The document discusses various techniques for measuring high voltages and currents, including: - Sphere gap voltmeters, which measure sparkover voltage between conducting spheres; - Electrostatic voltmeters, which measure the attraction force between charged parallel plates; - Generating voltmeters, which use a variable capacitor to generate a current proportional to input voltage. Peak reading voltmeters are also summarized, which use a capacitor to measure the peak voltage of AC waveforms. The document provides details on the principles, construction, advantages, and limitations of these different high voltage and current measurement methods.

1. Unit No.4:
Measurement of High Voltages
and Current

2. Topic Content
• Sphere gap voltmeter
• Electrostatic Voltmeter
• Generating Voltmeter
• Peak reading voltmeter
• Resistive, capacitive and mixed potential divider
• Capacitance voltage transformer
• Cathode ray oscilloscope for impulse voltage and current measurement
• Measurement of dielectric constant and loss factor
• Partial discharge measurement
• Measurement of high frequency a.c. using current transformer with electro-optical
signal converter
• Radio interference measurement.

3. Sphere gap measurement

4. 1. Sphere Gap Measurement can be arranged
(i) vertically with lower sphere grounded.
(ii) horizontally with both spheres connected to the source voltage or one sphere grounded.
2. Two sphere used are identical in size & shape. Schematic arrangement is shown in fig.(a) & (b)
3. The voltage to be measured is applied between two spheres & distance or spacing S between them gives
measure of spark over voltage.
4. Sphere Gap Measurement 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.
5. A series resistance is usually connected between the source and the sphere gap to
(i) To limit the breakdown current
(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 Ω in the case of impulse voltages.
6. Safety zone and height of h-v electrode from earth are kept as per table

5. D (cm) Value of A Value of B
(min)Min Max
Upto 6.25 7D 9D 14S
10 to 25 6D 8D 12S
25 5D 7D 10S
50 4D 6D 8S
100 3.5D 5D 7S
150 3D 4D 6S
200 3D 4D 6S
Table : Clearances for sphere gaps
Advantages:
1. They can be used for peak value of ac and dc also for
calibration of voltmeter.
2. This method is also used for measurement of voltage
in impulse tests.
3. This method is simple and can be used up to 2 to 2500
kV.
Disadvantages:
1. This method does not give continuous record of
voltage.
2. Voltage measurement carried out by sphere gap is
accurate within 2-3% only with precations taken.
Accuracy is less but acceptable.
3. Measurement is affected by humidity, pressure,
temperature, proximity of earthed object to testing side
etc. and these introduces error in measurement.

6. Precautions:
• Distance between two sphere should not exceed diameter of sphere.
• No measurement should be taken with spacing less than 5% of radius.
• The surface of sphere should be free from dust particles, shining surface and scratching should be
avoided.
• Spark over interval should not cause pitting and overheating of surface.
• No conducting masses, objects or surface should be region of safety zone.
Applications:
• Measurement of peak value of Dc,AC & impulse voltage.
• For calibration of other voltage measuring devices used in high voltage test circuits such as
oscilloscope.

7. Factors affecting on Measurement
Nearby Earthed
Object
Atmospheric
Conditions and
Humidity
Irradiation
Polarity and rise
time of voltage
waveform

8. Nearby Earthed Object
• The effect of earth object on measurement was first
observed by Kuffel.
• He observed that breakdown voltage reduces when gap is
surrounded by metal cage cylinder especially when gap
length exceeded sphere radius.
• The reduction in voltage is given by,
∆𝑉 = 𝑚𝑙𝑜𝑔
𝐵
𝐷
+C
Where, ∆𝑉- % reduction in breakdown voltage
B- diameter of earthed enclosing cylinder
D- Diameter of spheres. & m,c are constants
S- spacing
• If the specifications regarding the clearances are closely
observed the error is within the tolerances– and accuracy
specified. The variation of breakdown voltage with A/D
ratio is given in Figs a and b for a 50 cm sphere gap.

