"""""""FGRCTVOGPV"QH"GNGEVTKECN"GPIKPGGTKPI"
FKV"WPKXGTUKV["
VGG"246"JKIJ"XQNVCIG"GPIKPGGTKPI
Introduction to High Voltage Measurement – It is essential to measure the voltages and currents
accurately ensuring perfect safety to the personnel and equipment in industrial testing and research
laboratories. The location and layout of the devices are important as the person handling the equipment
must be protected against overvoltages and also against any induced voltages due to stray coupling. The
different used for high voltage measurements may be classified as in table 1 and table 2
Table 1 High Voltage Measurement Techniques
(a) D.C. Voltages
(i) Series resistance microammeter
(ii) Resistance potential divider
(iii) Generating voltmeter
(iv) Sphere and other spark gaps
(b) A.C. Voltages
(power frequency)
(i) Series impedance ammeters
(ii) Potential dividers (resistance or capacitance type)
(iii) Potential transformers (electromagnetic or CVT)
(iv) Electrostatic voltmeters
(v) Sphere gaps
(c) A.C. High frequency (i) Potential dividers with a cathode ray oscillograph (resistive or
voltages, impulse
(ii) Peak voltmeters
and other rapidly
(iii) Sphere gaps
changing voltages
Table 2
High Current Measurement Techniques
(a) Direct currents
(i) Resistive shunts with milliammeter
(ii) Hall effect generators
(iii) Magnetic links
(b) Alternating currents (i) Resistive shunts
(power frequency) (ii) Electromagnetic current transformers
(c) High frequency a.c.
Impulse and rapidly
changing currents
(i) Resistive shunts
(ii) Magnetic potentiometers or Rogowski coils
(ii) Magnetic links
(iv) Hall effect generators
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 with a microammeter as shown
in fig.1 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 small in comparison to the series resistance. A protective
device like a paper gap, neon glow tube, zener diode with a suitable series resistance is connected across
the meter as a protection against high voltages in case series resistance fails or flashes over.
The ohmic value of resistance R is so chosen such that a current of 1 to 10 µA is allowed for full scale
deflection. The limitations in the series resistance design are:
i.
Power dissipation and source loading.
1
http://eedofdit.weebly.com
Prepared by: Mohamed Samir
"""""""FGRCTVOGPV"QH"GNGEVTKECN"GPIKPGGTKPI"
FKV"WPKXGTUKV["
VGG"246"JKIJ"XQNVCIG"GPIKPGGTKPI
ii.
iii.
iv.
Temperature effects and long time stability.
Voltage dependence or resistive elements
Sensitivity to mechanical stresses.
HV
R
Protective
device
µA
Fig.1 Series resistance microammeter
Resistance Potential Dividers for d.c. Voltages – A resistance potential divider with an electrostatic
voltmeter is shown in fig.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 voltages, 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 necessary to avoid unnecessary discharges at high voltage ends. Potential
dividers are made with 0.05% accuracy upto 100 kV, with 0.01% accuracy upto 300 kV
HV
R1
R2
ESV
P
Fig.2 Resistance Potential Divider
ESV – Electrostatic Voltmeter
P – Protective Device
Generating Voltmeter – The generating principle is employed where source loading is prohibited 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.
2
http://eedofdit.weebly.com
Prepared by: Mohamed Samir
"""""""FGRCTVOGPV"QH"GNGEVTKECN"GPIKPGGTKPI"
FKV"WPKXGTUKV["
VGG"246"JKIJ"XQNVCIG"GPIKPGGTKPI
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
dq
dC
dV
i=
=V
+C
dt
dt
dt
For d.c. voltages dV/dt = 0 Hence
dq
dC
i=
=V
dt
dt
If the capacitance C varies between the limits C0 and (C0 + Cm) sinusoidally as C = C0 + Cm sin ω t and
ωVCm
the current is given by i = Im cos ω t where Im = ω VCm and the rms value is given by irms =
2
For a constant angular frequency ω , the current is proportional to the applied voltage V. The
generated current is rectified and measured by a moving coil meter. Generating voltmeter can also
be used for measuring a.c. voltage provided the angular frequency ω is the same or equal to half
that of the supply frequency.
