EE1110 - High Voltage Engineering Unit-4

EE1110 - High Voltage Engineering
UNIT IV - MEASUREMENT OF HIGH VOLTAGES AND CURRENTS
HVDC measurement techniques measurement of power

frequency A.C voltages-sphere gap measurement technique-
potential divider for impulse voltage measurements measurement
of high D.C, A.C and impulse currents
By
G.SANTHOSHKUMAR, M.E., (Ph.D)
Assistant Professor (O.G),
Department of EEE, SRM University.
Measurement of High Voltages and Currents
In industrial testing and research laboratories, it is essential to measure
the voltages and currents accurately, ensuring perfect safety to the
personnel and equipment.
Secondly, linear extrapolation of the devices beyond their ranges are not
valid for high voltage meters and measuring instruments, and they have to
be calibrated for the full range.
Electromagnetic interference is a serious problem in impulse voltage and
current measurements, and it has to be avoided or minimized.
Therefore, even though the principles of measurements may be same, the
devices and instruments for measurement of high voltages and currents
differ vastly from the low voltage and low current devices.
06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 2

MEASUREMENT OF HIGH DIRECT CURRENT VOLTAGES
Measurement of high d.c. voltages as in low voltage measurements, is
generally accomplished by extension of meter range with a large series
resistance.
The net current in the meter is usually limited to one to ten microamperes
for full-scale deflection.
For very high voltages (1000 kV or more) problems arise due to large power
dissipation, leakage currents and limitation of voltage stress per unit length,
change in resistance due to temperature variations, etc.
Hence, a resistance potential divider with an electrostatic voltmeter is
sometimes better when high precision is needed.
But potential dividers also suffer from the disadvantages stated above.
Both series resistance meters and potential dividers cause current drain
from the source.
Generating voltmeters are high impedance devices and do not load the
source.
They provide complete isolation from the source voltage (high voltage) as
they are not directly connected to the high voltage terminal and hence are
safer.
Spark gaps such as sphere gaps are gas discharge devices and give an
accurate measure of the peak voltage. These are quite simple and do not
require any specialized construction.
But the measurement is affected by the atmospheric conditions like
temperature, humidity, etc. and by the vicinity of earthed objects, as the
electric field in the gap is affected by the presence of earthed objects.
But sphere gap measurement of voltages is independent of the waveform
and frequency.

High Ohmic Series Resistance with Microammeter
High d.c. voltages are usually measured by connecting a
very high resistance (few hundreds of megaohms) in series Fig. 7.1 Series
with a microammeter as shown in Fig. 7. 1 . resistance micrometre
Only the current I flowing through the large calibrated
resistance R is measured by the moving coil microammeter.
The voltage of the source is given by V=IR.
The voltage drop in the meter is negligible, as the
impedance of the meter is only few ohms compared to few
hundred mega-ohms of the series resistance R.
A protective device like a paper gap, a neon glow tube, or a zener diode with a
suitable series resistance is connected across the meter as a protection against
high voltages in case the series resistance R fails or flashes over.
The ohmic value of the series resistance R is chosen such that a current of one
to ten microamperes is allowed for full-scale deflection. The resistance is
constructed from a large number of wire wound resistors in series.
The voltage drop in each resistor element is chosen to avoid surface flashovers
and discharges. A Value Iess than 5 kV/Cm in air or less than 2 kV/cm in good oil is
permissible.
The resistor chain is provided with corona free terminations. The material for
resistive elements is usually a carbon-alloy with temperature coefficient less than
10-4/0C. Carbon and other metallic film resistors are also used.
A resistance chain built with 1% carbon resistors located in an airtight
transformer oil filled P.V.C. tube, for 100 kV operation had very good temperature
stability.
The limitations in the series resistance design are
(i) power dissipation and source loading,
(ii) temperature effects and long time stability,
(iii) voltage dependence of resistive elements, and
(iv) sensitivity to mechanical stresses.
Series resistance meters are built for 500 kV d.c. with an accuracy better than 0.2%.

Resistance Potential Dividers for d.c. Voltages
A resistance potential divider with an electrostatic or high impedance
voltmeter is shown in Fig. 7.2. The influence of temperature and voltage on
the elements is eliminated in the voltage divider arrangement.
The high voltage magnitude is given by [(R1 + R2)/R2]V2, where V2 is the
d.c. voltage across the low voltage arm R2.
With sudden changes in voltage, such as switching operations, flashover of
the test objects, or source short circuits, flashover or damage may occur to
the divider elements due to the stray capacitance across the elements and
due to ground capacitances.
To avoid these transient voltages, voltage controlling capacitors are
connected across the elements. A corona free termination is also necessary
to avoid unnecessary discharges at high voltage ends.
A series resistor with a parallel capacitor connection for linearization of
transient potential distribution is shown in Fig. 7.3.
Potential dividers are made with 0.05% accuracy up to 100 kV, with 0.1%
accuracy up to 300 kV, and with better than 0.5% accuracy for 500 kV.
Generating Voltmeters
High voltage measuring devices employ generating principle when source
loading is prohibited (as with Van de Graaff generators, etc.) or when direct
connection to the high voltage source is to be avoided.
A generating voltmeter is a variable capacitor electrostatic voltage generator
which generates current proportional to the applied external voltage.
The device is driven by an external synchronous or constant speed motor
and does not absorb power or energy from the voltage measuring source.
Principle of Operation
The charge stored in a capacitor of capacitance C is given by q = CV.
If the capacitance of the capacitor varies with time when connected to the
source of voltage V, the current through the capacitor,
For d.c. voltages dV/dt = O. Hence,
If the capacitance C varies between the limits C0 and (C0 + Cm) sinusoidally as
C = C0 + Cm sin t
the current i is I= im cost, where im= Kcm
For a constant angular frequency , the current is proportional to the
applied voltage V. More often, the generated current is rectified and
measured by a moving coil meter.
Generating voltmeter can be used for a.c. voltage measurements also
provided the angular frequency is the same or equal to half that of the
supply frequency.
A generating voltmeter with a rotating cylinder consists of two excitating
field electrodes and a rotating two pole armature driven by a synchronous
motor at a constant speed n.
The a.c. current flowing between the two halves of the armature is rectified
by a commutator whose arithmetic mean may be calculated from:

This device can be used for measuring a.c. voltages provided the speed of the drive-
motor is half the frequency of the voltage to be measured.
Thus a four-pole synchronous motor with 1500 rpm is suitable for 50 Hz. For peak value
measurements, the phase angle of the motor must also be so adjusted that Cmax and the
crest value occur at the same instant.
Generating voltmeters employ rotating sectors or vanes for variation of capacitance.
Figure 7.4 gives a schematic diagram of a generating voltmeter.
The high voltage source is connected to a disc electrode S3 which is kept at a fixed
distance on the axis of the other low voltage electrodes S0, S1 and S2. The rotor S0
driven at a constant speed by a synchronous motor at a suitable speed (1500,1800,3000,
or 3600 rpm).
The rotor vanes of S0 cause periodic change in capacitance between the insulated disc S2
and the h.v. electrode S3.
The shape and number of the vanes of S0 and S1 are so designed that they produce
sinusoidal variation in the capacitance.

The generated a.c. current through the resistance R is rectified and read by a
moving coil instrument An amplifier is needed, if the shunt capacitance is
large or longer leads are used for connection to rectifier and meter.
The instrument is calibrated using a potential divider or sphere gap. The
meter scale is linear and its range can be extended. by extrapolation. Typical
calibration curves of a generating voltmeter are given in Figs. 7.5a and b.
Fig. 7.4 Schematic diagram of a generating voltmeter (rotating vane type)

Advantages of Generating Voltmeters
(i) No source loading by the meter,
(ii) no direct connection to high voltage electrode,
(iii) scale is linear and extension of range is easy, and
(iv) a very convenient instrument for electrostatic devices such as Van de
Graaff generator and particle accelerators.
Limitations of Generating Voltmeters
(i) They require calibration,
(ii) careful construction is needed and is a cumbersome instrument requiring
an auxiliary drive, and
(iii) disturbance in position and mounting of the electrodes make the
calibration invalid
MEASUREMENT OF HIGH A.C. AND IMPULSE VOLTAGES
Measurement of high a.c. voltages employ conventional methods like series
impedance voltmeters, potential dividers, potential transformers, or
electrostatic voltmeters.
But their designs are different from those of low voltage meters, as the
insulation design and source loading are the important criteria.
When only peak value measurement is needed, peak voltmeters and sphere
gaps can be used.
Often, sphere gaps are used for calibration purposes. Impulse and high
frequency a.c. measurements invariably use potential dividers with a cathode
ray oscillograph for recording voltage waveforms.
Sphere gaps are used when peak values of the voltage are only needed and
also for calibration purposes.
Series Impedance Voltmeters
For power frequency a.c. measurements the series impedance may be a
pure resistance or a reactance. Since resistances involve power losses, often
a capacitor is preferred as a series reactance.
Moreover, for high resistances, the variation of resistance with temperature
is a problem, and the residual inductance of the resistance gives rise to an
impedance different from its ohmic resistance.
High resistance units for high voltages have stray capacitances and hence a
unit resistance will have an equivalent circuit as shown below. At any
frequency of the a.c. voltage, the impedance of the resistance R is
Simplified lumped parameter equivalent circuit of a high ohmic resistance R

L Residual inductance; C Residual capacitance
If L and C are small compared to R,
The total phase angle is
This can be made zero and independent of frequency, if L/C=R2
For extended and large dimensioned resistors, this equivalent circuit is not
valid and each elemental resistor has to be approximated with this
equivalent circuit.
The entire resistor unit then has to be taken as a transmission line
equivalent, for calculating the effective resistance.
Also, the ground or stray capacitance of each element influences the current
flowing in the unit, and the indication of the meter results in an error.
The equivalent circuit of a high voltage resistor neglecting inductance and
the circuit of compensated series resistor using guard and timing resistors is
shown in Figs. 7.8a and b respectively.
Fig. 7.8 Extended series resistance
for high a.c. voltage measurements
Cg Stray capacitance to ground

Cs Winding capacitance
R Series resistor
Rs Guard resistor
Ra Tuning resistor
(a) Extended series resistance with inductance neglected (b) Series resistance with guard and tuning resistances

Stray ground capacitance effects (refer Fig. 7.8b) can be removed by
shielding the resistor R by a second surrounding spiral Rs, which shunts the
actual resistor but does not contribute to the current through the
instrument.
By tuning the resistors Ra, the shielding resistor end potentials may be
adjusted with respect to the actual measuring resistor so that the resulting
compensation currents between the shield and the measuring resistors
provide a minimum phase angle.