9. Effect of atmospheric conditions
• The sparkover voltages of a spark gap depends on the air
density which varies with the changes in both temperature
and pressure. If the sparkover voltage is V under test
conditions Temperature T and pressure p torr and if the
sparkover voltage is V0 under standard conditions of
temperature T= 20°C and pressure p = 760 torr, then
V=K𝑉0 where K is function of air density factor d,
given by
d =
𝑃
760
293
273+𝑇
The sparkover voltage increase with humidity.
(i) The humidity effect increases with the size of spheres
and is maximum for uniform field gaps, and
(ii) The sparkover voltage increases with the partial
pressure of water vapour in air, and for a given humidity
condition,
.
• The change in spark over voltage increases with the gap
length. As the change in spark over voltage with humidity
is within 3%, no correction is normally given for
humidity.

10. Effect of Irradiation
• Illumination of sphere gaps with ultra:violet or x-rays
aids easy ionization in gaps.
• The effect of irradiation is pronounced for small gap
spacings.
• A reduction of about 20% in sparkover voltage was
observed for spacings of 0.1 D to 0.3 D for a 1.3 cm
sphere gap with d.c. voltages.
• The reduction in sparkover voltage is less than 5%
for gap spacings more than 1 cm, and for gap
spacings of 2 cm or more it is about 1.5%.
• Hence, irradiation is necessary for smaller sphere
gaps of gap spacing less than 1 cm for obtaining
consistent values.
Effect of polarity and waveform
• It has been observed that the sparkover voltages for
positive and negative polarity impulses are different.
• Experimental investigation showed that for sphere gaps of
6.25 to 25 cm diameter, the difference between positive and
negative d.c. voltages is not more than 1%.
• For smaller sphere gaps (2 cm diameter and less) the
difference was about 8% between negative and positive
impulses of 1/50 μs waveform.
• Similarly, the wave front and wave tail duration also
influence the breakdown voltage. For wave fronts of less
than 0.5 μs and wave tails less than 5 μs the breakdown
voltages are not consistent and hence the use of sphere gap
is not recommended for voltage measurement in such
cases.

11. Electrostatic voltmeter
Electrostatic voltmeters are related to measurement of electric field force generated by voltages between pair of parallel
plane disc electrodes.
Principle of Operation
• Electrostatic voltmeter can refer to an electrostatic charge meter.
• Electrostatic Voltmeter to measure large electrical potential. ( Direct method of measuring HV)
• It can accurately measure surface potential (voltage) on materials without making physical contact.
• Electrostatic voltmeter utilizes the attraction force between two charged surfaces.
• Attraction between 2 charged surface create a deflection of a pointer directly calibrated in volts.
• Attraction Force is proportional to the square of the applied voltage.
• The measurement can be made for AC or DC voltages.
• When one of the electrodes is free to move, the force on the plate can be measured by controlling it by a spring or balancing
it with acounterweight.
• For high voltage measurements, a small displacement of one of the electrode by a fraction of a millimetres is usually sufficient
for voltage measurement.
• Electrostatic voltmeter is designed to measure high potential differences; typically from a few hundred to many
thousands volts.

12. Electrostatic voltmeter
Principle of Operation
• Electrostatic voltmeter utilizes the attraction force between two charged
surfaces to create a deflection of a pointer directly calibrated in volts.
• The pivoted sector NN is attracted to the fixed sector QQ
• The moving sector indicating the voltage by the pointer P and is
counterbalanced by the small weight w.
• Damping technique- Air friction damping

13. Electrostatic voltmeter
Principle of Operation
Schematic Diagram

14. Construction
• Electrostatic voltmeters are made with parallel plate configuration using
guard rings to avoid corona and maintain constant potential.This disc type
electrodes are separated by small distance. Diameter of each plate is about 1m.
• A resistive balance B is inside hemispherical metal dome D. It is used to
measure force of attraction between movable disc which changes from one of
its arm and larger plate P.
• M is movable electrode with clearance of above 0.01cm in a central opening
in upper plate union serves as a guard ring.
• Guard ring H surround space between discs F and M. Spacing between two
electrodes is large, uniformity in field is difficult to maintain hence guard ring
is used to achieve uniformity of electric field.
• An absolute voltmeter is made by balancing the plate with a counter weight
and is calibrated in terms of a small weight.
• The electrostatic voltmeters have a small capacitance (5 to 50 pF)
• Guard ring H are maintained at constant potential in space by C capacitance
divider ensuring uniform special potential distribution resistance.