A generating voltmeter with a rotating cylinder consists of two exciting field electrodes and a
rotating two pole armature driven by a synchronous motor at a constant speed n. The a.c. current
flowing between two halves of the armature is rectified by a commutator. The fig.3 shows 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 is driven at a constant speed by a synchronous motor at a suitable speed. 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. The instrument is calibrated using a potential divider or sphere gap. The
calibration curves of a generating voltmeter are shown in fig.4 (a) and 4(b)
S3
S0
S1
S2
Motor
Fig.3 Schematic Diagram of Generating Voltmeter
3
http://eedofdit.weebly.com
Prepared by: Mohamed Samir
"""""""FGRCTVOGPV"QH"GNGEVTKECN"GPIKPGGTKPI"
FKV"WPKXGTUKV["
VGG"246"JKIJ"XQNVCIG"GPIKPGGTKPI
S3 = High Voltage Electrode
S2
S1
S0
S3
S0 = Rotor
S1 & S2 = Fixed Electrodes
Advantages of Generating Voltmeter – The following are the advantages of a generating voltmeter:
i. No source loading by the meter.
ii. No direct connection is required with the high voltage electrode.
iii. The scale is linear and extension of range is easy.
iv. It is a very convenient instrument for electrostatic devices.
Limitations of Generating Voltmeter – The following are the limitations of a generating voltmeter:
i. Calibration of the voltmeter is required.
ii. It requires a careful construction and it is a cumbersome instrument requiring an auxiliary drive.
Capacitance Voltage Transformer – A capacitance voltage transformer (CVT) is basically a capacitor
divider used with a suitable matching or isolating potential transformer tuned for resonance conditions.
A CVT can be connected to a low impedance device like a wattmeter pressure coil or relay coil which is in
contrast to a simple capacitor divider which requires a high impedance meter like a TVM or electrostatic
voltmeter. The fig.4 shows the schematic diagram of CVT with its equivalent circuit as shown in fig.5
C1
L
T
C2
M
Fig. 4 Schematic diagram of CVT
The capacitor C1 is made of a few units of high voltage capacitors such that the total capacitance is of
few thousand picofarads. 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 maybe 10 or 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. The resonance condition is achieved when the following
condition is satisfied:
1
ω (L + LT ) =
(
ω C1 + C2 )
where L = inductance of the choke
4
http://eedofdit.weebly.com
Prepared by: Mohamed Samir
"""""""FGRCTVOGPV"QH"GNGEVTKECN"GPIKPGGTKPI"
FKV"WPKXGTUKV["
VGG"246"JKIJ"XQNVCIG"GPIKPGGTKPI
LT = equivalent inductance of the transformer referred to H.V. side
The meter reactance Xm is neglected and is taken as a resistance load Rm when the load is connected to
the voltage divider side.
Advantages of CVT: The following are the advantages of a capacitance voltage transformer:
i. Simple design and easy installation.
ii. Can be used both as a voltage measuring device for meter and relaying purposes as well as
coupling condenser for power line carrier communication & relaying.
iii. The voltage distribution along the elements is frequency independent as against conventional
potential transformers which requires additional insulation design against surges.
iv. Provides isolation between the high voltage terminal and low voltage metering.
Disadvantages of CVT: The following are the disadvantages of capacitor voltage transformer are:
i. The voltage ratio is susceptible to temperature variations.
ii. The problem of inducing ferro – resonance in power systems.
Electrostatic Voltmeter: It works on the principle that an attractive force develops between the
electrodes of a parallel plate capacitor in the presence of electrostatic fields. The magnitude of the force
is given by the expression as follows:
− δWS
δ 1
 1 δC
=  CV 2  = V 2
F=
δs  2
δs
ds
 2
2
2
1
A 1 V 
d 2 V 
= ε 0V 2 2 = ε 0   =
  gm. wt.
2
s
2 s
2825  s 
Where V = applied voltage between plates,
C = capacitance between the plates,
A = area of cross – section of the plates,
d = diameter of plates
s = separation between the plates,
ε0 = permittivity of the medium (air or free space)
Ws = work done in displacing a plate
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 a counter weight. The force is proportional to the square of the applied
voltage; the measurement can be made for a.c. or d.c. voltages.
The electrostatic voltmeter measures the force based on the above equations and is arranged such
that one of the plates is rigidly fixed whereas the other is allowed to move. Since it results in the
disturbance of electric field, therefore even for high voltages the movable electrode is allowed to move
by not more than a fraction of a millimetre to a few millimetres so that change in electric field is
negligibly small.
