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

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    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)

    Direct Currents(iii) Magnetic Links

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

    Frequency) (ii) Electromagnetic current transformers

    (i) Resistive shunts

    (ii) Magnetic potentiometers or Rogoswki coils

    (iii) Magnetic Links

    (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

    Adtt

    TS

    AdttF

    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.

    The paramount advantage is the extremely low loading effect, as only electrical fields

    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

    ===)( )(

    )()()(tA tA

    daaEdt

    ddaa

    dt

    d

    dt

    dqti (3.23)

    The integral boundary denotes the time-varying exposed areaA(t)and (a) as well as

    E(a) are also time dependent if the voltage is not constant. The field lines between H.V. and

    sensing electrode comprise a capacitive system. Thus the charge q can be computed by an

    Figure 3.11 Principle of generatingvoltmeters and field sensors

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    electrostatic field computation or by calibration of the system. The integration across the

    time-varying area A(t), however, provides a time-varying capacitance C(t), and also if the

    voltage changes with time, q(t) = C(t)V(t)and

    [ ])()()( tVtCdtd

    ti = (3.24)

    Various kinds of generating voltmeters use these basic equations and the manifold designs

    differ in the constructional means for providing C(t)and interpreting the current i(t).

    Generating voltmeters are very linear instruments and applicable over a wide range of

    voltages. The sensitivity may be changed by the area of the sensing electrodes (or iris) as well

    as by the current instrument or amplification. Their early application for the output voltage

    measurement of a Van-de-Graffs thus may well be understood. Excessive space charge

    accumulation within the gap between H.V. electrode and generating voltmeter, however,

    must be avoided. The presence of space charges will be observed if the voltage is switched

    off.

    Vibrating electrometers are also generating voltmeters, but will only be mentioned here as

    they are not widely used. The principle can well be understood with reference to Fig. 3.11

    neglecting the movable disc. If the sensing electrode would oscillate in the direction of the

    H.V. electrode, again a current i(t) = dq/dt is excited with constant voltage V due to a

    variation of the capacitance C =C(t). The sensing electrode may also pick up charges when

    placed just behind a small aperture drilled in a metal plate. Commercial types of such an

    instrument are able to measure D.C. voltages down to 10 V, or currents down to 10-17

    A, or

    charges down to 10-15

    pC, and its terminal resistance is as high as 1016.

    REFERENCE:REFERENCE:REFERENCE:REFERENCE:

    HVE Fundamentals Kuffel, Zaengl, Kuffel; 3.4; P-108-110

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    40. SERIES RESISTANCE MICRO AMMETER FOR HV DC

    MEASUREMENTS

    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. 7. 1. Only the

    current / flowing through the large calibrated resistance R is measured by the moving coil

    micro-ammeter. 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 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. The

    ohmic value of the series resistance R is chosen such that a current of

    one to ten microamperes is allowed for full-scale deflection. The

    resistance is constructed from a large number of wire wound resistors

    in series. The voltage drop in each resistor element is chosen to avoid

    surface flashovers and discharges.

    A Value of less than 5kV/cm in air or less than 20kV/cm in

    good oil is permissible. The resistor chain is provided with coronafree terminations. The material for resistive elements is usually a

    carbon-alloy with temperature coefficient less than 10-4/

    0C. Carbon and other metallic film

    resistors are also used. A resistance chain built with 1% carbon resistors located in airtight

    transformer oil filled P.V.C. tube, for 100 kV operations had very good temperature stability.

    The limitations in the series resistance design are:

    (i) Power dissipation and source loading,A

    (ii) Temperature effects and long time stability,

    (iii) Voltage dependence of resistive elements, and

    (iv) Sensitivity to mechanical stresses.

    Series resistance meters are built for 500 kV D.C. with an accuracy better than 0.2%.