15. Construction
• An upper frequency limit of about one MHz is achieved in careful designs.
• The accuracy for AC voltage measurements is better than DC voltage measurements.
• It consists of parallel plane disc type electrodes separated by a small distance.
• The moving electrode is surrounded by a fixed guard ring to make the field uniform in the central region.
• In order to measure the given voltage with precision, the disc diameter is to be increased, and the gap
distance is to be made less.
• The balancing weight gives controlling torque.
• Electrostatic voltmeters are constructed in an enclosed structure containing compressed air or SF6 or carbon
dioxide or nitrogen.
• The gas pressure may be in the order of 15atm.
• Working stress= 100kV/cm
Electrostatic voltmeter

16. • Light Reflected from mirror and carried by balanced
beam Magnifies its motion and is indicated when
equilibrium condition is replaced.
• In electrostatic fields, the attractive force between the
electrodes of a parallel plate condenser is given by
• F=
1
2
∈0 𝐴(
𝑉
𝑆
)2
Where, V- applied voltage between plates
C- capacitance between plates
A- area of cross section of plates.
d- Diameter of plates
S- separation between plates
∈0-permittivity of medium

17. Electrostatic voltmeter
Construction

18. Advantages
• Active power loss is negligibly small
• Low loading effect
• Voltage up to 600kV can be measured
Limitations
• The measurement of voltage lower than 50V is not possible because force become too small.
• For constant distance ‘s’, F α V2, the sensitivity is small. This can be overcome by varying the
gap distance d in appropriate steps.
Use
• It can be used AC and DC measurements.
• High precision type electrostatic voltmeters have been built for very high voltages up to
1000KV
Electrostatic voltmeter

19. Contents of presentation
i. What is Generating voltmeter….?
ii. Construction
iii. Principle of operation
iv. Advantages of Generating Voltmeters
v. Limitations of Generating Voltmeters
Generating voltmeter

20. What is Generating voltmeter….?
• 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 is no loading effect.
• Rate of change of capacitance of a capacitor w.r.t. time causes current to flow through it and this
current is proportional to voltage applied to variable capacitance.
• Generating volt meter can measure loss free AC voltage.
• Generating voltmeters are high impedance devices.
• No direct connection to the high voltage.

21. Construction
Generating voltmeter
• Fig. shows a schematic diagram of a generating voltmeter which
employs rotating vanes for variation of capacitance.
• High voltage source is connected to a disc electrode D3 which is
kept at a fixed distance on the axis of the other low voltage
electrodes D2, D1, & D0.
• The rotor D0 is driven at a suitable constant speed by a
synchronous motor. Rotor vanes of D0 cause periodic change in
capacitance between the insulated disc D2 and the high voltage
electrode D3.
• Number and shape of vanes are so designed that a suitable
variation of capacitance (sinusoidal or linear) is achieved.
• The AC current is rectified and is measured using moving coil
meters. If the current is small an amplifier may be used before the
current is measured.
• Generating voltmeters are linear scale instruments and applicable
over a wide range of voltages.
• The sensitivity can be increased by increasing the area of the pick
up electrode and by using amplifier circuits

22. Principle of operation
• We have charge stored in the capacitor q=CV
• If the capacitance of the capacitor varies with time when connected to the source of voltage V, the
current through the capacitor

23. Principle of operation
• We have charge stored in the capacitor 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 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 AC voltage
measurements also provided
the angular frequency is the
same or equal to half that of
the supply frequency.

