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High Voltage EngineeringHigh Voltage EngineeringHigh Voltage EngineeringHigh Voltage EngineeringUnitUnitUnitUnit----IVIVIVIV

Deependra SinghDeependra SinghDeependra SinghDeependra Singh95

UNIT-IV

MEASUREMENT OF HIGH VOLTAGES

37. INTRODUCTION

In industrial testing and research laboratories, it is essential to measure the voltages

and currents accurately, ensuring perfect safety to the personnel and equipment. Hence a

person handling the equipment as well as the metering devices must be protected against

over-voltages and also against any induced voltages due to stray coupling. Therefore, the

location and layout of the devices are important. Secondly, linear extrapolation of the devices

beyond their ranges is not valid for high voltage meters and measuring instruments, and they

have to be calibrated for the full range.

Electromagnetic interference is a serious problem in impulse voltage and current

measurements, and it has to be avoided or minimized. Therefore, even though the principles

of measurements may be same, the devices and instruments for measurement of high voltages

and currents differ vastly from the low voltage and low current devices. Different devices

used for high voltage measurements may be classified as in Tables 7.1 and 7.2.

Table 7.1: High Voltage Measurement Techniques

Type of Voltage Method or Technique(i) Series Resistance Micro-ammeter

(ii)Resistance Potential Divider

(iii)Generating VoltmetersDC Voltages

(iv)Sphere & spark gaps

(i) Series Impedance ammeters

(ii)Potential Dividers (R or C type)

(iii)Potential Transformers

(Electromagnetic or CVT)

(iv)Electrostatic Voltmeters

AC Power Frequency

(v)Sphere Gaps

(i) Sphere gaps

(ii)potential dividers with a cathode ray

oscillo-graph (capacitive, or resistive

dividers)

A.C. High Frequency Voltages, Impulse

Voltages, and other rapidly changing

voltages(iii)Peak voltmeters

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Table 7.2: High Current Measurement Techniques

Type of Current Method or Technique

(i) Resistive shunts with milli-ammeter

(ii)

Hall effect generators(a)

(i) Resistive shunts(b)Alternating currents (Power

Frequency) (ii) Electromagnetic current transformers

(i) Resistive shunts

(ii) Magnetic potentiometers or Rogoswki coils

(c)High frequency AC, impulse

and rapidly changing currents

(iv)

Hall effect generators

REFERENCE:REFERENCE:REFERENCE:REFERENCE:

HVE KAMARAJU NAIDU; 7.1; P-157-158

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38. ELECTROSTATIC VOLTMETER-PRINCIPLE, CONSTRUCTION AND

LIMITATION

Coulombs law defines the electrical field as a field of forces, and since electrical

fields may be produced by voltages, the measurement of voltages can be related to a force

measurement. In 1884 Lord Kelvin suggested a design for an electrostatic voltmeter based

upon this measuring principle. If the field is produced by the voltage V between a pair of

parallel plane disc electrodes, the force F on an area A of the electrode, for which the field

gradient E is the same across the area and perpendicular to the surface, can be calculated

from the derivative of the stored electrical energy Weltaken in the field direction (x). Since

each volume elementAdxcontains the same stored energy ( ) 2/2AdxEdWel = , the attracting

force dxdWF el/= becomes

2

2

2

22V

S

AAEF

== , (3.7)

Where = permittivity of the insulating medium and S = gap length between the parallel

plane electrodes.

The attracting force is always positive independent of the polarity of the voltage. If

the voltage is not constant, the force is also time dependent. Then the mean value of the force

is used to measure the voltage, thus:

( )2...20

2

0

2 2)(

1

2)(

1smr

TT

VS

TS

T

== , (3.8)

where T is a proper integration time. Thus, electrostatic voltmeters are r.m.s.-indicating

instruments.

The design of most of the realized instruments is arranged such that one of the

electrodes or a part of it is allowed to move. By this movement, the electrical field will

slightly change which in general can be neglected. Besides differences in the construction of

the electrode arrangements, the various voltmeters differ in the use of different methods of

restoring forces required to balance the electrostatic attraction; these can be a suspension of

the moving electrode on one arm of a balance or its suspension on a spring or the use of a

pendulous or torsional suspension. The small movement is generally transmitted and

amplified by a spotlight and mirror system, but many other systems have also been used. If

the movement of the electrode is prevented or minimized and the field distribution can

exactly be calculated, the electrostatic measuring device can be used for absolute voltage

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measurements, since the calibration can be made in terms of the fundamental quantities of

length and forces.

have to be built up. The atmospheric air, high-pressure gas or even high vacuum between the

electrodes provide very high resistivity, and thus the active power losses are mainly due to

the resistance of insulating materials used elsewhere. The measurement of voltages lower

than about 50 V is, however, not possible, as the forces become too small.

The measuring principle displays no upper frequency limit. The load inductance and

the electrode system capacitance, however, form a series resonant circuit, thus limiting the

frequency range. For small voltmeters the upper frequency is generally in the order of some

MHz.

In spite of the inherent advantages of this kind of instrument, their application for

H.V. testing purposes is very limited nowadays. For D.C. voltage measurements, the

electrostatic voltmeters compete with resistor voltage dividers or measuring resistors, as the

very high input impedance is in general not necessary. For A.C. voltage measurements, the

r.m.s. value is either of minor importance for dielectric testing or capacitor voltage dividers

can be used together with low-voltage electronic r.m.s. instruments, which provide acceptable

low uncertainties. Thus the actual use of these instruments is restricted and the number of

manufacturers is therefore extremely limited.

REFERENCE:REFERENCE:REFERENCE:REFERENCE:

High Voltage Engineering Fundamentals Kuffel, Zaengl, Kuffel; 3.2; P-94-96

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39. GENERATING VOLTMETER

PRINCIPLE & CONSTRUCTION

Similar to electrostatic voltmeters the generating voltmeter, also known as the rotary

voltmeter or field mill, provides a lossless measurement of D.C. and, depending upon the

construction, A.C. voltages by simple but mainly mechanical means.

The principle of operation is explained by Fig. 3.11. An adequately shaped, corona-

free H.V. electrode excites the electrostatic field within a highly insulating medium (gas,

vacuum) and ground potential. The earthed electrodes are subdivided into a sensing or pick-

up electrode A, a guard electrode G and a movable

electrode M, all of which are at same potential.

Every field line ending at these electrodes binds free

charges, whose density is locally dependent upon

the field gradient E acting at every elementary

surface area. For measurement purposes, only the

elementary surface areas dA = aof the electrode A

are of interest. The local charge density is then, (a)

= E(a),with the permittivity of the dielectric.

If the electrode M is fixed and the voltage V(or field-distributionE(a)) is changed, a current i(t)

would flow between electrode A and earth. This current results then from the time-dependent

charge density, (t,a), which is sketched as a one-dimensional distribution only. The amount

of charge can be integrated by

== AA

daatEdaattq ),(),()( ,

Where A is the area of the sensing electrode exposed to the field. This time-varying charge is

used by all kinds of field sensors, which use pick-up electrodes (rods, plates, etc.) only. If the

voltage V is constant, again a current i(t) will flow but only if M is moved, thus steadily

altering the surface field strength from full to zero values within the covered areas. Thus the

current is