    REFERENCE:REFERENCE:REFERENCE:REFERENCE:

    HVE KAMARAJU NAIDU; 7.1.1; P-158-159

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    41. STANDARD SPHERE GAP MEASUREMENTS OF HV AC, HV DC,

    AND IMPULSE VOLTAGESA uniform field spark gap will always have a sparkover voltage within a known

    tolerance under constant atmospheric conditions. Hence a spark gap can be used for

    measurement of the peak value of the voltage, if the gap distance is known. A sparkover

    voltage of 30 kV (peak) at 1 cm spacing in air at 200C and 760 torr pressure occurs for a

    sphere gap or any uniform field gap. But experience has shown that these measurements are

    reliable only for certain gap configurations. Normally, only sphere gaps are used for voltage

    measurements. In certain cases uniform field gaps and rod gaps are also used, but their

    accuracy is less. The spark gap breakdown, especially the sphere gap breakdown, is

    independent of the voltage waveform and hence is highly suitable for all types of waveforms

    from D.C. to impulse voltages of short rise times (rise time 0.5s). As such, sphere gaps

    can be used for radio frequency A.C. voltage peak measurements also (up to 1 MHz).

    Sphere Gap Measurements

    Sphere gaps can be

    arranged either (i) vertically

    with lower sphere grounded,

    or (ii) horizontally with both

    spheres connected to the

    source voltage or one sphere

    grounded. In horizontal

    configurations, it is generally

    arranged such that both

    spheres are symmetrically at

    high voltage above the ground. The two spheres used are identical in size and shape. The

    schematic arrangement is shown in Figs. 7.18a and 7.18b. The voltage to be measured is

    applied between the two spheres and the distance or spacing S between them 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 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

    500in the case of impulse voltages.

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    In the case of A.C. peak value and D.C. voltage measurements, the applied voltage is

    uniformly increased until sparkover occurs in the gap. Generally, a mean of about five

    breakdown values is taken when they agree to within 3%.

    In the case of impulse voltages, to obtain 50% flashover voltage, two voltage limits,

    differing by not more than 2% are set such that on application of lower limit value either 2 or

    4 flashovers take place and on

    application of upper limit value

    8 or 6 flashovers take place

    respectively. The mean of these

    two limits is taken as 50%

    flashover voltage. In any case, a

    preliminary sparkover voltage

    measurement is to be made

    before actual measurements are

    made.

    Sphere gaps are made

    with two metal spheres of identical diameters D with their shanks, operating gear, and

    insulator supports (Fig. 7.18a or b). Spheres are generally made of copper, brass, or

    aluminum; 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. The spacing is so designed and chosen

    such that flashover occurs near the sparking point P. The spheres are carefully designed and

    fabricated so that their surfaces are smooth and the curvature is uniform. The radius of

    curvature measured with a spherometer at various points over an area enclosed by a circle of

    0.3 D around the sparking point should not differ by more than 2% of the nominal value.

    The surface of the sphere should be free from dust, grease, or any other coating. The surface

    should be maintained clean but need not be polished. If excessive pitting occurs due to

    repeated sparkovers, they should be smoothened. The dimensions of the shanks used, the

    grading ring used (if necessary) with spheres, the ground clearances, etc. should follow the

    values indicated in Figs. 7.18a and 7.18b and Table 7.5. The high voltage conductor should

    be arranged such that it does not affect the field configuration. Series resistance connected

    should be outside the shanks at a distance 2D away from the high voltage sphere or the

    sparking point P.

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    Irradiation of sphere gap is needed when measurements of voltages less than 50kV are

    made with sphere gaps of 10 cm diameter or less. The irradiation may be obtained from a

    quartz tube mercury vapour lamp of 40 W rating. The lamp should be at a distance B or more

    as indicated in Table 7.5.

    REFERENCE:REFERENCE:REFERENCE:REFERENCE:

    HVE KAMARAJU NAIDU; 7.1.1; P-176-180

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    42. FACTORS AFFECTING THE MEASUREMENTS

    Factors Influencing the Sparkover Voltage of Sphere Gaps:

    Various factors that affect the sparkover voltage of a sphere gap are:

    (i) EFFECT OF NEARBY EARTHED OBJECTS:

    The effect of nearby earthed objects was investigated by Kuffel (14) by enclosing the

    earthed sphere inside an earthed cylinder. It was observed that the sparkover voltage is

    reduced. The reduction was observed to be

    V= m log (B/D) + C (7.21)

    where, V= percentage reduction,

    B= diameter of earthed enclosing cylinder,

    D= diameter of the spheres,

    And m and C are constants.

