Mesurement Of High Voltages
& High Currents
Unit 4
High Voltage Measurement Techniques
2
Sphere Gaps

Applicatios:


Voltage Measurement (Peak) - Peak values of voltages may be measured from 2 kV up to
about 2500 kV by means of spheres.
Arrangements:
3
1.
Vertically with lower sphere grounded (For Higher Voltages)
2.
Horizontally with both spheres connected to the source voltage or one sphere grounded
(For Lower Voltages).
Sphere Gaps

The arrangement is selected based on the relation between the peak voltage,
determined by sparkover between the spheres, and the reading of a voltmeter on the
primary or input side of the high-voltage source. This relation should be within 3%
(IEC, 1973).

Standard values of sphere diameter are 6.25, 12.5, 25, 50, 75, 100, 150, and 200 cm.

The Clearance around the sphere gaps:
Fig C :Breakdown voltage characteristic of
sphere gaps
4
Sphere Gaps

The effect of humidity is to increase the breakdown voltage of sphere gaps by up to
3%.

Temperature and pressure, however, havea significant influenceo n breakdown
voltage.

Breakdown Voltage under normal atmospheric conditions is, Vs=kVn where k is a
factor related to the relative air density (RAD) δ.

The relation between the RAD(δ) and the correction factor k:

Under impulse voltages, the voltage at which there is a 50% breakdown probability
is recognized as the breakdown level.
5
Sphere Gaps

Factors Influencing the Sparkover Voltage of Sphere Gaps

i.
Nearby earthed objects,
ii.
Atmospheric conditions and humidity,
iii.
Irradiation, and
iv.
Polarity and rise time of voltage waveforms.
The limits of accuracy are dependant on the ratio of the spacing d to the sphere
diameter D, as follows:


d < 0.5 D
Accuracy = ± 3 %

0.75 D > d > 0.5 D
Accuracy = ± 5 %
For accurate measurement purposes, gap distances in excess of 0.75D are not used
6
High Ohmic Series Resistance with Microammeter

Resistance (R) :






Constructed with large wire wound
Value: Few hundreds of Mega ohms –Selected to give (1-10μA) for FSD.
Voltage drop in each element is chosen to avoid surface flashovers and discharges
(5kV/cm in air, 20kV/cm in oil is allowed)
Provided with corona free terminals.
Material: Carbon alloy with temperature coefficient of 10-4/oC .
Resistance chain located in air tight oil filled PVC tube for 100kV operation with
good temp stability.
Mircoammeter – MC type
Voltage of source, V=IR
Impedance of the meter is few ohms. i.e, very less compared to R so the
drop across the meter is negligible.
Protection: Paper gap, Neon Glow tube, a zener diode with series resistance
– Gives protection when R fails.




7
High Ohmic Series Resistance with Microammeter
Maximum voltage: 500kV with  0.2% accuracy.
Limitations:




8
Power dissipation & source loading
Temp effects &
Measurement Of High Dc Voltage

Series resistance micrometer
Resistance potential divider
Generating voltmeter
Sphere and other gaps



9
SERIES RESISTANCE MICROMETER







A very high resistance in series with a micrometer.
Current through R is measured using micrometer.
Voltage of source, V = IR
The resistance is constructed from a large no. of wire
wound resistors in series.
Can be operated upto 500kV (D.C)
Accuracy = ±0.2%
Selection of R value:




10
Current allowed: 1 to 10A
Corona free termination
Temp. coefficient<10-4/0C : Carbon Alloy
Placed in airtight, oil filled PVC tube to maintain temp.
stability
RESISTANCE POTENTIAL DIVIDER



It uses electrostatic voltmeter.
Can be placed near the test object which might not
always be confined to one location
Let, V2-Voltage across R2
V2  V1X
R2
(R 1  R 2 )
High voltagemagnitude, V1  V2X

(R 1  R 2 )
R2
Sudden voltage changes during transients due to:



11
Switching operation
Flashover of test objects
Damage due to stray capacitance across the elements & ground
capacitance
GENERATING VOLTMETER

Generating principle is used where direct loading or
direct connection is to be avoided.

Generating voltmeter: A variable
electrostatic voltage generator.

It generates current proportional to voltage under
measurement

This arrangement provides loss free measurement of
DC and AC voltages

It is driven by synch. motor, so doesn’t observe
power from the voltage measuring source

The high voltage electrode and the grounded
electrode in fact constitute a capacitance system.