Principle of Operation of Electrostatic Voltmeter: The electric field is produced by voltage and therefore
if the field force could be measured, the voltage can also be measured. The basic principle of an
electrostatic voltmeter is that whenever a voltage is applied to a parallel plate electrode arrangement,
an electric field is set up between the plates. Since the two plates are oppositely charged there is always
a force of attraction between the plates. If the voltage is time dependent, the force developed is also
time dependent.
5
http://eedofdit.weebly.com
Prepared by: Mohamed Samir
"""""""FGRCTVOGPV"QH"GNGEVTKECN"GPIKPGGTKPI"
FKV"WPKXGTUKV["
VGG"246"JKIJ"XQNVCIG"GPIKPGGTKPI
Construction of electrostatic voltmeter: Various designs of the voltmeter have been developed which
differ in the construction of electrode arrangement and in the use of different methods of restoring
forces required to balance the electrostatic forces of attraction. Some of the methods are as follows:
(i) Suspension of moving electrode on one arm of a balance.
(ii) Suspension of moving electrode on a spring.
(iii) Pendulous suspension of moving electrode.
(iv) Torsional suspension of moving electrode.
The fig. 5 shows a schematic diagram of an absolute electrostatic voltmeter. The hemispherical metal
dome E encloses sensitive balance D which measures force of attraction between the movable disc
which hangs from one of its arms and the lower plate B.
HV
D
Light Source
F
G
B
G
M
HV
R
m
M
Scale
H
H
C
F
(a) Absolute electrostatic voltmeter
M – Mounting plate
G – Guard plate
F – Fixed plate
H – Guard hoops or rings
(b) Light beam arrangement
m – Mirror
B – Balance
C – Capacitance divider
D – Dome
R – Balancing weight
Fig. 5 Electrostatic Voltmeter
The movable electrode M hangs with a clearance of above 0.01 cm in a central opening in the upper
plate which serves as a guard ring. The diameter of each plate is 1 m. Light reflected from a mirror
carried by the balance beam serves to magnify its motion. The uniformity of electric field is maintained
by guard rings H which surround the space between the discs M and F. The guard rings are maintained at
a constant potential in space by a capacitance divider ensuring a uniform special potential distribution.
Usually the electrostatic voltmeters have a small capacitance (5 to 50 pF) and high insulation resistance
(R ≥ 1013 Ω). The area of the plates should be large, spacing between plates should be small and some
dielectric medium other than air should be used to achieve higher force for a given voltage.
The gap length cannot be made very small as this is limited by the breakdown strength of the
dielectric medium between the plates. A limit is imposed on frequency range as the load inductance and
6
http://eedofdit.weebly.com
Prepared by: Mohamed Samir
"""""""FGRCTVOGPV"QH"GNGEVTKECN"GPIKPGGTKPI"
FKV"WPKXGTUKV["
VGG"246"JKIJ"XQNVCIG"GPIKPGGTKPI
the measuring system capacitance forms a series resonance circuit. The greatest advantage of
electrostatic voltmeter is its extremely low loading effect and negligibly small power loss.
Sphere Gap: It is considered as one of the standard methods for the measurement of peak value of d.c.,
a.c. and impulse voltages and is used for checking the voltmeters and other voltage measuring devices
used in H.V. test circuits. When an electric field across a gap exceeds the static breakdown strength of
the gap it results in complete breakdown of the gaseous gap having uniform field. A uniform electric field
is created in the gaseous gap between two spherical electrodes of equal diameter, if the electrodes are
separated by a distance much smaller than the electrode radius. It can be used for measurement of
impulse voltage of either polarity provided that the impulse is of standard waveform having wavefront
time atleast 1 µs and wavetail time 5 µs. The sphere gaps can be arranged either vertically with lower
sphere grounded, or horizontally with both spheres connected to source voltage or one sphere
grounded. The two spheres used are identical in shape and size. The fig. 6(a) and 6(b) shows the
schematic arrangement.
B
2
d
≤0.5d
P
S
A
1
Fig. 6(a) Horizontal Arrangement of Sphere gap
The horizontal arrangement is usually preferred for sphere diameters d < 50 cm. This arrangement is
used for measurement at lower voltage ranges. With larger spheres the vertical arrangement is chosen
where the lower electrode is earthed. In both the arrangements one of the spheres is static and the
other is movable so that the spacing between them can be adjusted. A minimum clearance around the
spheres must be available within which no external objects such as walls, ceilings, transformer tanks,
impulse generators or supporting framework for the spheres are allowed. The minimum clearance is
dependent on the gap spacing.