    The reduction was less than 2% for S/D < 0.5 and BID time 0.8. Even for S/D 1.0

    and B/D 1.0 the reduction was only 3%. Hence, if the specifications regarding the

    clearances are closely observed the error is within the tolerances and accuracy specified. The

    variation of breakdown voltage with AID ratio is given in Figs. 7.19a and b for a 50 cm

    sphere gap. The reduction in voltage is within the accuracy limits, if S/D is kept less than

    0.6.Ain the above ratio A/D is the distance from sparking point to horizontal ground plane

    (also shown in Fig. 7.19)

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    (ii) ATMOSPHERIC CONDITIONS AND HUMIDITY:

    The sparkover voltage 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

    of temperature T and pressure p torr and if the sparkover voltage is V0 under standard

    conditions of temperature T = 200C and pressure p = 760 torr, then

    V = kVo

    where k is a function of the air density factor d, given by:

    +

    =

    T

    pd

    273

    293

    760 (7.22)

    (iii) 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 0.1D to 0.3D 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.

    (iv) POLARITY AND RISE TIME OF VOLTAGE WAVEFORMS:

    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 s waveform. Similarly, the wave front and wave tail

    durations also influence the breakdown voltage. For wave fronts of less than 0.5 s and wave

    tails less than 5 s the breakdown voltages are not consistent and hence the use of sphere gap

    is not recommended for voltage measurement in such cases.

    REFERENCE:REFERENCE:REFERENCE:REFERENCE:

    HVE KAMARAJU NAIDU; 7.1.1; P-181-183

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    43. POTENTIAL DIVIDERS RESISTANCE DIVIDERS, CAPACITANCE

    DIVIDERS & MI!ED RC POTENTIAL DIVIDERS

    In a resistance potential

    divider, R1 and R2 are

    considered as resistors of small

    dimensions in the previous

    section. For voltages above 100

    kV, R1 is no longer small in

    dimension and is usually made

    of a number of sections. Hence

    the divider is no longer a small

    resistor of lumped parameters, but has to be considered as an equivalent distributed network

    with its terminal to ground capacitances and inter-sectional series capacitances as shown in

    Fig. 7.26. The total series resistance R1is made of n resistors of value R1 and R = nR1'. Cgis

    the terminal to ground capacitance of each of the resistor elements R1, and CS is the

    capacitance between the terminals of each section. The inductance of each element (L1) is

    not shown in the figure as it is usually

    small compared to the other elements

    (i.e. R1, CS and Cg). This type of

    divider produces a non-linear voltage

    distribution along its length and also

    acts like an R-C filter for applied

    voltages. The output of such a divider

    for various values of Cg/Cs ratio is

    shown in Fig. 7.27 for a step input. By

    arranging guard rings at various elemental points, the equivalent circuit can be modified,

    where Chrepresents the stray capacitance introduced between the high voltage lead and the

    guard elements. This reduces the distortion introduced by the original divider.

    CAPACITANCE VOLTAGE DIVIDERS: Capacitance voltage dividers are ideal for

    measurement of fast rising voltages and pulses. The capacitance ratio is independent of the

    frequency, if their leakage resistance is high enough to be neglected. But usually the dividersare connected to the source voltage through long leads which introduce lead inductances and

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    residual resistances. Also, the capacitance used for very high voltage work is not small in

    dimension and hence cannot be considered as a lumped element. Therefore, the output of the

    divider for high frequencies and impulses is distorted as in the case of resistance dividers.

    MIXED RC POTENTIAL DIVIDERS:Passive circuits are nowadays rarely used in the

    measurement of peak values of high A.C. or impulse voltages. The rapid development of very

    cheap integrated operational amplifiers and circuits during the last decades has offered many

    possibilities to sample and hold such voltages and thus displace passive circuits.

    Nevertheless, a short treatment of basic passive crest voltmeters will be included because the

    fundamental problems of such circuits can be shown. The availability of excellent

    semiconductor diodes has eliminated the earlier difficulties encountered in the application of

    the circuits to a large extent. Simple, passive circuits can be built cheaply and they are

    reliable. And, last but not least, they are not sensitive to electromagnetic impact, i.e. their

    electromagnetic compatibility (EMC) is excellent. In contrast, sophisticated electronic

    instruments are more expensive and may suffer from EMC problems. Passive as well as

    active electronic circuits and instruments as used for peak voltage measurements are unable

    to process high voltages directly and they are always used in conjunction with voltage

    dividers which are preferably of the capacitive type.