The capacitance is a function of time as the area A
varies with time and, therefore, the charge q(t) is
given as,
12
capacitor
GENERATING VOLTMETER
and,
For d.c. Voltages,
Hence
If the capacitance varies linearly with time and reaches its peak value Cm is time Tc /2 and
again reduces to zero linearly in time Tc /2, the capacitance is given as
If the capacitance C varies sinusoidally between the limits C0 and (C0 + Cm) then
C = C0 + Cm sin ωt
and the current ‘i' is then given as, i(t) = im cos ω t , where im = VCmω
Here ω is the angular frequency of variation of the capacitance.
 Generally the current is rectified and measured by a moving coil meter
 Generating voltmeters can be used for a.c. voltage measurement also provided the angular
frequency ω is the same or equal to half that of the voltage being measured.
 Above fig. shows the variations of C as a function of time together with a.c. voltage, the
frequency of which is twice the frequency of C (t).
13
Generating Voltmeter

Instantaneous value of current i(t) = Cm fvV(t)


where fv = 1/Tv the frequency of voltage.
Since fv = 2fc and fc = 60/n we obtain,
I(t) = n/30 CmV(t)

Fig. shows a schematic diagram of a generating voltmeter
which employs rotating vanes for variation of capacitance

High voltage electrode 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, and 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 (sinusodial
or linear) is achieved.
The a.c. 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.
14
Generating Voltmeter

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

Advantages:
i.
scale is linear and can be extrapolated
ii.
source loading is practically zero
iii.
15
no direct connection to the high voltage electrode.
Measurement Of High Ac Voltage





Electrostatic voltmeter
Series impedance voltmeter
Potential dividers : Resistance or Capacitance type
Potential transformers : Electromagnetic or CVT
Sphere gaps
16
Electrostatic Voltmeter

One of the direct methods of measuring high
voltages is by means of electro-static voltmeters.

For voltages above 10 kV, generally the attracted
disc type of electrostatic voltmeter is used.

When two parallel conducting plates (cross
section area ‘A’ and spacing ‘s’) are charged q and
have a potential difference V, then the energy
stored in the is given by
1
1
W  CV 2  dW  V 2 dC  F ds
2
2
1
dC
 Force,F  V 2
Newton
2
ds
For uniform field capacitance, C 
Aε
dC
 Aε 

  2 
s
ds
s 
1 V 2 
 F   Aε 2  Newton
2 s 
 It is thus seen that the force of attraction is proportional to the square of the potential difference
applied, so that the meter reads the square value (or can be marked to read the rms value).
17
Electrostatic Voltmeter

Electrostatic voltmeters of the attracted disc type may be connected across the high
voltage circuit directly to measure up to about 200 kV, without the use of any
potential divider or other reduction method. [The force in these electrostatic
instruments can be used to measure both a.c. and d.c. voltages].

The right hand electrode forms the high voltage plate.

The centre portion of the left hand disc is cut away and encloses a small disc which
is movable and is geared to the pointer of the instrument.

The range of the instrument can be altered by setting the right hand disc at premarked distances.

The force of attraction F(t) created by the applied voltage causes the movable part-to
which a mirror is attached-to assume a position at which a balance of forces takes
place.

An incident light beam will therefore be reflected toward a scale calibrated to read
the applied voltage magnitude.
18
Electrostatic Voltmeter

Advantages:
i.
Low loading effect
ii.
Active power losses are negligibly small
iii.
Voltage source loading is limited to the reactive power needed to
charge the system capacitance.(i.e., For 1V VoltmeterCapacitance is few Pico farad)
iv.
Voltages upto 600kV can be measured.

Disadvantage:
i.
For constant distance ‘s’, F α V2, the sensitivity is small. This can
be overcome by varying the gap distance d in appropriate steps.
Absolute Electrostatic Voltmeter
19
Series Impedance Voltmeter

For power frequency a.c. measurements the series impedance may be a pure
resistance or a reactance.

But use of resistances yields the followings,

Power losses

Temperature problem

Residual inductance of the
resistance.
resistance gives rise to an impedance different from its ohmic

High resistance units for high voltages have stray capacitances and hence a unit
resistance will have an equivalent circuit as shown in Fig.

