HV Engineering

High Voltage Engineering

LECTURE – 9 & 10

Measurement of High-Voltages

1

Measurement of High Voltages

Types of Voltages to be measured

i.ii. iii.

A.C.H.VD.C.H.V Impulse/Transient

H.V

2

Measurement

Summary.

A.C.H.V

of High Voltages

i)

a)b) c) d) e) f)

ElectrostaticSpheregaps.

Voltmeter.

Uniform field Gaps.Milliammeter in Series with Resistance.Series Capacitance Voltmeters.Potential Dividers (resistance orCapacitance type). Oscilloscope (CRO) Potential Transformers.

g)h)

3


Summary.

ii) D.C.H.V

a)b) c) d) e) f) g) h)

ElectrostaticSpheregaps.

Voltmeter.

Uniform field Gaps.series resistance mircoammeters. Resistance potential Dividers. Generating Voltmeter.Series Impedance Ammeter. Oscilloscope (CRO).

4


Summary.

iii) Transient / Impulses High Voltage.

a)b) c) d)

Sphere Gaps.Uniform field Gaps.Oscilloscope (CRO). With delay Cable.Potential dividers.

5


Electrostatic Voltmeters

Pointer

Voltmeter to measure AC + DC voltage, but notImpulse/Transient voltages.

The mechanical force between two charged electrodeshas frequently been used for the measurement of highvoltages and the attracted disc electrometer designed byLord Kelvin is one of the first instruments based on thisprinciple.

6

Scale Fixed Electrode

Spring

Gap

H.V



An electrostatic voltmeter consists essentially of a pair ofparallel plane disc electrodes separated by a smalldistance. The moving disc is surrounded by a fixed guard-ring and the two are at the same potential. Thisarrangement makes the electrostatic field uniform in thecentraldiscs.

portion of the gap between the moving and fixed

7


1.

on


the disc is given byThe force

d F

V

2

Where

dlV

===

diameter of the moving disc, cm,gap length, cm,potential difference between the electrodes, kV.

8

2

2825l 2


1. Electrostatic Voltmeters

In order to measure a given voltage with great precision,the disc diameter has to be increased and the gap lengthdecreased. An increase ina corresponding increase

the disc diameter necessitatesin the diameter of the guard-

ring and of the opposing electrode. A decrease in theelectrode spacing reduces the working voltage in orderthat the gradient remains below 5 kV/cm for voltmetersoperating in atmospheric air.

9



The main difference between the various types of electro-statictorque

voltmeters is the manner in which theof

restoringis obtained and the movement the disc

indicated. In the simplest method the restoring torque isobtained by a spring control which actuates a pointermoving on the scale of the instrument.

10


1. Electrostatic VoltmetersElectrostatic voltmeters for higher ranges have been con-structed with compressed

electrodes.gasOne

as the insulating mediumsuch instrument

developedforby

betweenvoltages

theup to 600 kV has been

Bocker. The insulating medium is generally carbon dioxidegas at a pressure of 15 atm. Also vacuum has beenemployed as the insulating medium in voltmeters which giveworking gradients, in the range of 100 kV/cm, and theinstrument is, therefore, smaller than an open-air instrumentof the same voltage rating.

11


1.

Voltmeters


using atmospheric air have beenmade for use up to 400-kV peak and those withcompressed gas up to 600-kV peak. The accuracyof these instruments is about 0.1 %.

Electrostaticespecially in

voltmetersthe case

absorb no appreciable powerof direct voltages. After a

momentary charge flow ofinsulation leakage current small. The insulation

a few micro coulombs only thewill flow, whichresistance

should be veryis, however,

affected by humidity and the current consumption willincrease with increasing humidity.

12


1.

alternating


On voltages there is a small currentconsumption which is proportional to the frequency, andconsequently such instruments are not suitable for highfrequencies (above few megacycles).

As the operating force in such instruments is proportionalto the square of the applied voltage, the scale is non-uniform and it is not possible to obtain the sameaccuracy of observation throughout the working range.Instruments which cover a wide range of voltagemeasurements are provided with different sizes of themovable discs for different ranges.

13



Bowdler designed an attracted disc voltmeter, operatingin compressed nitrogen at a pressure of 150 Ib / in2, or a mixture of nitrogen and arcton-6 at a pressure of 50 Ib/in2 for a voltage range up to 350 kV (peak). The accuracy ofthe instrument was tested at various voltages and wasabout 1 % at 1 kV and 0.1 % at 30 kV and above.