Series Capacitance Voltmeter
To avoid the
drawbacks pointed
out earlier, a series
capacitor is used
instead of a resistor
for a.c. high voltage
measurements. The
current IC through
the meter is: IC=jCV

With a 10% fifth harmonic only, the current is 11.2% higher, and hence the
error is 11.2% in the voltage measurement.
This method is not recommended when a.c. voltages are not pure sinusoidal
waves but contain considerable harmonics.
Series capacitance voltmeters were used with cascade transformers for
measuring rms values up to 1000 kV.
The series capacitance was formed as a parallel plate capacitor between the
high voltage terminal of the transformer and a ground plate suspended
above it.
A rectifier ammeter was used as an indicating instrument and was directly
calibrated in high voltage rms value.
The meter was usually a 0-100 A moving coil meter and the over all error
was about 2%.

Capacitance Potential Dividers and Capacitance Voltage Transformers
The errors due to harmonic voltages can be eliminated by the use of
capacitive voltage dividers with an electrostatic voltmeter or a high
impedance meter such as a T.V.M.
If the meter is connected through a long cable, its capacitance has to be
taken into account in calibration.
Usually, a standard compressed air or gas condenser is used as C1 (Fig. 7.10),
and C2 may be any large capacitor (mica, paper, or any low loss condenser).
C1 is a three terminal capacitor and is connected to C2 through a shielded
cable, and C2 is completely shielded in a box to avoid stray capacitances. The
applied voltage V1 is given by V1 = V2((C1+C2+Cm)/C1))
where Cm is the capacitance of the meter and the connecting cable and the
leads and V2 is the meter reading.

Capacitance Voltage TransformerCVT
Capacitance divider with a suitable matching or
isolating potential transformer tuned for
resonance condition is often used in power
systems for voltage measurements.
This is often referred to as CVT. In contrast to
simple capacitance divider which requires a
high impedance meter like a V.T.V.M. or an
electrostatic voltmeter, a CVT can be connected
to a low impedance device like a wattmeter
pressure coil or a relay coil CVT can supply a
load of a few VA.
The schematic diagram of a CVT with its
equivalent circuit is given in Fig. 7.11.
C1 is made of a few units of high voltage condensers, and the total capacitance will be
around a few thousand picofarads as against a gas filled standard condenser of about
100 pF.
A matching transformer is connected between the load or meter M and C2. The
transformer ratio is chosen on economic grounds, and the h.v. winding rating may be
10 to 30 kV with the l.v. winding rated from 100 to 500 V.
The value of the tuning choke L is chosen to make the equivalent circuit of the CVT
purely resistive or to bring resonance condition. This condition is satisfied when
where, L= inductance of the choke, &
LT = equivalent inductance of the transformer referred to h.v. side.
The voltage V2 (meter voltage) will be in phase with the input voltage V1.
The phasor diagram of CVT under resonant conditions is shown in Fig. 7.11. The meter
is taken as a resistive load, and X ' m is neglected
The voltage across the load referred to the divider side will be V'2 = (Im Rm) and VC2
= V'2 + Im(Re + Xe).
FIg. 7.11 Capacitive voltage transformer (CVT)
(a) Schematic representation (b) Equivalent circuit
It is clear from the phasor diagram that V1. (input voltage) = (VC1 + VC2) and is in phase
with V2, the voltage across the meter.
Re and Xe include the potential transformer resistance and leakage reactance. Under
this condition, the voltage ratio becomes a= (V1 /V2)(Vc1 + VRi + V2)/ V2.
(neglecting the voltage drop Im Xe, which is very small compared to the voltage Vc1)
where VRi is the voltage drop in the transformer and choke windings.
The advantages of a CVT are:
(i) simple design and easy installation,
(ii) can be used both as a voltage measuring device for meter and relaying purposes and
also as a coupling condenser for power line carrier communication and relaying.
(iii) frequency independent voltage distribution along elements as against conventional
magnetic potential transformers which require additional insulation design against surges,
and
(iv) provides isolation between the high voltage terminal and low voltage metering.
The disadvantages of a CVT are:
(i) the voltage ratio is susceptible to temperature variations, and
(ii) the problem of inducing ferro-resonance in power systems.
Resistance Potential Dividers
Resistance potential dividers suffer from the same disadvantages as series resistance
voltmeters for a.c. applications.
Moreover, stray capacitances and inductances associated with the resistances make them
inaccurate, and compensation has to be provided. Hence, they are not generally used.
30-03-2017

Potential Transformers (Magnetic Type)
Magnetic potential transformers are the oldest devices for a.c.
measurements. They are simple in construction and can be
designed for any voltage. For very high voltages, cascading of the
transformers is possible. The voltage ratio is: (V1/V2) = a = (N1/N2).
V1 & V2 are the primary and secondary voltages,
N1 & N2 the respective turns in the windings.
These devices suffer from the ratio and phase angle errors caused
by the magnetizing and leakage impedances of the transformer
windings.
The errors are compensated by adjusting the turns ratio with the
tappings on the high voltage side under load conditions.
Potential transformers (PT) do not permit fast rising transient or
high frequency voltages along with the normal supply frequency,
but harmonic voltages are usually measured with sufficient
accuracy.
With high voltage testing transformers, no separate potential transformer is
used, but a PT winding is incorporated with the high voltage windings of the
testing transformer.
With test objects like insulators, cables, etc. which are capacitive in nature,
a voltage rise occurs on load with the testing transformer, and the potential
transformer winding gives voltage values less than the actual voltages
applied to the test object.
If the percentage impedance of the testing transformer is known, the
following correction can be applied to the voltage measured by the PT
winding of the transformer.
V20 = open circuit voltage of the PT winding; VX =% reactance drop in the transformer.
CN as load capacitance used for testing; C = test object capacitance (C CN) and