At any frequency ω of the a.c. voltage, R+jXL is connected in parallel with –jXC.
R  jωL  1
R  jωL 
jωC
Z

2
1
1

ω
LC  jωCR
R  jωL  
jωC
Since, ω 2 LC  jωCR,
R  jωL 
Z
1  jωCR
20
Series Impedance Voltmeter
Z
R 
jωL  1  jωCR

1  jωCR 1  jωCR
R  jωL  jωCR 2  ω 2 LCR
Z
1  ω 2C 2 R 2

 ωL

Z  R  jωL  jωCR 2  R 1  j 
 ωCR 
 R


 ωL

where, Phase angle, φ  t an1 
 ωCR 
 R


Extended Series Resistance neglecting inductance is shown in figures.

Resistor unit then has to be taken as a transmission line equivalent, for calculating
the effective resistance.

Ground or stray capacitance of each element influences the current flowing in the
unit, and the indication of the meter results in an error.

Stray ground capacitance effects 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.
21
Series Impedance Voltmeter

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.
22
Series Capacitance Voltmeter

To avoid the drawbacks pointed out Series impedance voltmeter, a series
capacitor is used instead of a resistor for a.c. high voltage measurements.

Current through the instrument, Ic=jωCV

The rms value of the voltage V with harmonics is given by,

Vrms  V12  V22    Vn2
where V1,V2 ,... ,Vn represent the rms value of the fundamental, second...
and nth harmonics.
The currents due to these harmonics are
I1=ωCV1 , I2=2ωCV2 , ……In=nωCVn
I rms  ωC V12  2V2     nVn 
2
2

With a 10% fifth harmonic only, the current is 11.2% higher, and hence
the error is 11.2% in the voltage measurement

Not recommended when a.c. voltages are not pure sinusoidal waves but
contain considerable harmonics.

Used for measuring rms values up to 1000 kV.
23
Series Capacitance Voltmeter

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)μA moving coil meter and the over all error was
about 2%.
24
Peak Reading Voltmeters

For Sine wave,





Peak Value=RMS Value X 2
Maximum dielectric strength may be obtained by non-sine wave. In that case,
Peak Value ≠ RMS Value X 2
Therefore, peak measurement is important.
Types:



25
Series Capacitance Peak Voltmeter (Chubb-Frotscue Method)
Digital Peak Voltmeter
Peak Voltmeter with potential divider
Peak Reading Voltmeters
Chubb Frotscue Method:

Chubb and Fortescue suggested a simple and accurate
method of measuring peak value of a.c. voltages.

The basic circuit consists of a standard capacitor, two diodes
and a current integrating ammeter (MC ammeter) as shown
in Fig. 4.11 (a).

The displacement current ic(t), Fig. 4.12 is given by the rate
of change of the charge and hence the voltage V(t) to be
measured flows through the high voltage capacitor C and is
subdivided into positive and negative components by the
back to back connected diodes
 The voltage drop across these diodes can be neglected (1 V for Si diodes) as compared with
the voltage to be measured
 The measuring instrument (M.C. ammeter) is included in one of the branches. The
ammeter reads the mean value of the current,
 An increased current would be obtained if the current reaches zero more than once during
one half cycle
26
Peak Reading Voltmeters
(Chubb Frotscue Method Continued…)

This means the wave shapes of the voltage would contain more than one maxima per half cycle.

The standard a.c. voltages for testing should not contain any harmonics and, therefore, there could
be very short and rapid voltages caused by the heavy predischarges, within the test circuit which
could introduce errors in measurements.

To eliminate this problem filtering of a.c. voltage is carried out by introducing a damping resistor
in between the capacitor and the diode circuit, Fig. 4.11 (b).

The measurement of symmetrical a.c. voltages using Chubb and Fortescue method is quite
accurate and it can be used for calibration of other peak voltage measuring devices.
27
Peak Reading Voltmeters
Digital Peak Voltmeter:
 In contrast to the method discussed just now, the rectified current is not
measured directly, instead a proportional analog voltage signal is derived
which is then converted into a proportional medium frequency for using a
voltage to frequency convertor (Block A in Fig. 4.13).
 The frequency ratio fm/f is measured with a gate circuit controlled by the a.c.
power frequency (supply frequency f) and a counter that opens for an
adjustable number of period Δt = p/f. The number of cycles n counted during
this interval is

where ‘p’ is a constant of the instrument.
28
Peak Reading Voltmeters
Digital Peak Voltmeter continued….
Voltage to frequency 
fm
fm
A



convertion factor 
Ri m R 2 Vm f C

im  Rectified Current throughR 
fm
1

f 2R Vm C
fm
 2Vm CR  A
f
Therefore, n  2Vm CR  AP
i.e.,
im 
Vm
 Vm 2 π f C
XC
i.e., im proportional to 2 Vm f C
By proper selection of R and P, Voltage can be measured immediately.
 Accuracy is less than 0.35%