14



The author has estimated errors arising from factors suchas non-uniformity of field, parallelism and flatness ofelectrodes, dimensionsof attractive force. The

of electrodes and measurementresultant error was found to be

smalleststrength

for the largest spacing and the highest electricbetween the electrodes.

15



Broadbent, Cooper and Elliott(S) have recently describedan attracted disc voltmeter which measures voltages upto 30 kV to accuracy within 0.25 %. The principle ofoperation of the voltmeter is illustrated in Fig. 2. Theelectrodes consist of a movable circular disc surroundedby a fixed guard ring. Both the guard ring and themovable disc are placed opposite a parallel planeelectrode terminated in curved edges.

16

Measurement of High Voltages1. Electrostatic Voltmeters

Fig: 2.

17



The disc is attached to one arm of a mechanical balanceand the guard ring is earthed while the voltage to bemeasured is applied to the lower plane electrode. Thecontrol torque is obtained from the force of attractionbetween a coil attached toand a permanent magnet.

the other arm of the balance

The balance arm carries a small mirror which reflects abeam of light on to a pair of photoconductive cells. Thesystem is adjusted in such a way that when the balance arm tilts, the spot of light covers more of one cell and less of the other according to the direction of deflection of thebalance arm.

18



This causes an unbalance in a bridge circuit containingthe two photoconductive cells. The out-of-balance voltageis used to control the current through the coil and thesystem acts as an error controlled closed-loop electro-optical circuit. A fixed relationship exists between the coilcurrent and applied voltage, and thus the unknownvoltage can be read directly.

The instrument was calibrated absolutely by balancingknown weights against the attractive force between theelectrodes produced by known voltages.

19


1.

attractive


The force corresponding to any given coilcurrent was first determined by applying weights to thediscwas of a and

electrode in the absence of voltage. The coil currentsimultaneously measured to within 0.02 % by means d.c. potentiometer. The weights were then removed the voltage to be measured was applied and the coil

current measured.

Knowing the relation between the coil current and theattractive force, the voltage was determined absolutely bythe use of the equation relating the electrostatic force and the dimensions of the electrode.

20


2. Sphere Gaps

Used for measurement of A.C, D.C and Impulse voltage.

Sphere gaps are commonly used for the measurement ofthe peak value of high voltages and as a result ofextensive investigations calibration tables givingbreakdown voltages corresponding to various gap lengthsfor different sizes of spheres have been prepared.

21

H.V

Sphere gap


2. Sphere Gaps

The calibration data recommended in the British StandardRules for Measurement of Voltage with

andSphere Gaps,

thereafter referred to as B.S. 358, are included inTables 1 and 2.

22


2. Sphere GapsTables 1.

23


2. Sphere Gaps

24



25

Measurement of High Voltages2. Sphere Gaps

26



27


28

Measurement of High Voltages2.

and

Sphere Gaps

For alternating 1/50 μsec impulse voltages forspacing up to 0.5D (sphere diameter) the tables areconsidered to be accurate within ±3%.

Values in the tables for spacing between 0.5D and 0.75Dare regarded as being of doubtful accuracy.

For direct voltage measurement in the absence ofexcessive dust the results are considered accuratewithin ±5% for spacing not greater than 0.4D.

29


2.

mechanism

Sphere Gaps

The of electric breakdown in air wasdiscussed previously. The factors which effect thebreakdown voltage and the procedure of measurementwill be discussed now.The spheres maybe made of aluminum, brass, bronze orlight alloys and the surface should be free fromirregularities.The spheres should be cleaned immediately before use,otherwise dust or deposited moisture may affect theaccuracy of measurement.For spheres of a particular diameter, the field in the gapbecomes less uniform as the gap length

diameter.should

not

increases from asphere radius to a sphere B.S. 358: 1960recommend that the spacingradius.

exceed a sphere30


a) Effect of Nearby Earthed ObjectsThe accuracy of measurement of voltage with sphere gapis considerably affected by earthed objects around thegap.

Kuffel and Husbands have studied the influence ofnearby earthed objects on the direct voltageof horizontal gaps.

breakdown

They studied the effect of surrounding the gap by acylindrical metal cage and found that the breakdownvoltage was lowered appreciably, particularly when thegap length exceeded a sphere radius.

31

2.

Effect

Sphere

Nearby

Gaps

Eartheda) of Objects

Fig: 4.

