05-May-23

Introduction
 Different types of high voltages generated in a high voltage laboratory are applied across insulators to
determine essentially two things: 1) voltage at which an insulator breaks down and 2) the maximum
. .
voltage which the insulation can withstand. Measurements of these voltages form the basis of testing of
high voltage equipment and research in the area of high voltage techniques.
 There are difficulties in measurement of such high voltages. The measurements should be able to control
high electric fields so that flashover does not take place. Sometimes heat dissipation within the measuring
circuits have to be controlled.
 Generally mean value of DC voltages and peak values of AC and impulse voltages are of practical
interest. Voltage wave shapes of AC and impulse voltages and ripple factor of DC voltages may have to
be determined in certain cases.
 The present chapter is devoted to measurement of voltages applied for testing of high voltage equipment.
The measuring method will be classified according to the types of voltages to be measured.

Peak voltmeter (Chubb-Fortescue method)

. .
• Chubb and Fortescue suggested the following circuit (fig. a) for the measurement
of peak value of high ac voltage. The circuit comprises of standard capacitor C,
two diodes, D1 and D2 connected in anti-parallel and a moving coil millimeter
(mA).
• The drop across diodes D1 and D2 may be ignored while measuring the high
voltage. As the drop across the silicon diode is only 1V.
• Simple and accurate method for the peak measurement of a.c. voltages
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• PROBLEM
• Determine the breakdown voltage for air gaps of 2 mm and 15 mm lengths under uniform field and
. . . standard atmospheric conditions. Also, determine the voltage if the atmospheric pressure is 750 mm Hg
and temperature 35°C.

Voltage dividers
. . Voltages dividers for a.c., d.c. or impulse voltages may consist of resistors or capacitors
or a convenient
combination
The height of a voltage
divider depends upon the flash over voltage and this follows from the rated maximum voltage applied.
Now, the potential distribution may not be uniform and hence the height also depends upon the design
of the high voltage electrode, the top electrode. For voltages in the megavolt range, the height of the
divider becomes large. As a thumb rule following clearances between top electrode and ground may be
assumed.
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• 2.5 to 3 metres/MV for d.c. voltages.

Resistive voltage dividers
.• More than 5 m/MV rms for a.c. voltages.
• 2 to 2.5 m/MV for lightning impulse voltages.
.
• More than 4 m/MV for switching impulse voltage.
• The potential divider is most simply represented by two impedances Z1and Z2 connected in series
and the sample voltage required for measurement is taken from across Z2 Fig.

• This means that the time constant of both the arms should be the same. This compensation is used
for the construction of high voltage dividers. Resistance Potential Divider for very high impulse voltages:
. .

Fig. 2 resistance potential dividers

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A resistance divider of 1400 kV (impulse) has a high voltage arm of 16 kilo-

. ohms and a low voltage arm consisting 16 members of 250 ohms, 2 watt
resistors in parallel. The divider is connected to a CRO through a cable of
. .
surge impedance 75 ohms and is terminated at the other end through a 75
ohm resistor.
Calculate the exact divider ratio.

Electrostatic Voltmeters:
The electric field is produced by voltage and, therefore, if the field force could be measured, the voltage
can also be measured. Whenever a voltage is applied to a parallel plate electrode arrangement, an electric
field is set up between the plates.
It is possible to have uniform electric field between the plates with suitable arrangement of the plates.

CAPACITIVE VOLTAGE DIVIDERS Series resistance volt meter.

• Ohm’s law provides a method to reduce high voltages to measurable quantities, i.e. adequate .
currents or low voltages. The simplest method employs a micrometer in series with a resistor R of
sufficiently high value to keep the loading of an h. v. source as small as possible (Fig.(a)).
• Thus for a pure resistance R, the measured quantities are related to the unknown high voltage by;
• V (t) = R i(t)
• High d.c. voltages are usually measured by connecting a very high resistance (few
hundreds of mega ohms) in series with a micro ammeter as shown in Fig. Only the current
I flowing through the large calibrated resistance R is measured by the moving coil micro
ammeter. The voltage of the source is given by 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.
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• Electrostatic voltmeters measure the force based on the above equations and are arranged such that one of the
plates is rigidly fixed whereas the other is allowed to move.
Problem
.• With this the electric field gets disturbed. For this reason, the movable electrode is allowed to move by not
more than a fraction of a millimeter to a few millimeters even for high voltages so that the change in electric
field is negligibly small. As the force is proportional to square of , the meter can be used both for a.c. and
• An absolute electrostatic voltmeter has a movable circular plate 8 cm in diameter. If the distance
d. c. voltage measurement. The measurement of voltages lower than about 50 volt is, however, not possible, between the plates during a measurement is 4 mm, determine the potential difference when the
as the forces become too small. force of attraction is 0.2 gm wt.
• Schematic diagram of static voltmeter is shown below •

• It consists of two parallel plate electrodes 1 and 2, separated by small distance. The electrode 1 is moveable
while electrode 2 is fixed. High tension/voltage terminal is connected to plate 2 which is to be properly
.
insulated. Fix guard ring 3 surrounds the moving electrode to ensure that the field between plates 1 & 2 is
uniform. The dielectric medium between the plats may be atmospheric air, nitrogen or SF6. The taut -band
suspension 7 provides the restoring force.

High voltage terminal