Peak Reading A.C. Voltmeters
In some occasions, the peak value of an a.c. waveform is more important.
This is necessary to obtain the maximum dielectric strength of insulating
solids, etc.
When the waveform is not sinusoidal, rms value of the voltage multiplied by
2 is not correct. Hence a separate peak value instrument is desirable in
high voltage applications.
Series Capacitor Peak Voltmeter
When a capacitor is connected to a sinusoidal voltage source, the charging
current 0 = 0 = jCV where V is the rms value of the voltage and is
the angular frequency.
If a half wave rectifier is used, the arithmetic mean of the rectifier current is
proportional to the peak value of the a.c. voltage.
The schematic diagram of the circuit arrangement is shown in Fig. 7.14. The d.c.
meter reading is proportional to the peak value of the value Vm = I/2fc.
I is the d.c. current read by the meter and C is the capacitance of the capacitor.
This method is known as the Chubb-Frotscue method for peak voltage
measurement.
The diode D1 is used to rectify the a.c. current in one half cycle while D2 by-
passes in the other half cycle.
This arrangement is suitable only for positive or negative half cycles and hence is
valid only when both half cycles are symmetrical and equal.
Fig. 7.14 Peak voltmeter with a series capacitor

C Capacitor D1, D2 Diodes
v(t) Voltage waveform P Protective device
T Period IC (t) Capacitor current waveform
I Indicating meter (rectified current indicated)

This method is not suitable when the voltage waveform is not sinusoidal but
contains more than one peak or maximum as shown in Fig. 7.14.
The charging current through the capacitor changes its polarity within one half
cycle itself.
The shaded areas in Fig. 7.15 give the reverse current in any one of the half cycles
and the current within that period subtracts from the net current.
Hence the reading of the meter will be less and is not proportional to Vm as the
current flowing during the intervals (t1 - t2) etc will not be included in the mean
value.
The 'second or the false maxima is easily spotted out by observing the waveform
of the charging current on an oscilloscope.
Under normal conditions with A.C. testing, such waveforms do not occur and as
such do not give rise to errors.
But pre-discharge currents within the test circuits cause very short duration
voltage drops which may introduce errors.

This problem can also be overcome by using a resistance R in series with
capacitor C such that CR 1/ for 50 Hz application. The error due to the
resistance is
V = actual value, Vm = measured value
Fig. 7.15 Voltage waveform with harmonic

content showing false maxima

In determining the error, the actual value of the angular frequency co has to be
determined. The different sources that contribute to the error are
(i) the effective value of the capacitance being different from the measured value of C
(ii) imperfect rectifiers which allow small reverse currents
(iii) non-sinusoidal voltage waveforms with more than one peak or maxima per half cycle
(iv) deviation of the frequency from that of the value used for calibration.
As such, this method in its basic form is not suitable for waveforms with more than one
peak in each half cycle.
A digital peak reading meter for voltage measurements is shown in Fig. 7.16. Instead of
directly measuring the rectified charging current, a proportional analog voltage signal is
derived which is then convened into a proportional medium frequency fm.
The frequency ratio fm/f is measured with a gate circuit controlled by the a.c. power
frequency (f) and a counter that opens for an adjustable number of periods t= p/f.

During this interval, the number of impulses counted, n, is
Where p is a constant of the instrument and A represents the conversion factor of the a.c.
to d.c. converter. A = fm/Rim ; im is the rectified current through resistance R.
An immediate reading of the voltage in kV can be obtained by suitable choice of
the parameters R and the number of periods p.
The total estimated error in this instrument was less than 0.35%. Conventional
instruments of this type are available with less than 2% error.
Fig. 7.16 Digital peak voltmeter

C Series capacitor D1, D2 Diodes
p Input resistor 1 Voltage to frequency converter
2 Gate circuit 3 Read out counter (indicator)

Peak Voltmeters with Potential Dividers
Peak voltmeters using capacitance dividers designed by Bowlder et al., are
shown in Fig. 7.17a.
The voltage across C2 is made use of in charging the storage capacitor CS. Rd
is a discharge resistor employed to permit variation of Vm whenever V2 is
reduced.
CS is charged to a voltage proportional to the peak value to be measured.
The indicating meter is either an electrostatic voltmeter or a high
impedance V.T.V.M.
The discharge time constant CS Rd is designed to be about 1 to 10 s. This
gives rise to a discharge error which depends on the frequency of the supply
voltage.
To compensate for the charging and discharging errors due to the
resistances, the circuit is modified as shown in Fig. 7.17b.
Measurement of the average peak is done by a microameter. Rabus'
modification to compensate the charging errors is given in Fig. 7.17c.