The height of the sparking point P above the horizontal ground plane A, minimum clearance B are
related to sphere diameter d and gap spacing S respectively. The voltage to be measured is applied
between the two spheres and the distance between the gap 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 source voltage when breakdown occurs. The value
of the series may vary from 100 Ω to 1000 kΩ for a.c. and d.c. voltages but not more than 500 Ω in the
case of impulse voltages.
7
http://eedofdit.weebly.com
Prepared by: Mohamed Samir
"""""""FGRCTVOGPV"QH"GNGEVTKECN"GPIKPGGTKPI"
FKV"WPKXGTUKV["
VGG"246"JKIJ"XQNVCIG"GPIKPGGTKPI
2
1
B
d
P
S
A
1 – Insulating Support
2 – High Voltage Connection with series resistor
A – Height of sparking point P above the ground plane
B – Radius of space free from external structures
Fig. 6(b) Vertical Arrangement of Sphere gap
The spheres are made of copper, brass or aluminium. The standard diameters for the spheres are 2, 5,
6.25, 10, 12.5, 15, 25, 50, 75, 100, 150 and 200 cm. The spheres are carefully designed and fabricated so
that their surfaces are smooth and the curvature is uniform. The surfaces should be free from dust,
grease or any other coating. Irradiation of gap is needed when measurements of voltage are less than 50
kV are made with sphere gaps of 10 cm diameter or less.
Factors Influencing Sparkover Voltage of Sphere Gaps: There are various factors that affect the
sparkover voltage of a sphere gap like nearby earthed objects, atmospheric conditions and humidity,
irradiation, polarity and rise time of voltage waveforms.
(a) Influence of nearby earthed objects: The effect of nearby objects was investigated by Kuffel by
enclosing the earthed sphere inside an earthed cylinder. It was observed that there was
reduction in breakdown voltage given by the empirical formula.
B
∆V = m ln + C
D
Where ∆V = reduction in breakdown voltage
B = diameter of earthed enclosing cylinder
D = diameter of spheres
m and C = factors depending on S/D ratio
S = spacing between spheres
The relation was less than 2% for S/D ≤ 0.5 and B/D ≥ 0.8. The reduction was only 3% for S/D ≈ 1.0
and B/D ≥ 1.0
(b) Influence of humidity: Kuffel studied the effect of humidity on breakdown voltage by using
spheres of 2 cm to 25 cm of diameter and uniform field electrodes. It was concluded that the
sparkover increases with the partial pressure of water vapour in air, and for a given humidity
condition, the change in breakdown voltage increases with the gap length. This is due to water
particles which readily attach with free electrons thus forming negative ions. These ions therefore
8
http://eedofdit.weebly.com
Prepared by: Mohamed Samir
"""""""FGRCTVOGPV"QH"GNGEVTKECN"GPIKPGGTKPI"
FKV"WPKXGTUKV["
VGG"246"JKIJ"XQNVCIG"GPIKPGGTKPI
(c)
(d)
(e)
(f)
slow down and are unable to ionise neutral molecules under field conditions in which electrons
will readily ionize. It has been observed that within the humidity range of 4 to 17 g/m3 the
relative increase of breakdown voltage is found to be between 0.2 to 0.35 % per gm/m3 for the
largest sphere of diameter 100 cm and gap length upto 50 cm.
Influence of dust particles: The presence of dust particles between the gap results in erratic
breakdown in homogeneous or slightly homogeneous electrode configurations. The dust particle
comes in contact with one electrode getting charged to polarity of that electrode when d.c.
voltage is applied. It then gets attracted by the opposite electrode due to field forces triggering
early breakdown. The gaps subjected to a.c. voltages are sensitive to dust particles but the
probability of erratic breakdown is less.
Influence of atmospheric conditions: The breakdown voltage of a spark gap depends on the air
density which varies with the changes in both temperature and pressure. If the breakdown
voltage is V under test conditions of temperature T and pressure p , if sparkover voltage is VO
under standard conditions of temperature and pressure then V = k VO where k is a function of d
where
p  293 
d=
760  273 + T 
The following table gives the relation between k and d
d
0.70 0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15
k
0.72 0.77 0.82 0.86 0.91 0.95 1.00 1.05 1.09 1.12
Influence of irradiation: The illumination of sphere gaps with ultraviolet or X rays aids easy
ionization in gaps. It was observed that for spacings of 0.1D to 0.3D for a 1.3 cm sphere gap with
d.c. voltages there was reduction of 20% in breakdown voltage. The reduction in breakdown
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% Thus irradiation is necessary for smaller sphere gaps of gap spacing less than 1 cm for
obtaining consistent values.