D 32

V m log B C

2.

Effect

Sphere Gaps

a) of Nearby Earthed Objects

Fig: 4.

33


2. Sphere Gaps

a) Effect of Nearby Earthed Objects

A similar reduction was observed when an earthedconducting plane was placed parallel to the gap axis. Thiseffect is shown in Fig.5 where a reference13.4D was taken.

clearance of

The clearance A to the plane had a very small effectwhen the distance was l0D or greater. At a9.8D the breakdown voltage was reduced by

clearance of0.2-0.7 %.

34

2.

Effect

Sphere Gaps

a) of Nearby Earthed Objects

Fig: 5(a).

35

2. Sphere Gaps


Fig: 5(b).

36

2. Sphere Gaps


A comparison of results for sphere spacings larger thanthetheby

sphere radius showed that the ratio of the changes inbreakdown voltage, caused by a cylindrical shield andan equally distant plane, respectively, varied from

about 1.1 to 1.8 when the clearance was varied from 4Dto 12D.The results showed that the effect of objects surroundinga sphere gap on the breakdown voltage can be maderelatively small by bringing an earthed plane sufficientlynear to the gap. Then the plane will have a greater and more consistent influence on the breakdown voltage than the combined effect of a number of objects which are at a comparable or greater distance from the gap.

37

2. Sphere Gaps


It was pointed out that the accuracy with which voltagesmay be measured by a sphere gap depends on theaccuracy of the accepted calibrations. Thus the clearanceconditions for the calibration should be specified withinvery close limits although the limits may be extendedwhen the gap is used for voltage measurement.

Table:3 gives values, suggestedfor suitable standard clearances

by Kuffel and Husbands,and the use of the same

standard clearancesspheres. An accuracy

is recommended for all sizes ofof ±1% is claimed to be obtainable,

as far as clearance effect is concerned, by observingthese clearance limits.

38

2.

Effect

Sphere Gaps

a) of Nearby Earthed ObjectsTable: 3

39


2. Sphere Gaps

b) Effect of Humidity

The breakdown voltage of an air gap is affected by theatmospheric conditions and a correction factor has been-worked out to convert the breakdown voltage to thestandard atmospheric conditions of 760 mm Hg pressureand 200C temperature. The breakdown voltage V at arelative air density ℓ, and a voltage Vn at standardatmospheric conditions are related by:

40

V KVn

2. Sphere Gaps

b) Effect of HumidityWhere K is a factor depending upon ℓ .

The air density ℓ is expressed as

760 273 t

Where

p = air pressure in mm Hg,t = air temperature in centigrade.

41

ℓ p 293


b) Effect of HumidityTable5. gives the values of K for different values of ℓ .

42



The B.S. 358: 1960 takes no account of the effect ofhumidity on the flashover voltage of sphere gaps,although it is indicated that an error of up to 3 % couldarise from differences in the air humidity.

Kuffel has studied the influence of humidity on the break-down voltage of air gaps using 2.0, 6.25, 12.5 and 25-cmdiameter spheres and uniform field electrodes. Withsphere gaps, the influence of humidity varied with the

the the

sphere diameter and with gap lengths. Fig: 6 showspercentage increase in breakdown voltage whenwater-vapour pressure was changed from 0 to 17 mm Hg.

43

2. Sphere Gaps


Fig: 6.

44



The humidity effect increased with gap length until amaximum effect was reached at a well defined gaplength.reduced

Furthereffect,

increases in the gap lengthgave

gave abut the larger spheres an effect

which again increased with still longer gaps. Theobserved maximum change was smallest with the 2-cmdiameter spheres; it increased with the sphere size andwas largest for uniform field electrodes.

45



The maximum rate of change in breakdownhumidity was found to be in the region 0-4there after the change was slightly less.

voltagemm Hg

withand

Furthermore, the relationship between the breakdownvoltage and the humidity between 4 and 17 mm Hg was

that which gavesubstantially linear for spacingsthe maximum humidity effect.

less than

The relationship was less linear at the larger spacings.

46


2. Sphere Gaps


The effect of humidity on the breakdown voltage with 25-cmdiameter spheres was also studied with alternating voltageover the range 6-15 mm Hg of water vapour. Fig: 7 showsthe two sets of results for a l-cm gap, and it can be seen

generalthat the alternating breakdown voltagesvoltage

were inslightly lower than the direct values but thedifference did not exceed 0.5 %.