FIg. 7.17b Peak voltmeter as modified by Haefeely
Fig. 7.17a Peak voltmeter with a capacitor

potential divider and electrostatic voltmeter
Fig. 7.17c Peak voltmeter with equalizing branch as

designed by Rabus
M Electrostatic voltmeter or V.T.V.M. of high impedance
CS2 CS1 + C meter; Rd1 = Rd2,

Spark Gaps for Measurement of High D.C., A.C. and
Impulse Voltages (Peak Values)
A uniform field spark gap will always have a sparkover voltage within a known
tolerance under constant atmospheric conditions. Hence a spark gap can be used
for measurement of the peak value of the voltage, if the gap distance is known.
A sparkover voltage of 30 kV (peak) at 1 cm spacing in air at 2O0C and 760 torr
pressure occurs for a sphere gap or any uniform field gap. But experience has
shown that these measurements are reliable only for certain gap configurations.
Normally, only sphere gaps are used for voltage measurements. In certain cases
uniform field gaps and rod gaps are also used, but their accuracy is less.
The spark gap breakdown, especially the sphere gap breakdown, is independent
of the voltage waveform and hence is highly suitable for all types of waveforms
from d.c. to impulse voltages of short rise times (rise time > 0.5s).
As such, sphere gaps can be used for radio frequency a.c. voltage peak
measurements also (up to 1 MHz).
Sphere Gap Measurements
Sphere gaps can be arranged either (i) vertically with lower sphere grounded,
or (ii) horizontally with both spheres connected to the source voltage or one
sphere grounded.
In horizontal configurations, it is generally arranged such that both spheres are
symmetrically at high voltage above the ground. The two spheres used are
identical in size and shape.
The schematic arrangement is shown in Figs. 7.18a and 7.18b. The voltage to
be measured is applied between the two spheres and the distance or spacing S
between them gives a measure of the sparkover voltage.
A series resistance is usually connected between the source and the sphere
gap to (i) limit the breakdown current, and (ii) to suppress unwanted
oscillations in the source voltage when breakdown occurs (in case of impulse
voltages).
The value of the series resistance may vary from 100 to 1000 kilo ohms for a.c.
or d.c. voltages and not more than 500 in the case of impulse voltages.
1 Insulator support; 2 Sphere shank; 3 Operating gear and motor for changing gap distance
4 H.V. connection; 5 Spacing; P Sparking point; D Diameter of the sphere
A Height of P above earth; B Radius of the clearance fromexternal structures
X High voltage lead should not pass through this plane within a distance B from P
In the case of a.c. peak value and d.c. voltage measurements, the applied voltage
is uniformly increased until sparkover occurs in the gap.
Generally, a mean of about five breakdown values is taken when they agree to
within 3%.
In the case of impulse voltages, to obtain 50% flashover voltage, two voltage
limits, differing by not more than 2% are set such that on application of lower
limit value either 2 or 4 flashovers take place and on application of upper limit
value 8 or 6 flashovers take place respectively.
The mean of these two limits is taken as 50% flashover voltage. In any case, a
preliminary sparkover voltage measurement is to be made before actual
measurements are made.
The flashover voltage for various gap distances and standard diameters of the
spheres used are given in Tables 7.3 and 7.4 respectively.
The values of sparkover voltages are specified in BS : 358, EEC Publication 52 of
1960 and IS : 1876 of 1962.
The clearances necessary are shown in Figs. 7.18a and 7.18b for measurements to
be within 3%. The values of A and B indicated in the above figures are given in
Table 7.5.
Sphere Gap Construction and Assembly
Sphere gaps are made with two metal spheres of identical diameters D with their shanks,
operating gear, and insulator supports (Fig. 7.18a or b).
Spheres are generally made of copper, brass, or aluminium; the latter is used due to low
cost The standard diameters for the spheres are 2,5,6.25,10,12.5,15,25,50,75,100,150, and
200 cm.
The spacing is so designed and chosen such that flashover occurs near the sparking point
P.
The spheres are carefully designed and fabricated so that their surfaces are smooth and
the curvature is uniform.
The radius of curvature measured with a spherometer at various points over an area
enclosed by a circle of 0.3 D around the sparking point should not differ by more than 2%
of the nominal value.
The surface of the sphere should be free from dust, grease, or any other coating. The
surface should be maintained clean but need not be polished. If excessive pitting occurs
due to repeated sparkovers, they should be smoothened.
The dimensions of the shanks used, the grading ring used (if necessary) with spheres, the
ground clearances, etc. should follow the values indicated in Figs. 7.18a and 7.18b and
Table 7.5
The high voltage conductor should be arranged such that it does not affect the field
configuration.
Series resistance connected should be outside the shanks at a distance 2D away from the
high voltage sphere or the sparking point P.
Irradiation of sphere gap is needed when measurements of voltages less than 50 kV are
made with sphere gaps of 10 cm diameter or less.
The irradiation may be obtained from a quartz tube mercury vapour lamp of 40 W rating.
The lamp should be at a distance B or more as indicated in Table 7.5.
Factors Influencing the Sparkover Voltage of Sphere Gaps.
Various factors that affect the sparicover voltage of a sphere gap are:
(i) nearby earthed objects, (ii) atmospheric conditions and humidity,
(iii) irradiation, and (iv) polarity and rise time of voltage waveforms.
(i) Effect of nearby earthed objects

FIg. 7.19 Influence of ground planes on sparkover voltage
(ii) Effect of atmospheric conditions

increases with the gap length. As the change in
sparkover voltage with humidity is within 3%,
no correction is normally given for humidity.
Fig. 7.20 Influence of humidity on

d.c. and a.c. breakdown voltages
(25 cm dia sphere gap, 1 cm
spacing)