24. 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
(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
(iii) Disturbance in position and mounting of the electrodes make the calibration invalid.

25. Peak Reading Voltmeter
• In some cases it is necessary to measure peak value of an ac wave e.g. to obtain maximum dielectric strength
of insulating solids etc.
• When waveform is not sinusoidal RMS value of voltage multiplied by 2 is not correct. Hence separate peak
value instrument is needed.
Series Capacitor Peak Voltmeter
• when a capacitor is connected to sinusoidal voltage source, the charging current
𝑖0 = 0
𝑡
𝑉𝑑𝑡 = 𝑗𝑤𝐶𝑉 where V is rms value of voltage and w is angular frequency.
• In positive half cycle capacitor charges up to peak value and when voltage falls it discharges through
millimetres and so that voltage across capacitor is very nearly constant at peak value and current is thus
proportional to peak value.

26. • The diode D1 is used to rectify the a.c.
current in one half cycle while
D2 bypasses in the other half cycle.
• This arrangement is suitable only of
positive or negative half cycles and
hence is valid only when both half,
cycles are symmetrical and equal.
• This method is not suitable when the
voltage waveform is not sinusoidal but
contains more than one peak or
maximum as shown in Fig.
• The charging current through the
capacitor changes its polarity within
one half cycle and current within that
period subtracts from net current.
Peak Reading Voltmeter

27. Peak Reading Voltmeter
• 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/ω for 50 Hz application.
• The error due to the resistance is

28. • In determining the error, the actual value of the angular frequency ω 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 allows 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.

29. • Instead of directly measuring the rectified charging current, a proportional analog voltage signal is
derived which is then converted into a proportional mean 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 Δt =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.

30. What is potential divider…?
Types of potential dividers
Potential dividers

31. Potential dividers
What is potential dividers…?
• Potential or voltage dividers useful for
high voltage DC and AC measurement.
• Potential dividers are usually either resistive or
capacitive or mixed element type.
• The low voltage arm of the divider is usually
connected to a fast recording oscillograph or a
peak reading instrument through a delay cable
or a coaxial cable.

32. • Z1 is usually a resistor or a series of resistors in case of a resistance potential divider.
• Z1 is usually a single or a number of capacitors in case of a capacitance divider.
• Z1 can able to use the combination of resistance & Capacitor in case of a mixed RC potential divider.
• Z2 will be a resistor or a capacitor or an R-C impedance depending upon the type of the divider.
Potential dividers

33. Resistance potential divider
• A simple resistance potential divider
consists of two resistances R1 and R2 in
series. (R1>>R2)
• Voltage ratio or attenuation factor is given
by
• The divider element R2, in practice, is
connected through the coaxial cable to
the oscilloscope.
Sudden switching action causes Flash over
voltage and that causes damage to divider
circuit
In order to protect the dividers from flash
over voltage, voltage controlled capacitors
are used.

34. Resistance potential divider

35. • The cable will generally have a surge impedance Z0
• Surge impedance will come in parallel with the oscilloscope input impedance (Rm , Cm).
• Rm will generally be greater than one megaohm and Cm may be 10 to 50 picofarads.
• For high frequency and impulse voltages, the ratio in the frequency domain will be given by
Resistance potential divider

36. Capacitive potential divider
• Harmonic Effects can be eliminated by use of
Capacitive Potential Dividers (CPD) with Electro
Static Voltmeter (ESV).
• Gas filled condensers C1 and C2 are used as shown
in figure.
• C1 is a three terminal capacitor, connected to C2 by
shielded cable.
• C2 is shielded to avoid stray capacitance
• Applied voltage V1 is given by,
• where,
– Cm - Capacitance of the meter and cableleads
– V2 - Reading ofVoltmeter
• Impulse voltage can be measured by using
capacitive potential dividers
C - Standard Compressed Gas H.V.Condenser1
C2 - Standard Low Voltage Condenser
ESV- Electrostatic Voltmeter
P -Protective Gap
C.C - Connecting Cable