47

2. Sphere Gaps


Fig: 7

48



The breakdown voltage/humidity relationships (Fig.6)display three main features:

1) The breakdown voltage increasespressure of water vapour.

with the partial

2) The total voltage change for a given humiditychange increases with gap length.

3) The humidity effect increases with the size ofspheres and is largest for uniform field electrodes.

49

2. Sphere Gaps


The first two effects may be attributed to the relativevalues of ionization and attachment coefficients in air ofdifferent humidities and at different voltage gradients. The increase of humidity effect with the size of spheres was explained by Kuffel) by taking into account the distributionof the ionization and attachment coefficients along theaxis of the sphere gaps.An analysis of the distribution of

forthese coefficients

showed that the maximum effect different sizesof

ofsphere occurred when the minimum values theeffective ionization coefficient (α - n)min in the middle ofthe gaps were approximately the same.

50


c)The

Effect of Irradiation and of Polarityreliability of the sphere gap can be increased by

irradiating the spheres with ultraviolet light. Irradiationincreases the availability of the initiating electrons andtherefore reduces. the statistical time lag of breakdown.

Experiments have shown that irradiation becomesnecessary when voltages lower than 50 kV are beingmeasured and also when spheres of 12.5 cm diameterand less are used for the measurement.

51


c) Effect of Irradiation and of Polarity

Irradiation can be obtained by placing a quartz-mercuryvapour lamp ateffective source

a distance B given inof obtaining irradiation

Table:4. A moreis by inserting a

capsule of 0,5 mg of radium inside the high-voltagesphere close to the sparking-point.

Occasionally light from another spark-usually the triggergap of an impulse generator-produces the requiredirradiation.

52



Theand

influence of irradiation on the breakdown of directalternating voltages was studied by Edwards and

Smee and the results for gaps between 1.3-cm diameterspheres are shown in Fig.8.

When the gap was irradiated by 0.5 mg of radium, thebreakdown voltage became

breakdown

consistent. Irradiation,however, reduces the voltage slightly.

53

2. Sphere Gaps


Fig: 8.

54


2. Sphere Gaps

c)

The

Effect of Irradiation and of Polarity

effect of irradiation on direct voltage calibration ofsphere gaps was studied by Kuffel. The gap wasirradiated by placing 0.5 mg of radium inside one sphereof approximate thickness of 1 mm of copper.

Fig:9 shows the percentagegap

lengths.

lowering of breakdownvoltages of various

55

2.

EffectSphere Gaps

c) of Irradiation and of Polarity

Fig: 9.

56


2. Sphere Gaps


The effect is most marked with 2-cm diameter spheres atsmall spacings where a reduction in breakdownof about 4.5 % was obtained for a gap length of mm.

voltageabout 1

A second maximum was observed at about 1 cm spacingand thereafter the effect of irradiation steadily diminisheswith gap lengths. The gap length at which the maximum effect is obtained varies according to the size of spheres and generally increases with sphere diameter.

57


2. Sphere Gaps


Alternating voltage breakdowns were found to be lessaffected by irradiation as compared with directreduction of 1.5 % in the alternating voltage

voltages. Abreakdowna 6'25-cmwas observed for a gap length of 2 cm of

diameter sphere gap.

The corresponding reduction under direct voltage was 2.5%. These findings supported the recommendation made in B.S. 358: 1960.

58



When impulse voltages are measured with sphere gaps,it becomes more important to eliminate the statistical time lag. Under proper irradiation a gap may break down if thepeak voltage exceeds the d.c. breakdown value.However, in the case of poorly irradiated gap, the peakvalue of the applied impulsebreakdown of the gap.

has to be increased to cause

59


2. Sphere Gaps


Several workers have studied thepower

flashoverfrequency for shorter

characteristics of sphere gaps underand impulse voltages. These studies show thatspacings the breakdown curves coincided, but in longergaps the positive impulse breakdown curve rose abovethe negative impulse breakdown curve by

gap

an amountdepending upon the sphere diameter and length.

60


2. Sphere Gaps

c)The

Effect of Irradiation and of Polaritysource of irradiation was the illuminating spark

caused by the discharge of a 0.1-F capacitance at 10 kVthrough a gap between zinc spheres. Fig: 15 shows the lowering of breakdown voltage as a function of the

distance between the gap and the illuminating source.The gap lengths are adjusted such that the d.c.breakdown voltages are 5 kV and 10 kV respectively.