Field Controlled Voltage Dividers
The electrostatic or capacitive field distribution of a shield or guard ring
placed over a resistive divider to enforce a uniform field in the
neighbourhood and along the divider may be adopted for high voltage
measurements.
The schematic diagram is shown in Fig. 7.31 and its equivalent circuit is
same as that given in Fig. 7.28.
The shield is of the form of a cone. R1 is a non-linear resistance in the sense
the resistance per unit length is not the same but is variable.
The main advantage is that the capacitance per unit length is small and
hence loading effect is reduced.
Sometimes the parallel resistance R2 together with the lead inductance and
shunt capacitances cause oscillations as shown in Fig. 7.32a.

The oscillations can be reduced by adding a damping resistance Rd as shown in Fig.
7.31. Such dividers are constructed for very high voltages (up to 2 MV) with
response times less than 30 ns.
The resistance column, R1 is made of woven resistance of 20 kilo ohms. The step
response of such a divider is shown in Fig. 7.32, with and without a damping resistor.
With a proper damping resistor (Rd) the response time is much less and the
overshoot is reduced.
Fig. 7.31 Field controlled resistance divider with a damping resistor
Rd Damping resistor
L Lead inductance
Cp Capacitance of the shield to ground
S Shield

MEASUREMENT OF HIGH DC,AC AND IMPULSE CURRENTS

Measurement of High Direct Currents

Hall Generators for d.c. Current Measurements
The principle of the "Hall effect" is made use of in measuring very high
direct currents.
If an electric current flows through a metal plate located in a magnetic field
perpendicular to it, Lorenz forces will deflect the electrons in the metal
structure in a direction normal to the direction of both the current and the
magnetic field.
The charge displacement generates an emf in the normal direction, called
the HaIl voltage.
The Hall voltage is proportional to the current i, the magnetic flux density B,
and the reciprocal of the plate thickness d; the proportionality constant R is
called the HaIl coefficient.

For metals the Hall coefficient is very small, and hence semi-conductor
materials are used for which the Hall coefficient is high.
In large current measurements, the current carrying conductor is
surrounded by an iron cored magnetic circuit, so that the magnetic field
intensity H = (I/) is produced in a small air gap in the core.
The Hall element is placed in the air gap (of thickness ), and a small
constant d.c. current is passed through the element. The schematic
arrangement is shown in Fig. 7.43.
The voltage developed across the Hall element in the normal direction is
proportional to the d.c. current I.
It may be noted that the Hall coefficient R depends on the temperature and
the high magnetic field strengths, and suitable compensation has to be
provided when used for measurement of very high currents.

Measurement of High Power Frequency Alternating Currents
Measurement of power frequency currents are normally done using current
transformers only, as use of current shunts involves unnecessary power loss.
Also the current transformers provide electrical isolation from high voltage circuits
in power systems.
Current transformers used few extra high voltage (EHV) systems are quite different
from the conventional designs as they have to be kept at very high voltages from
the ground.
A new scheme of current transformer measurements introducing electro-optical
technique is described in Fig. 7.44.
A voltage signal proportional to the measuring current is generated and is
transmitted to the ground side through an electro-optical device.
Light pulses proportional to the voltage signal are transmitted by a glass-optical
fibre bundle to a photodetector and converted back into an analog voltage signal.
Accuracies better than 0.5% have been obtained at rated current as well as for high
short circuit currents. The required power for the signal converter and optical device are
obtained from suitable current and voltage transformers as shown in the Fig. 7.44.
Fig. 7.44 Current transformer with electro-optical
signal converter for EHV systems

Measurement of High Frequency and Impulse Currents
In power system applications as well as in other scientific and technical fields, it is often
necessary to determine the amplitude and waveforms of rapidly varying high currents.
High impulse currents occur in lightning discharges, electrical arcs and post arc
phenomenon studies with circuit breakers, and with electric discharge studies in plasma
physics.
The current amplitudes may range from a few amperes to few hundred kiloamperes. The
rate of rise for such currents can be as high as 106 to 1012 A/s, and rise times can vary from
few microseconds to few nano seconds.
In all such cases the sensing device should be capable of measuring the signal over a wide
frequency band.
The methods that are frequently employed are
(i) resistive shunts,
(ii) magnetic potentiometers or probes, and
(iii) the Faraday and Hall effect devices.
The accuracy of measurement varies from 1 to 10%. In applications where only peak value
measurement is required, peak reading voltmeters may be employed with a suitable
shunt.
Resistive shunts
The most common method employed for high impulse current measurements is a
low ohmic pure resistive shunt shown in Fig. 7.45.
The equivalent circuit is shown in Fig. 7.45b. The current through the resistive
element R produces a voltage drop v(t)=i(t)R.
The voltage signal generated is transmitted to a CRO through a coaxial cable of
surge impedance Z0. The cable at the oscilloscope end is terminated by a resistance
Ri= Z0 to avoid reflections.
The resistance element, because of its large dimensions will have a residual
inductance L and a terminal capacitance C.
The inductance L may be neglected at low frequencies (), but becomes
appreciable at higher frequencies () when L is of the order of R.
Similarly, the value of C has to be considered when the reactance 1/C is of
comparable value. Normally L and C become significant above a frequency of 1MHz.