 1C
V1V2 
 C1  C2  Cm 

37. • Capacitive potential divider can measure fast rising voltage
& pulse and impulse voltage.
• Capacitance ratio is independed of frequency.
• Ratio of the divider (Attenuation factor) is given by
• Capacitance C1 is formed between the HV terminal of
the source.
Capacitive potential divider

38. Capacitive potential divider
• Suitable for measuring the impulse voltage up to 1 MV
• C1 is the standard compressed air or gas
condenser- HV Capacitor.
• Value of C2 is very high, C2 may be mica capacitor,
paper capacitor etc
• C1 is connected to C2 by using a shield cable
• C2 is completely covered by using a box, for
avoiding stray capacitance.
• Voltage can measure by using VTVM (Vacuum
Tube Volt Meter) or ESV- testing purpose for
impulse voltage

39. Impulse voltage measurement by using capacitive
V1- Voltage to be measured
V2-Meter reading
C1-Standard compressed gas HV condenser
C2-Standard low voltage condenser
Cm-Capacitance of the meter
Capacitive potential divider
Advantages
• Loading on the source is negligible
• Capacitance ratio independent of frequency

40. • Mixed potential dividers use R-C elements in series or in parallel.
• Improved step response
Mixed RC potential divider

41. Different arrangement
Mixed RC potential divider

42. Mixed RC potential divider
Step response is the time behavior of the output of a general
system when the input changes from zero to one in a very short
time.

43. The following elements mainly constitute the different errors in the measurement:
(i) Residual inductance in the elements
(ii)Stray capacitance occurring
(a) between the elements
(b) from sections and terminals of the elements to ground
(c) from the high voltage lead to the elements or sections
(iii)The impedance errors due to
(a) connecting leads between the divider and the test objects
(b) ground return leads and extraneous current in ground leads
(iv) Parasitic oscillations due to lead and cable inductances and capacitance of high voltage terminal
to ground.
Potential dividers

44. Note1 -surge impedance
• The characteristic impedance or surge impedance of a uniform transmission line, usually written Z0,
is the ratio of the amplitudes of voltage and current of a single wave propagating along the line.
• That is, a wave travelling in one direction in the absence of reflections in the other direction.
• Characteristic impedance is determined by the geometry and materials of the transmission line and, for a
uniform line, is not dependent on its length. The SI unit of characteristic impedance is the ohm.
Note 2-Parasitic capacitance
• In electrical circuits, parasitic capacitance, stray capacitance or, when relevant, self-capacitance, is an
unavoidable and usually unwanted capacitance that exists between the parts of an electronic component
or circuit simply because of their proximity to each other.
• All actual circuit elements such as inductors, diodes, and transistors have internal capacitance, which can
cause their behavior to depart from that of 'ideal' circuit elements.
• Additionally, there is always non-zero capacitance between any two conductors; this can be significant at
higher frequencies with closely spaced conductors, such as wires or printed circuit board traces.

45. Capacitance voltage transformer
• The high voltage measurement is possible
by utilizing the concept of resonance. The
set up used for it is often known as
capacitance voltage transformer.
• The CVT is shown in fig.(a) & (b)
• It consist of-
C1-High Voltage Capacitance
C2- standard low voltage capacitance.
M-Electrostatic Voltmeter
L-Variable inductor
T-Resonant transformer

46. • 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 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..
• C1 is made of a few units of high voltage capacitors, and the total Capacitance Voltage Transformer
will be around a few thousand picofarads as against a gas filled standard capacitor 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 I.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

47. • The phasor diagram of the CVT under
resonant conditions is shown in Fig. The meter
reactance, Xm is neglected and is taken as a
resistance load Rm
• when the load is connected to the voltage
divider side. The voltage across the potential
transformer V2 = Im.Rm and the voltage across
the capacitor VC2 = V2 + Im (Re + j Xe). The
Phasor diagram is written taking V1 as the
reference phasor.
• It is can be seen resistance and reactance are
not shown separately and are included in
Ri and Xi, the resistance and reactance of
tuning inductor L.
• The Voltage ratio, Neglecting the reactance drop ImXe, VRi is the voltage
drop across the tuning inductor and the transformer
resistance. The voltage V2 (meter voltage) will be in
phase with the input voltage V1.

