(ii)
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D.D
H
V
TESTING OF CABLES
If the coupling capacitor connected, transient wave will be
received directly from the discharge cavity and second
wave from the wave end i.e., two transient pulses is
detected
In circuit shown in fig (ii), no severe reflection is occurred
except a second order effect of negligible magnitude.
Two transients arrive at both ends of the cable-super
imposition of the two pulses detected-give serious error in
measurement of discharge
ii.
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Location of discharges
Voltage dip caused by discharge or fault is travelled along the length
& determined at the ends
Time duration b/w the consecutive pulses can be determined
The shape of the voltage gives information on the nature of
discharges
TESTING OF CABLES
Scanning Method:
iii.
iv.
Em Kt
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1
-
n
TESTING OF TRANSFORMERS
Transformer is one of the most expensive and important
equipment in power system.
If it is not suitably designed its failure may cause a
lengthy and costly outage.
Therefore, it is very important to be cautious while
designing its insulation, so that it can withstand transient
over voltage both due to switching and lightning.
The high voltage testing of transformers is, therefore,
very important and would be discussed here. Other tests
like temperature rise, short circuit, open circuit etc. are
not considered here.
However,these can be found in the relevant standard
specification.
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TESTING OF TRANSFORMERS
Induced over voltage test:
Transformer secondary is excited by HFAC(100 to 400Hz) to about
twice the rated voltage
This reduces the core saturation and also limits the charging current
necessary in large X-mer
The insulation withstand strength can also be checked
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TESTING OF TRANSFORMERS
Impulse Testing of Transformer:
To determine the ability of the insulation to withstand
transient voltages
In short rise time of impulses, the voltage distribution
along the winding will not be uniform
The equivalent circuit of the transformer winding for
impulses is shown in Fig.1.
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TESTING OF TRANSFORMERS
Impulse voltage applied to the equivalent sets up
uneven voltage distribution and oscillatory voltage
higher than the applied voltage
Impulse tests:
Full wave standard impulse
Chopped wave standard impulse (Chopping time: 3 to 6S)
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TESTING OF TRANSFORMERS
Procedure for Impulse Test:
The schematic diagram of the transformer connection for
impulse test is shown in Fig.2
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TESTING OF TRANSFORMERS
Impulse testing consists of the following steps:
i.
ii.
iii.
iv.
v.
TESTING OF TRANSFORMERS
If an arc occurs between the turns or from turn to the
ground, a train of high frequency pulses are seen on
the oscilloscope and wave shape of impulse changes.
If it is only a partial discharge, high frequency
oscillations are observed but no change in wave shape
occurs.
Impulse strength of the transformer winding is same for
either polarity of wave whereas the flash over voltage
for bushing is different for different polarity.
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ii.
iii.
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INSULATION CO-ORDINATION
Insulation Coordination:
The process of bringing the insulation strengths of
electrical equipment and buses into the proper
relationship with expected overvoltages and with the
characteristics of the insulating media and surge
protective devices to obtain an acceptable risk of failure.
Basic lightning impulse insulation level (BIL):
The electrical strength of insulation expressed in terms of
the crest value of a standard lightning impulse under
standard atmospheric conditions.
Basic switching impulse insulation level (BSL)
The electrical strength of insulation expressed in terms of
the crest value of a standard switching impulse.
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INSULATION CO-ORDINATION
Factor of Earthing:
This is the ratio of the highest r.m.s. phase-to-earth
power frequency voltage on a sound phase during an
earth fault to the r.m.s. phase-to-phase power
frequency voltage which would be obtained at the
selected location without the fault.
This ratio characterizes, in general terms, the earthing
conditions of a system as viewed from the selected
fault location.
Effectively Earthed System :
A system is said to be effectively earthed if the factor
of earthing does not exceed 80%, and non-effectively
earthed if it does.
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INSULATION CO-ORDINATION
Statistical Impulse Withstand Voltage:
This is the peak value of a switching or lightning impulse test
voltage at which insulation exhibits, under the specified conditions,
a 90% probability of withstand.
In practice, there is no 100% probability of withstand voltage. Thus
the value chosen is that which has a 10% probability of breakdown.
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INSULATION CO-ORDINATION
Statistical Impulse Voltage:
This is the switching or lightning overvoltage applied to
equipment as a result of an event of one specific type on the
system (line energising, reclosing, fault occurrence, lightning
discharge, etc), the peak value of which has a 2% probability of
being exceeded.
INSULATION CO-ORDINATION
Necessity of Insulation Coordination:
i.