61

2. Sphere Gaps

d) Procedure of MeasurementWhenmade,series

measurements with direct and alternating voltages area resistance of 100 kΩ to 1MΩ should be placed inwith the sphere gap. This will reduce the pitting of the

spheres and damp the superimposed oscillations whichcause erratic flashover of the gap.

may

Protectingpresence

resistance becomes more important inthe

thetestof any discharge in the specimen

produceor in

circuit, as these discharges may transient over-voltages.

If the circuit is free from discharges, the value of the seriesresistance may be reduced within the limits mentioned above.

62


d) Procedure of Measurement

To prevent oscillations in the sphere-gap circuit underimpulse voltage measurement a non-inductive resistanceof not more than 500Ω should be used.

When measurements of direct and alternating voltagesare made, the applied voltage is increased gradually untilthe flash over of the gap takes place. Generally the mean value of three successive readings agreeing within 3 % is taken to be the breakdown value of the gap.

63


3. Uniform Field Gaps

For the measurement of A.C, D.C and Impulsevoltage.

Sphere gap, though extensively used for high-voltagemeasurements, produces a field which is uniform oververy limited region of the gap namely along the axis

only aof thewouldgap and it is not possible to ensure that sparking

always take place along the uniform field region.

Rogowski presented designs for profiles for electrodesgiving uniform field for voltages up to 600 KV.

64



It has been shown theoretically that the field between twoequal spheres is very nearly uniform for values of

thespacing/radius ratio less than about 1/10 andaccurate theoretical interpretation of the experimentalresults is possible only under this condition.

If the spark takes placevoltage could be defined

in a uniform field, the sparkoverby an equation of the form:

65

V = AS+B S



Where A and Bvoltage for a gap

are constants, and Vlength S.

is the sparkover

This limitation of the sphere gap led to the developmentof uniform-field gap and one of the early designs was putforward by Stephenson.Fig:16 shows the design of the electrodes used, formeasurement of 50 c/s alternating voltages up to 400 kV(peak).The portion AC is plane and is of a diameter equal to or greater than the maximum spacing S. The radius from A to B and C to D should be greater than 10S.

66

Measurement of High-Voltages3. Uniform Field Gaps

Fig: 16.

67


To avoid sparking at the edges, the curvature from B to Eand D to F should be of continuously increasingmagnitude. Under this arrangement, it was foundexperimentally that sparking always occurred in theuniform-field region of the gap for all spacings up to 16cm under normal atmospheric conditions.

From a theoretical analysisStephenson has shown that spacing are related, at 25°C gap length in centimetres.

of the experimental results,the breakdown voltage and and 760 mm Hg by S is the

68

v = 24.4S+7.50 S kV(peak)



It has also been shown that the sparking potential is afunction of both the spacing S and the gas density ℓ, sothat the density correction factor at a given density varies with the spacing, and the above relation is accordingly modified as:

+ 7.50 (ℓS)1/2 KW (Peak) [Stephenson)V = 24.4ℓS

Where e is the gas density which is unity at normalatmospheric conditions.

69


Bruce has also studied the characteristics of uniform-fieldelectrode for power frequency alternating voltages up to315 kV. A half contour of one of the electrodes used byBruce is shown in Fig.17. The flat portion AB is of adiameterused. BCsuch that

not less than the maximum gap length to beis a sine curve, based on the axes OB and OC,XY = OC sin (BX/ BOXπ/2). CD forms the arc of

circle with centre at O.

70


Fig: 17

71


In order to maintain the uniformity of the field at different gaplengths, different pairs of electrodes were used.The diameters of the flat surfaces were 2.25, 4.4 and 7.8 in. for the measurement of voltages up to 140, 280 and 420 kVrespectively. The corresponding overall diameters of theelectrodes were 4.5, 9.0 and 15.0 in.UsingBrucelength

these electrodes for voltages from 9 to 315 kV(peak),has shown that the breakdown voltage of a gap of

%S cm in air at 25°C and 760 mm Hg is within ± 0.2of the value given by the relation

72

BruceV = 24.22S + 6.08 S kV (peak).


The influence of nearby objects on the breakdownalso been studied by Bruce and he recommendedthe clearance between the edges of the electrodes

hasthat and gapthe nearest earthed conductor in the plane of the

should be .wore than four times the maximum gap length.The clearance between the edges of the electrodes and adischarging conductor should' bethe maximum spacing.

not less than ten times

To account for the air density the above expressionbecomes

73

V = 24.22ℓS + 6.08 (ℓS)1/2

Measurement of High-Voltages3.