48. The advantages of a CVT:
1. simple design and easy installation,
2. 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.
3. Frequency independent voltage distribution along elements as against conventional magnetic
potential transformers which require additional insulation design against surges.
4. Provides isolation between the high voltage terminal and low voltage.
The disadvantages of a CVT:
1. The voltage ratio is susceptible to temperature variation.
2. The problem Of inducing ferro-resonance in power systems

49. Cathode ray oscilloscope for impulse measurement
• Modern Cathode Ray Oscillograph for Impulse Measurements are sealed tube, hot cathode oscilloscopes
with photographic arrangement for recording the waveforms.
• The cathode ray oscilloscope for impulse work normally has input voltage range from 5 m V/cm to about
20 V/cm. In addition, there are probes and attenuators to handle signals up to 600 V (peak to peak).
• The bandwidth and rise time of the oscilloscope should be adequate. Rise times of 5 ns and bandwidth as
high as 500 MHz may be necessary. Sometimes high voltage surges test oscilloscopes do not have vertical
amplifier and directly require an input voltage of 10 V. They can take a maximum signal of about 100 V
(peak to peak) but require suitable attenuators for large signals.
• Oscilloscopes are fitted with good cameras for recording purposes.
• With rapidly changing signals, it is necessary to initiate or start the oscilloscope time base before the signal
reaches the oscilloscope deflecting plates, otherwise a portion of the signal may be missed.
• Such measurements require an accurate initiation of the horizontal time base and is known as triggering.
Oscilloscopes are normally provided with both internal and external triggering facility.
• When external triggering is used, as with recording of impulses, the signal is directly fed to actuate the
time base and then applied to the vertical or Y deflecting plates through a delay line. The delay is usually
0.1 to 0.5 μs.

50. The delay is obtained by:
1. A long interconnecting coaxial cable 20 to 50
in long. The required triggering is obtained
from an antenna whose induced voltage is
applied to the external trigger terminal.
2. The measuring signal is transmitted to the
CRO by a normal coaxial cable. The delay is
obtained by an externally connected coaxial
long cable to give the necessary delay. This
arrangement is shown in Fig.
3. The impulse generator and the time base of the
CRO are triggered from an electronic tripping
device. A first pulse from the device starts the
CRO time base and after a predetermined time
a second pulse triggers the impulse generator.
Cathode ray oscilloscope for impulse measurement

51. Measurement of dielectric constant and loss
• Many insulating substances have dielectric constant greater than unity and have Dielectric Constant and Loss
when subjected to a.c. voltages.
• The dielectric constant and the loss depend on the magnitude of the voltage stress and on the frequency of the
applied voltage.
• When a dielectric is used in an electrical equipment such as cable or a capacitor, the variation of these quantities
with frequency is of importance. The microscopic properties of the dielectric are described by combining the
variation of the above two quantities into one “complex quantity” known as “complex permittivity” and
determining them at various frequencies.
• A capacitor connected to a sinusoidal voltage source v = v0 exp (jωt) with an angular frequency ω = 2πf stores a
charge Q = C0v and draws a charging current Ic = dQ/dt = jωC0v. When the dielectric is vacuum, C0 is the
vacuum capacitance or geometric capacitance of the capacitor
• If the capacitor is filled with a dielectric of permittivity ε′, the capacitance of the capacitor is increased to C =
C0ε′/ε0 = C0K′ where K′ is the relative Dielectric Constant and Loss of the material with respect to vacuum.
• Under these conditions, if the same voltage V is applied, there will be a charging current Ic and loss component
of the current, I1. I1 will be equal to GV where G represents the conductance of the dielectric material. The total
current I = Ic + I1 = (jωC + G)V.. The current leads the voltage by an angle θ which is less than 90°. The loss
angle δ is equal to (90 – θ)°. The phasor diagrams of an ideal capacitor and a capacitor with a lossy dielectric are
shown in Figs a and b.