Fig. 7.45 Calibrated low ohmic shunt and its equivalent circuit for impulse current measurements

The resistance value usually ranges from 10 to few milliohms, and the voltage
drop is usually about a few volts. The value of the resistance is determined by the
thermal capacity and heat dissipation of the shunt.
The voltage drop across the shunt in the complex frequency domain may be
written as:
where s is the complex frequency or Laplace transform operator and V(s) and I(s)
are the transformed quantities of the signals v(t) and i(t). With the value of
C neglected it may be approximated as: V(s)= (R+ Ls)I(S).
It may be noted here that the stray inductance and capacitance should be made as
small as possible for better frequency response of the shunt. The resistance shunt
is usually designed in the following manner to reduce the stray effects.
(a) Bifilar flat strip design,
(b) coaxial tube or Park's shunt design, and
(c) coaxial squirrel cage design
Bifilar Strip Shunt
The bifilar design (Fig. 7.46) consists of resistor elements wound in opposite directions
and folded back, with both ends insulated by a teflon or other high quality insulation.
The voltage signal is picked up through a ultra high frequency (UHF) coaxial connector.
The shunt suffers from stray inductance associated with the resistance element, and its
potential leads are linked to a small pan of the magnetic flux generated by the current
that is measured. To overcome these problems, coaxial shunts are chosen.
Fig. 7.46 Bifilar flat strip resistive shunt
1. Metal base
2. Current terminals (C1 and C2)
3. Bifilar resistance strip
4. Insulating spacer (teflon or bakelite)
5. Coaxial UHF connector P1, P2 Potential terminals
(b) Connection for potential and current terminals

06-04-2017 (a) Schematic arrangement G.Santhoshkumar, AP/EEE, SRM University 70
Coaxial Tubular or Park's Shunt
In the coaxial design (Fig. 7.47) the current is made to enter through an inner cylinder or
resistive element and is made to return through an outer conducting cylinder of copper
or brass.
The voltage drop across the resistive element is measured between the potential pick-up
point and the outer case.
The space between the inner and the outer cylinder is air and hence acts like a pure
insulator. With this construction, the maximum frequency limit is about 1000 MHz and
the response time is a few nanoseconds.
The upper frequency limit is governed by the skin effect in the resistive element.
Fig. 7.47 Schematic arrangement of a coaxial ohmic shunt
1. Current terminals
2. Coaxial cylindrical resistive element
3. Coaxial cylindrical return conductor (copper or brass tube)
4. Potential pick up lead
5. UHF coaxial connector

The equivalent circuit of the shunt is given in Fig. 7.48. The step response and the
frequency response are shown in Fig. 7.49. The inductance L0 shown in Fig. 7.48 may be
written as:
where
= 0 r;
the magnetic permeability, 0 = 4 x 10-9 Vs/A cm is the magnetic field constant of vacuum;
d = thickness of the cylindrical tube, l = length of the cylindrical tube, r is radius of the
cylindrical tube Fig. 7.48 Simplified and exact equivalent circuits of a coaxial tubular shunt
(a) Exact equivalent circuit (b) Simplified circuit
L0 - Inductance, R0 - d.c. resistance, n - Number of sections per unit length L 0.43L0

Fig. 7.49 Step and frequency responses of a coaxial tubular shunt
(a) Step response (b) Frequency response
B = Band width
fc = Maximum frequency limit
The effective resistance is given by R= V(t)/I0 = R0(t)

where, R0 = d.c. resistance; L0=inductance for d.c. currents and (t) is the theta function
of type 3 and is equal to
V(t) = signal developed; and I0 is the step current.

The effective impedance of the shunt for any frequency f according to Silsbee is given by:

The simplified equivalent circuit shown in Fig. 7.48 is convenient to calculate the rise time of
the shunt The rise time accordingly is given by,
and the bandwidth is given by
The coaxial tubular shunts were constructed for current peaks up to 500 kA; shunts constructed
for current peaks as high as 200 kA with di/dt of about 5x1010 A/s have induced voltages less
than 50Vand the voltage drop across the shunt was about 100 V.

Fig. 7.50 Response of squirrel cage shunt for different number of rods
(i) number of rods too small
(ii) ideal number of rods
(iii) number of rods too high

Squirrel Cage Shunts
In certain applications, such as post arc current measurements, high ohmic
value shunts which can dissipate larger energy are required.
In such cases tubular shunts are not suitable due to their limitations of heat
dissipation, larger wall thickness, and the skin effect.
To overcome these problems, the resistive cylinder is replaced by thick rods
or strips, and the structure resembles the rotor construction of double
squirrel cage induction motor.
The equivalent circuit for squirrel cage construction is different, and
complex.
The shunts show peaky response for step input, and a compensating
network has to be designed to get optimum response.
In Fig. 7.50, the step response (Fig. 7.50a) and frequency response (Fig.
7.50b) characteristics are given.

Rise times of better than 8ns with bandwidth more than 400 MHz were
obtained for this type of shunts.
A typical R-C compensating network used for these shunts is shown in Fig. 7.51.
Fig. 7.51 Compensating network for squirrel cage shunts

Materials and Technical Data for the Current Shunts
The important factor for the materials of the shunts is the variation of the resistivity of
the material with temperature.
In Table 7.11 physical properties of some materials with low temperature coefficient,
which can be used for shunt construction are given.