Influence of polarity and waveform: It has been observed that the breakdown voltages for
positive and negative polarity impulses are different. It has been experimentally investigated that
for sphere gaps of 6.25 cm to 25 cm diameter, the difference between positive and negative d.c.
voltages is not more than 1%. For smaller sphere gaps ( 2 cm or less diameter) the difference is
8% between negative and positive impulses of 1/50 µs waveform. For the wavefronts of less than
0.5 µs and wavetails less than 5 µs the breakdown voltages are not consistent. Hence the use of
sphere gap is not recommended for voltage measurement in such cases.
Cathode Ray Oscilloscope for Impulse Measurements: The modern oscilloscopes are sealed tube hot
cathode oscilloscopes provided with photographic arrangement for recording the waveforms. The CRO
for impulse work normally has input voltage range from 5 mV/cm to about 20 V/cm. The bandwidth and
rise time of the oscilloscope should be adequate. Rise time of 5 ns and bandwidth as high as 500 MHz
maybe necessary. Sometimes high voltage surge 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.
It is necessary to start the oscilloscope time base before the signal reaches the oscilloscope
deflection plates in case of rapidly changing signals otherwise a portion of the signal maybe missed. Such
measurements require an accurate initiation of horizontal time base which is known as triggering. The
oscilloscopes are normally provided with both internal and external triggering facility. In case of external
triggering, the signal is directly fed to start the time base and then applied to the vertical deflecting
plates through a delay line. The delay is usually 0.1 to 0.5 µs. The fig. 7(a) and 7(b) shows the block
diagram of surge test oscilloscope where measuring signal is transmitted to the CRO by a normal coaxial
9
http://eedofdit.weebly.com
Prepared by: Mohamed Samir
"""""""FGRCTVOGPV"QH"GNGEVTKECN"GPIKPGGTKPI"
FKV"WPKXGTUKV["
VGG"246"JKIJ"XQNVCIG"GPIKPGGTKPI
cable. The delay is obtained by an externally connected coaxial long cable. Another method to obtain
delay is using an electronic tripping device to trigger the impulse generator and CRO time base. A first
pulse from the device starts the CRO time base and after a predetermined a second pulse triggers the
impulse generator.
c
ZO
3
b
a
2
1
Fig. 7(a) Surge Test Oscilloscope Block Dig.
1 – Trigger amplifier
2 – Sweep generator
3 – External delay line
a – Vertical amplifier input
b – Input to delay line
c – output of delay line to CRO Y plates
1
3
2
V(t)
4
5
6
Fig. 7(b) Surge Test Oscilloscope Block Dig.
1 – Plug in amplifier
4 – Trigger amplifier
2 – Y amplifier
5 – Sweep generator
3 – Internal delay line
6 – X amplifier
The electromagnetic interference is to be avoided and therefore it is essential that leads, layout and
connections from the signal sources to CRO are to be arranged in such a manner which avoids induced
voltages and stray pick-ups. The connecting cables behave as either capacitive or inductive depending on
the load at the end of the cable. In case of fast rising signals to avoid unnecessary reflections at the cable
ends, it has to be terminated properly by connecting a resistance equal to surge impedance of cable. The
oscilloscopes have finite input impedance usually about 1 to 10 MΩ resistance in parallel with a 10 to 50
pF capacitance thus acting as a load at the end of a surge cable and it attenuates the signal at CRO end.
To eliminate noise voltages due to ground loop currents, electromagnetic coupling, cable shields,
multiple shielding arrangement as shown in fig. 8 may have to be used.
10
http://eedofdit.weebly.com
Prepared by: Mohamed Samir
"""""""FGRCTVOGPV"QH"GNGEVTKECN"GPIKPGGTKPI"
FKV"WPKXGTUKV["
VGG"246"JKIJ"XQNVCIG"GPIKPGGTKPI
HV
2
3
1
4
CRO
Fig.8 Multiple shielding arrangement for noise
voltage eliminaion
1 – Potential divider
2 – Triple shielded cable
3 – Inner shielded enclosure
4 – Terminating impedance
11
http://eedofdit.weebly.com
Prepared by: Mohamed Samir