52. • It would be premature to conclude that the Dielectric
Constant and Loss material corresponds to an R-C parallel
circuit in electrical behaviour. The frequency response of this
circuit which can be expressed as the ratio of the loss current
to the charging current, i.e. the loss tangent.
• may not at all agree with the result actually observed, because
the conductance need not be due to the migration of charges
or charge carriers but may represent any other energy
consuming process. Hence, it is customary to refer the
existence of a loss current in addition to the charging current
by introducing “complex permittivity“
• so that current I may be written as

53. • K* is called the complex relative permittivity or complex dielectric constant, ε′ and K’ are called the
permittivity, and relative permittivity and ε′′ and K” are called the loss factor and relative loss factor
respectively.
• The loss tangent
• The product of the angular frequency and ε′′ is equivalent to the dielectric conductivity σ,
• The dielectric conductivity sums up all the dissipative effects and may represent the actual conductivity as
well as the energy loss associated with the frequency dependence (dispersion) of e’, i.e. the orientation of
dipoles in a dielectric.
• In dielectric measurements, often, the geometrical capacitance and the capacitance of the system with a
dielectric material are obtained. The ratio of the above two measurements gives the relative permittivity
ε′/ε0 = K′. This is sometimes referred to as the dielectric constant or εr.

54. Measurement of High Direct Current
High magnitude direct currents are measured using a resistive shunt of low ohmic value. The voltage drop
across the resistance is measured with a millivoltmeter. The value of the resistance varies usually between
10 μΩ and 10 mΩ. This depends on the heating effect and the loading permitted in the circuit.
Measurement of High Direct Current resistors are usually oil immersed and are made as three or four
terminal resistances. The voltage drop across the shunt is limited to a few millivolts (< I Volt) in power
circuits.
Hall Generators for DC Measurements
The principle of the “Hall effect” is made use of in Measurement of High Direct Current. 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 magnetic field. The charge displacement generates an emf in the normal
direction, called the “Hall 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 “Hall
coefficient“

55. • 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—(1/δ) is produced in a small air gap in the core. The
Hall elements is placed in the air gap (of thickness d), and a small
constant d.c. current is passed through the element. The schematic
arrangement is shown in Fig.
• The voltage developed across the Hall element in the normal
direction is proportional to the d.c. current I. 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 High Direct Current.
• Hall generators can be used for measurement of unidirectional a.c.
and impulse currents also. With proper design of Hall element
dimensions and addition of compensating circuits, the bandwidth of
the Hall generator can be increased to about 50 MHz. As such, these
generators can be used for the measurement of post arc currents and
unidirectional impulse currents.

56. 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.
• current transformers provide electrical isolation from high voltage circuits in power systems.
• Current transformers used for 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.
• A voltage signal proportional to the measuring current is generated and is transmitted to the ground through
an electro optical device. Light pulses proportional to the voltage signal are transmitted by a glass-optical
fibre bundle to a photo detector 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 Fig.