29
Peak Reading Voltmeters
Peak voltmeter with Potential divider:
 Diode D is used for rectification
 Voltage across C2 is used to charge C3
 Resistance Rd permits the variation of Vm when
V2 is reduced
 Electrostatic Voltmeter as indicating instrument




Voltage across Cs  Peak value to be measured
Discharge time constant=CsRd1 to 10 sec
This arrangement gives discharge error.
Discharge error depends on frequency of the supply
30
Measurement of High Currents
Type of Current
Method used
D.C Current
1. Resistant shunt
2. Hall Generator
High Power frequency A.C
Current Transformer with electro-optical
technique
High frequency and impulse currents
1.
2.
3.
4.
5.
Impulse Voltages and Currents
Cathode Ray Oscilloscope
31
Resistive shunts
Magnetic potentiometers or probes
Magnetic links
Hall generators
Faraday Generators
Hall Generators



Hall effect is used to measure very
high direct current.
Whenever electric current flows
through a metal plate placed in a
magnetic field perpendicular to it,
Lorenz force will deflect the electrons
in the metal structure in a direction
perpendicular to the direction of both
the magnetic field and the flow of
current.
The change in displacement generates
an e.m.f called “Hall Voltage”
32
Hall Generators

Hall Voltage,VH α
BI
d
BI
d
where, B-Magnetic Flux density
I-Current
d-Thickness of the metal plate
R-Hall Coefficient (depends on
Material of the plate &
temperature)
R is small for metals and High for
semiconductors
VH  R 
 When large d.c. currents are to be measured the current
carrying conductor is passed through an iron cored magnetic
circuit
33
Hall Generators



The magnetic field intensity produced by the conductor in the
air gap at a depth ‘d’ is given by,
1
H
2 d
The Hall element is placed in the air gap and a small constant
d.c. current is passed through the element.
The voltage developed across the Hall element is measured and
by using the expression for Hall voltage the flux density B is
calculated and hence the value of current I is obtained.
34
Faraday Generator or Magneto Optic Method


These methods of current measurement use the rotation of the plane
of polarisation in materials by the magnetic field which is
proportional to the current (Faraday effect).
When a linearly polarised light beam passes through a transparent
crystal in the presence of a magnetic field, the plane of polarisation
of the light beam undergoes rotation. The angle of rotation is given
by,
θ = α Bl
where,
α = A constant of the cyrstal which is a function of the wave length of the
light.
B = Magnetic flux density due to the current to be measured in this case.
l = Length of the crystal.
35
Faraday Generator or Magneto Optic Method





Fig. shows a schematic diagram of Magneto-optic method.
Crystal C is placed parallel to the magnetic field produced by the
current to be measured.
A beam of light from a stabilised light source is made incident on the
crystal C after it is passed through the polariser P1.
The light beam undergoes rotation of its plane of polarisation.
After the beam passes through the analyser P2, the beamis focussed on a
photomultiplier, the output of which is fed to a CRO.
36
Faraday Generator or Magneto Optic Method


The filter F allows only the monochromatic light to pass through it.
Photoluminescent diodes too, the momentary light emission of which is
proportional to the current flowing through them, can be used for
current measurement.
Advantages:
1.
2.
3.
37
It provides isolation of the measuring set up from the main current circuit.
It is insensitive to overloading.
As the signal transmission is through an optical system no insulation problem is
faced. However, this device does not operate for D.C current.
Magnetic Potentiometer(Rogowski Coil)

If the current to be measured is flowing through a conductor which is
surrounded by a coil as shown in Fig.

and M is the mutual inductance between the coil and the conductor, the
voltage across the coil terminals will be:
di
dt
Usually the coil is wound on a non-magnetic former in the form of a
toroid and has a large number of turns, to have sufficient voltage
induced which could be recorded.
v(t)  M

38
Magnetic Potentiometer(Rogowski Coil)



The coil is wound cross-cross to reduce the leakage inductance.
If N is the number of turns of the coil, A the coil area and lm its mean
length, the mutual inductance is given by
μ NA
M 0
lm
Usually an integrating circuit RC is employed as shown in Fig to obtain
the output voltage proportional to the current to be measured. The
output voltage is given by
t
1
1
di
M
M
v0 (t) 
v(t)dt