    A.C. VOLTAGES:

    A capacitor divider reduces the high voltage V to a low magnitude. If R2and Rd are neglected

    and the voltage V increases, the storage capacitor Cs is charged to the crest value of V2

    neglecting the voltage drop across the diode. Thus the D.C. voltage Vm+V2max could be

    measured by a suitable instrument of very high input resistance. The capacitor Cs will not

    significantly discharge during a period, if the reverse current through the diode is very small

    and the discharge time constant of the storage capacitor very large. If V2is now decreased, C2

    will hold the charge and the voltage across it and thus Vmno longer follows the crest value of

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    V2. Hence, a discharge resistor Rdmust be introduced into the circuit. The general rules for

    the measuring technique require that a measured quantity be indicated within a few seconds.

    Thus the time constant RdCsshould be within about 0.5 1 sec. Three new errors, however,

    are now introduced: an experiment would readily show that the output voltage Vmdecreases

    steadily if a constant high voltage V is switched to the circuit. This effect is caused by a

    continuous discharge of Cs as well as of C2. Thus the mean potential of V2(t) will gain a

    negative D.C. component, which finally equals to about +V2max. Hence a leakage resistor R2

    must be inserted in parallel with C2to equalize these unipolar discharge currents. The second

    error refers to the voltage shape across the storage capacitor. Almost independent of the type

    of instrument used (i.e. mean or r.m.s. value measurement), is due to the ripple and recorded

    as the difference between peak and mean value of Vm. The error is approximately

    proportional to the ripple factor (see eqn (2.2)) and thus frequency dependent as the discharge

    time constant is a fixed value. For RdCs = 1 sec, this discharge error amounts to ~1 per cent

    for 50 Hz, ~0.33 per cent for 150 Hz and ~0.17 per cent for 300 Hz. The third source of

    systematic error is related to this discharge error: during the conduction time of the diode the

    storage capacitor is recharged to the crest value and thus Cs is in parallel to C2. If the

    discharge error is ed, this recharge error eris approximately given by:

    s

    s

    dr CCC

    C

    ee ++

    212

    IMPULSE VOLTAGES:

    The measurement of peak values of impulse voltages with short times to crest (lightning

    impulses) with passive elements only was impossible up to about 1950. Then the availability

    of vacuum diodes with relatively low internal resistance and of vacuum tubes to build active

    D.C. amplifiers offered the opportunity to design circuits for peak impulse voltage

    measurement but of relatively low accuracy. Now, active highly integrated electronic devices

    can solve all problems involved with passive circuits. The problems, however, shall shortly

    be indicated by the following explanations. Impulse voltages are single events and the crest

    value of an impulse is theoretically available only during an infinitely short time. The actual

    crest value may less stringently be defined as a crest region in which the voltage amplitude is

    higher than 99.5 per cent. For a standard 1.2/50 sec wave the available time is then about

    1.1 sec. Consider now the simple crest voltmeter circuit of Fig. 3.14 discussed earlier,

    omitting the discharge resistor Rdas well as R2. The diode D will then conduct for a positive

    voltage impulse applied to the voltage divider, and the storage capacitor must be charged

    during the rising front only. But instantaneous charging is only possible if the diode has no

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    forward (dynamic) resistance. The actual forward resistance RD gives rise to a changing time

    constant RdCsand it is known that a response time, which is equal to the time constant RdCs

    for such an RC circuit, of about 0.2 sec would be necessary to record the crest value with

    adequate accuracy. For a low Csvalue of 1000pF the required RD = 200. As also the diodes

    junction capacitance must be very small in comparison to Cs, diodes with adequate values

    must be properly selected. The more difficult problem, however, is the time required to read

    the voltage across Cs. The voltage should not decrease significantly, i.e. 1 per cent for at least

    about 10 sec. Hence

    the discharge time

    constant of Cs must

    be longer than 103

    sec, and thus the

    interaction between

    the diodes reverse

    resistance and the

    input resistance of the instrument necessary to measure the voltage across Csshould provide a

    resultant leakage resistance of 1012. A measurement of this voltage with electrostatic or

    electronic electrometers is essential, but the condition for the diodes reverse resistance can

    hardly be met. To avoid this problem, a charge exchange circuit shown in Fig. 3.15 was

    proposed.