57. Measurement of High Power Frequency Alternating Currents

58. Partial Discharge Measurements:
• Earlier the testing of insulators and other equipment was based on the insulation resistance
measurements, dissipation factor measurements and breakdown tests.
• It was observed that the dissipation factor (tan δ) was voltage dependent and hence became a criterion
for the monitoring of the high voltage insulation.
• In further investigations it was found that weak points in an insulation like voids, cracks, and other
imperfections lead to internal or intermittent discharges in the insulation.
• These imperfections being small were not revealed in capacitance measurements but were revealed as
power loss components in contributing for an increase in the dissipation factor. In modern terminology
these are designated as “Partial Discharge Measurements” which in course of time reduce the strength
of insulation leading to a total or partial failure or breakdown of the insulation.
• Electrical insulation with imperfections or voids leading to Partial Discharge Measurements can be
represented by an electrical equivalent circuit shown in Fig.
• Consider a capacitor with a void inside the insulation (Ca). The capacitance of the void is represented
by a capacitor in series with the rest of the insulation capacitance (Cb). The remaining void-free
material is represented by the capacitance Cc. When the voltage across the capacitor is raised, a critical
value is reached across the capacitor Ca and a discharge occurs through the capacitor, i.e. it becomes
short circuited. This is represented by the closure of the switch.

59. • Generally Ca << Cb << Cc. A charge
Aga which was present in the
capacitor Ca flows through Cb and
Cc giving rise to a voltage pulse
across the capacitor Cc. A measure
of the voltage pulse across the
capacitor gives the amount of
discharge quality. But this
measurement is difficult in practice,
and an apparent charge
measurement across a detecting
impedance is usually made.

60. • The circuit arrangement shown in Fig. gives a simplified
circuit for detecting “partial discharges“.
• The high voltage transformer shown is free from internal
discharges. A resonant filter is used to prevent any
pulses starting from the capacitance of the windings and
bushings of the transformer. Cx is the test object, Cc is
the coupling capacitor, and Zm is a detection impedance.
• The signal developed across the impedance Z is passed
through a band pass filter and amplifier and displayed
on a CRO or counted by a pulse counter multi-channel
analyzer unit.
Discharge Detection Using Straight Detectors

61. Discharge Detection Using Straight Detectors
Figure (B)

62. • In Fig(B), the discharge pattern displayed on the CRO screen of a Partial Discharge Measurements detector
with an elliptical display is shown.
• The sinusoidal voltage and the corresponding ellipse pattern of the discharge are shown in Fig. a and a single
corona pulse in a point-plane spark gap geometry is shown in Figs b and c.
• When the voltage applied is greater than that of the critical inception voltage, multiple pulses appear ( Fig.c),&
all the pulses are of equal magnitude.
• A typical discharge pattern in cavities inside the insulation is shown in Fig.d. This pattern of discharge appears
on the quadrants of the ellipse which correspond to the test voltage rising from zero to the maximum, either
positively or negatively.
• The discharges usually start near the peaks of the test voltage but spread towards the zero value as the test
voltage is increased beyond the inception level.
• The number and magnitude of the discharges on both the positive and negative cycles are approximately the
same. A typical discharge pattern from a void bounded on one side by the insulation and the other side by a
conductor is shown in Fig.c.
• This pattern of discharge is common in insulated cables (like polyethylene and XLPE cables) when the
discharge is made up of a large number of pulses of small magnitude on the positive cycle and a much smaller
number of large magnitude pulses on the negative half-cycle.

63. • In the narrow band detection scheme Zm is a parallel L-C circuit tuned to 500 kHz. The bandpass
filter has a bandwidth of about ± 10 kHz. The pulses after amplification are displayed in an
elliptical time base of a CRO, and the resolution for the pulses is about 35 per quadrant.
• In the wide band detection scheme Zm is an R-C network connected to a double tuned transformer.
The bandwidth is about 250 kHz with centre frequency between 150 and 200 kHz. A wide band
amplifier is used, and the signal is displayed on the CRO as in the previous case. The resolution is
about 200 pulses per quadrant.
• With tuned narrow band detectors, the Partial Discharge Measurements can be detected with a
sensitivity less than one pico coulomb for a testpiece capacitance of 100 pF. Testpieces with
capacitances in the range 100 pF to 0.1 pF can be tested. With the wide band detector samples up
to 250 μF capacitance can be tested. Sensitivity of the measurement varies from 0.005 pico
coulomb at 6 pF sample capacitance to about 15 pc at a sample capacitance of 250 μF