To ensure the reliability & continuity of service
ii. To minimize the number of failures due to over
voltages
iii. To minimize the cost of design, installation and
operation
Requirements of Protective Devices:
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INSULATION CO-ORDINATION
Volt-Time Curve
The breakdown voltage for a particular insulation of flashover
voltage for a gap is a function of both the magnitude of voltage
and the time of application of the voltage.
Volt-time curve is a graph showing the relation between the
crest flashover voltages and the time to flashover for a series of
impulse applications of a given wave shape.
Construction of Volt-Time Curve:
Waves of the same shape but of different peak values are applied to the
insulation whose volt-time curve is required.
If flashover occurs on the front of the wave, the flashover point gives one
point on the volt-time curve.
The other possibility is that the flashover occurs just at the peak
value of the wave; this gives another point on the V-T curve.
The third possibility is that the flashover occurs on the tail side of the
wave.
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INSULATION CO-ORDINATION
To find the point on the V-T curve, draw a horizontal line from the
peak value of this wave and also draw a vertical line passing through
the point where the flashover takes place
The intersection of the horizontal and vertical lines gives the point
on the V-T curve.
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INSULATION CO-ORDINATION
Steps for Insulation Coordination:
1.
2.
3.
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INSULATION CO-ORDINATION
Conventional method of insulation coordination:
In order to avoid insulation failure, the insulation level of different
types of equipment connected to the system has to be higher than
the magnitude of transient overvoltages that appear on the
system.
The magnitude of transient over-voltages are usually limited to a
protective level by protective devices.
Thus the insulation level has to be above the protective level by a
safe margin. Normally the impulse insulation level is established at
a value 15-25% above the protective level.
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INSULATION CO-ORDINATION
Consider the typical co-ordination of a 132 kV transmission line
between the transformer insulation, a line gap (across an insulator
string) and a co-ordinating gap (across the transformer bushing).
[Note: In a rural distribution transformer, a lightning arrester may not be used on account of the
high cost and a co-ordinating gap mounted on the transformer bushing may be the main surge
limiting device]
INSULATION CO-ORDINATION
For the higher system voltages, the simple approach
used above is inadequate. Also, economic considerations
dictate that insulation coordination be placed on a more
scientific basis.
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INSULATION CO-ORDINATION
Statistical Method of Insulation Co-ordination
At the higher transmission voltages, the length of insulator strings
and the clearances in air do not increase linearly with voltage but
approximately to V1.6 The required number of suspension units for
different overvoltage factors is shown below.
INSULATION CO-ORDINATION
Thus, while it may be economically feasible to protect the
lower voltage lines up to an overvoltage factor of 3.5 (say), it
is definitely not economically feasible to have an overvoltage
factor of more than about 2.0 or 2.5 on the higher voltage
lines.
Switching overvoltages is predominant in the higher voltage
systems. However, these may be controlled by proper design
of switching devices.
In a statistical study, the statistical distribution of
overvoltages has to be known instead of the possible highest
overvoltage.
In statistical method, experimentation and analysis carried
to find probability of occurrence of overvoltages and
probability of failure of insulation.
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INSULATION CO-ORDINATION
The
aim
of
statistical
methods is to quantify the risk
of failure of insulation through
numerical
analysis
of
the
statistical
nature
of
the
overvoltage magnitudes and of
electrical withstand strength of
insulation.
The risk of failure of the insulation is dependant on the
integral of the product of the overvoltage density function f0(V)
and the probability of insulation failure P(V).
Thus the risk of flashover per switching operation is equal to
f 0 (V) P(V)
dV.
the areaunder
the
curve
Since we cannot find suitable insulation such that the
withstand distribution does not overlap with the overvoltage
distribution, in the statistical method of analysis, the insulation
is selected such that the 2% overvoltage probability coincides
with the 90% withstand probability as shown.
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K.Rajkumar
Surge Arresters :Modern Surge arresters are of the gapless Zinc Oxide
type. Previously, Silicon Carbide arresters were used, but their use has
been superceeded by the ZnO arresters, which have a non-linear
resistance characteristic. Thus, it is possible to eliminate the series gaps
between the individual ZnO block making up the arrester.
Selection Procedure for Surge arresters:
1. Determine the continuous arrester voltage. This is usually the system
rated voltage.
2. Select a rated voltage for the arrester.
3. Determine the normal lightning discharge current. Below 36kV, 5kA rated
arresters are chosen. Otherwise, a 10kA rated arrester is used.
4. Determine the required long duration discharge capability.
For rated voltage < 36kV, light duty surge arrester may be specified.
For rated voltage between 36kV and 245kV, heavy duty arresters may be
specified.
For rated voltage >245kV, long duration discharge capabilities may be
specified.
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