    REFERREFERREFERREFERENCE:ENCE:ENCE:ENCE:

    HVE KAMARAJU NAIDU; P-189-191

    HVE KAMARAJU NAIDU; P-191

    HVE Fundamentals Kuffel, Zaengl, Kuffel; 3.4; P-116-113

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    44. SURGE CURRENT MEASUREMENT

    In power system the amplitude of currents may vary between a few amperes to a few

    hundred kilo-amperes and the rate of rise of currents can be as high as 1010

    A/sec and the rise

    time can vary between a few microseconds to a few macro seconds. Therefore, the device to

    be used for measuring such currents should be capable of having a good frequency response

    over a very wide frequency band. The methods normally employed are(i) resistive shunts;

    (ii) elements using induction effects; (iii) Faraday and Hall Effect devices. With these

    methods the accuracy of measurement varies

    between 1 to 10%. Fig. 4.27 shows the

    circuit diagram of the most commonly used

    method for high impulse current

    measurement. The voltage across the shunt

    resistance R due to impulse current i(t) is fed

    to the oscilloscope through a delay cable D.

    The delay cable is terminated through an impedance Z equal to the surge impedance of the

    cable to avoid reflection of the voltage to be measured and thus true measurement of the

    voltage is obtained.

    Since the dimension of the resistive element is large, it will have residual inductance

    L and stray capacitance C. The inductance could be neglected at low frequencies but at higher

    frequencies the inductive reactance would be comparable with the resistance of the shunt.

    The effect of inductance and capacitance above 1 MHz usually should be considered. The

    resistance values range between 10 micro ohm to a few milliohms and the voltage drop is of

    the order of few volts. The resistive shunt used for measurements of impulse currents of large

    duration is achieved only at considerable expense for thermal reasons. The resistive shunts

    for impulse current of short duration can be built with rise time of a few nanoseconds ofmagnitude. The resistance element can be made of parallel carbon film resistors or low

    inductance wire resistors of parallel resistance wires or resistance foils.

    REFERENCE:REFERENCE:REFERENCE:REFERENCE:

    HVE C L WADHWA; 4.9; P-141

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    PROBLEMS

    Q. 1 What are the requirements of a sphere gap for measurement of high voltages? Discuss

    the disadvantages of sphere gap for measurements.Q. 2 Explain clearly the procedure for measurement of (i) impulse; (ii) A.C. high voltages

    using sphere gap.

    Q. 3 Discuss the effect of (i) nearby earthed objects (ii) humidity and (iii) dust particles on

    the measurements using sphere gaps.

    Q. 4 Describe the construction of a uniform field spark gap and discuss its advantages and

    disadvantages for high voltage measurements.

    Q. 5 Explain with neat diagram how rod gaps can be used for measurement of high

    voltages. Compare its performance with a sphere gap.

    Q. 6 Explain with neat diagram the principle of operation of an Electrostatic Voltmeter.

    Discuss its advantages and limitations for high voltage measurements.

    Q. 7 Draw a neat schematic diagram of a generating voltmeter and explain its principle of

    operation. Discuss its application and limitations.

    Q. 8 Discuss the problems associated with peak voltmeter circuits using passive elements.

    Draw circuit developed by Rabus and explain how this circuit overcomes these

    problems.

    Q. 9 What are the problems associated with measurement of very high impulse voltages?

    Explain how these can be taken care of during measurements.

    Q. 10 Discuss and compare the performance of (i) resistance (ii) capacitance potential

    dividers for measurement of impulse voltages.

    Q. 11 Discuss various resistance potential dividers and compare their performance of

    measurement of impulse voltages.

    Q. 12

    Discuss various capacitance, potential dividers and compare their performance for

    measurement of impulse voltages.

    Q. 13 Draw a simplified equivalent circuit of a resistance potential divider and discuss its

    step response.

    Q. 14 Discuss various methods of measuring high D.C. and A.C. currents.

    Q. 15 Discuss various methods of measuring high impulse currents.