The importance of the skin effect has been pointed out in the coaxial shunt
design. The skin depth d for a material of conductivity a at any frequency f is
given by
Skin depth, d, is defined as the distance or depth from the surface at which
the magnetic field intensity is reduced to 1/e (e = 2.718 ...) of the surface
value for a given frequency f.
Materials of low conductivity (high resistivity materials) have large skin
depth and hence exhibit less skin effect It may be stated that low ohmic
shunts of coaxial type or squirrel cage type construction permit
measurements of high currents with response times less than 10 ns.

Measurement of High Impulse Currents Using Magnetic Potentiometers
(Rogowskl Colls) and Magnetic Links

Rogowski coils with electronic or active integrator circuits have large bandwidths
(about 100 MHz).
At frequencies greater than 100 MHz the response is affected by the skin effect, the
capacitance distributed per unit length along the coil, and due to the electromagnetic
interferences.
However, miniature probes having nanosecond response time are made using very
few turns of copper strips for UHF measurements.
Fig. 7.52 Rogowski coil for high impulse current measurements

Magnetic Links
Magnetic links are short high retentivity steel strips arranged on a circular wheel
or drum.
These strips have the property that the remanent magnetism for a current pulse
of 0.5/5 s is same as that caused by a d.c. current of the same value.
Hence, these can be used for measurement of peak value of impulse currents.
The strips will be kept at a known distance from the current carrying conductor
and parallel to it.
The remanent magnetism is then measured in the laboratory from which the
peak value of the current can be estimated.
These are useful for field measurements, mainly for estimating the lightning
currents on the transmission lines and towers.
By using a number of links, accurate measurement of the peak value, polarity,
and the percentage oscillations in lightning currents can be made.
Other Techniques for Impulse Current Measurements
(a) Hall Generators
Hall generators described earlier can be used for a.c. and impulse current
measurements also. The bandwidth of such devices was found to be about
50 MHz with suitable compensating devices and feedback. The saturation
effect in magnetic core can be minimized, and these devices are
successfully used for post arc and plasma current measurements.
(b) Faraday Generator or Ammeter
When a linearly polarized light beam passes through a transparent crystal
in the presence of a magnetic field, the plane of polarization of the light
beam undergoes rotation. The angle of rotation a is given by:
= VBl
V= a constant of the crystal which depends on thewavelength of the light,
B = magnetic flux density, and l = length of the crystal.
To measure the waveform of a large current in a EHV system an arrangement shown in Fig.
7.53 may be employed.
A beam of light from a stabilized light source is passed through a polarizer P1 to fall on a
crystal F placed parallel to the magnetic field produced by the current I.
The light beam undergoes rotation of its plane of polarization. After passing through the
analyser, the beam is focused on a photomultiplier, the output of which is fed to a CRO.
The output beam is filtered through a filter M, which allows only the monochromatic light.
The relation between the oscillograph display and the current to be measured are complex
but can be determined.
The advantages of this method are that
(i) there is no electric connection between the source and the device,
(ii) no thermal problems even for large currents of several kiloamperes, and
(Iii) as the signal transmission is through an optical system, no insulation problems or
difficulties arise for EHV systems.
However, this device does not operate for d.c. currents.
FIg. 7.53 Magneto-optical method of measuring impulse currents
L Light source F Crystal CPhoto-multiplier

P1 Polarizer CRO Recording oscillograph
P2 Analyser M Filter

EE1110 - High Voltage Engineering Unit-4

Uploaded by

Copyright:

Available Formats

EE1110 - High Voltage Engineering Unit-4

Uploaded by

Document Information

Original Description:

Original Title

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

EE1110 - High Voltage Engineering Unit-4

Uploaded by

Copyright:

Available Formats

EE1110 - High Voltage Engineering

UNIT IV - MEASUREMENT OF HIGH VOLTAGES AND CURRENTS

HVDC measurement techniques measurement of power

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 2

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 6

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 8

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 12

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 13

Fig. 7.4 Schematic diagram of a generating voltmeter (rotating vane type)

Simplified lumped parameter equivalent circuit of a high ohmic resistance R

Cg Stray capacitance to ground

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 20

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 21

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 22

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 23

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 24

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 29

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 31

Fig. 7.14 Peak voltmeter with a series capacitor

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 33

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 34

Fig. 7.15 Voltage waveform with harmonic

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 35

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 36

Fig. 7.16 Digital peak voltmeter

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 37

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 38

Fig. 7.17a Peak voltmeter with a capacitor

Fig. 7.17c Peak voltmeter with equalizing branch as

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 39

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 49

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 51

Fig. 7.20 Influence of humidity on

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 52

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 55

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 57

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 59

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 60

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 61

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 63

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 65

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 67

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 68

Fig. 7.46 Bifilar flat strip resistive shunt

(b) Connection for potential and current terminals

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 71

(a) Exact equivalent circuit (b) Simplified circuit

L0 - Inductance, R0 - d.c. resistance, n - Number of sections per unit length L 0.43L0

The effective resistance is given by R= V(t)/I0 = R0(t)

V(t) = signal developed; and I0 is the step current.

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 73

and the bandwidth is given by

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 74

(ii) ideal number of rods

(iii) number of rods too high

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 75

06-04-2017 G.Santhoshkumar, AP/EEE, SRM University 76

Fig. 7.51 Compensating network for squirrel cage shunts