M

dt

di

i(t)



RC 0
RC
dt
RC
RC

The frequency response of the Rogowski coil is flat upto 100 MHz but
beyond that it is affected by the stray electric and magnetic fields and
also by the skin effect.
39
Resistive Shunt
40
SERIES IMPEDANCE VOLTMETER
Extended series
impedance with
inductance
neglected
41
SERIES CAPACITOR PEAK VOLTMETER
C – capacitor
D1,D2 – Diodes
OP – Protective devices
I – indicating meter
V(t) – voltage waveform
Ic(t) – capacitor current
waveform
T – period
42
PEAK READING AC VOLTMETER
43
PEAK READING AC VOLTMETER
44
SPHERE GAPS MEASUREMENT
Ub = kd Ub0
45
SPHERE GAPS
46
Potential divider for impulse voltage measurement
47
MEASUREMENT OF HIGH DIRECT CURRENTS
HALL GENERATORS FOR D.C CURRENT MEASUREMENTS
 Hall effect principle is used.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 magnetic field.
 The charge displacement generates an emf in the normal direction (Hall
voltage).
 VH=RBi/d
 H=I/δ
48
MEASUREMENT OF HIGH POWER FREQUENCY
ALTERNATING CURRENTS



Current transformer is used.it uses electro optical technique.
A voltage signal proportional to the measuring current is generated and it is
transmitted to the ground side through 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.
49
SPHERE GAP

The sphere gap method of measuring high voltage is the most reliable and is used as the
standard for calibration purposes.

breakdown strength of a gas depends on the ionisation of the gas molecules, and on the density
of the gas.

As such, the breakdown voltage varies with the gap spacing; and for a uniform field gap, a
high consistency could be obtained, so that the sphere gap is very useful as a measuring
device.

In the measuring device, two metal spheres are used, separated by a gas-gap. The potential
difference between the spheres is raised until a spark passes between them.

The breakdown strength of a gas depends on the size of the spheres, their distance apart and a
number of other factors.

A spark gap may be used for the determination of the peak value of a voltage wave, and for the
checking and calibrating of voltmeters and other voltage measuring devices.

The density of the gas (generally air) affects the spark-over voltage for a given gap setting.

Thus the correction for any air density change must be made. The air density correction factor
δ must be used.
50
SPHERE GAP

The spark over voltage for a given gap setting under the standard conditions (760
torr pressure and at 20oC) must be multiplied by the correction factor to obtain the
actual spark-over voltage.

The breakdown voltage of the sphere gap (Figure:a) is almost independent of
humidity of the atmosphere, but the presence of dew on the surface lowers the
breakdown voltage and hence invalidates the calibrations.
Figure. a:- Measuring spheres

The breakdown voltage characteristic (figure 6.3) has been determined for similar
pairs of spheres (diameters 62.5 mm, 125 mm, 250 mm, 500 mm, 1 m and 2 m)
51
Fig.b:Breakdown voltage characteristic of sphere gaps
SPHERE GAP

In sphere gaps used in measurement, to obtain high accuracy, the
minimum clearance to be maintained between the spheres and the
neighbouring bodies and the diameter of shafts are also specified, since
these also affect the accuracy (Figure:d).

There is also a tolerance specified for the radius of curvature of the
spheres.

Peak values of voltages may be measured from 2 kV up to about 2500
kV by means of spheres.

One sphere may be earthed with the other being the high voltage
electrode, or both may be supplied with equal positive and negative
voltages with respect to earth (symmetrical gap).

Needle gaps may also be used in the measurement of voltages up to
about 50 kV, but errors are caused by the variation of the sharpness of
the needle gaps, and by the corona forming at the points before the gap
actually sparks over.

Also the effect of the variation of the humidity of the atmosphere on
such gaps is much greater.
52
Figure. d: Sphere gap

When the gap distance is increased, the uniform field between the spheres becomes
distorted, and accuracy falls.

The limits of accuracy are dependant on the ratio of the spacing d to the sphere
diameter D, as follows:

d < 0.5 D
Accuracy = ± 3 %

0.75 D > d > 0.5 D
Accuracy = ± 5 %

For accurate measurement purposes, gap distances in excess of 0.75D are not used

Breakdown voltage characteristic is also dependant on the polarity of the high
voltage sphere in the case of asymmetrical gaps, in a symmetrical gap, then the
polarity has no effect.
53
Figure . C:- Breakdown voltage characteristics