The effect ofby Ritz who

Uniform Field Gaps

humidity on uniform fieldobserved an increase of

gaps was studiedabout 2 % in the

breakdown of a 1 cm air gap when the water vapourpressure varied from 10 to 25 mm Hg. He suggested arelationship of the following form.

74


Kohrmann observed a greater humidity effect and foundthat the breakdown voltage of 0'5-cm gap increased by2.4 % when the water vapour pressure was increasedfrom zero to 10 mm Hg.Kuffel has studied the effect of humidity for a number of gap lengths. Fig: 18 illustrates the percentage increase in breakdown voltage when humidity was varied from 0 to17 mm Hg, and it is seen that the breakdown voltage for

gaps up to 2The change humidity for

cm increases by 4-5.5% at 17 mm Hg.in voltage is not linearly related to either the a given gap length, or the gap length for agiven humidity.

75


Fig: 18

76


3. Uniform Field Gapsnearly linearly related toThe voltage change is, however,

the humidity between the rangegap length is constant. Between

4-17thes

e

mm Hg when thehumidity limits thegaps, giving 0.19voltage increase is greater for longer

%/mm Hg for2-cm gap.

a 0.5-cm spacing and 0.27 %/mm Hg for a

The increasewater vapour

in breakdown voltage withmay be explained partly by

the quantity ofconsidering the

higher electron attachment in moist air.

77


The problem has been discussed by Kuffel and anestimation of the increase in the breakdown voltage gavea reasonable agreement with the observed values over arange of gaps from 0.5 to 2.0 cm.An increase with gap length in the voltage change for agiven humidity change also follows from electron-attachment considerations. The ionization coefficientvaries more rapidly with the field gradient than does theattachment coefficient.The field gradient at breakdown decreases withincreasing gap length and consequently the influence ofthe attachment coefficient on the ionization efficiency willbe greater for longer gaps.

78



Prasad and Craggs have recently shown that the rate ofincrease of the attachment coefficient is considerablygreater than that of the ionization coefficient as the partialpressure of water vapour is increased.

In addition, the secondary ionization coefficient wasobservedpressure.

toIn

decrease with increasing water vapourconsequence the breakdown voltage in

creases with increasing humidity.

79


3.

and

Uniform Field Gaps

Holzer Ritz studied uniform field breakdown underprofilealternating voltages using the electrodes with the

suggested by Rogowski.

The relation between the breakdown voltage and gaplengthHolzer

8is

in cm, at atmospheric conditions, obtained by

80

V = 23.85S+ 7.85 S kV (peak), Holzer and that given by Ritz is

V = 24.558+6.66 S kV (peak). Ritz



Results obtained by various workers are given in Table:7andalso

the values obtained by sphere-gap measurements areinserted for comparison.

The discrepancies observed inbe

thedue

results of thesetest

weremeasurements are likely to to

the

differentvoltagesconditions

measured.and the accuracy with which

81


Fig: 17.

82



From the experimental resultslimits ofdetecte

d

available, it may beassumed that within the experimental accuracynoa.c.

difference has been between the d.c. andbreakdown voltages.

83



The impulsefield gap has

breakdown characteristics of the uniform-not yet been studied extensively.

Holzer made a study with impulse voltages ofdifferentrate of rise and observed an increase of about4% in the breakdown voltage for a 12-cm gap above thestatic breakdown voltagerate of rise of 9.3X 108 kV

when an impulse/s was used.

voltage of a

84



At smaller gaps the difference became less. Also whenX 106the rate of rise was reduced to 1.6 kV /s the

impulse values became equal to static breakdown values.Cooper made measurements by applying recurrently avoltage impulse of 1 μsectime intervals between the the instant of breakdown.

front duration and studied theapplication of the impulse and

85


3. Uniform Field Gapspolarity were applied to rate of 400 impulses/so

Rectangular impulses of negativea parallel-plate electrode gap at aThe maximum amplitude of the voltage was about 25 kV.The impulse voltages applied to the gap were measuredby means of a cathode-ray oscillograph in conjunctionwith a resistance potential divider.

The results of his finding are summarized in Table. 8which compares the effect of irradiation on breakdownvoltage.

Irradiation of the gap was obtained by inserting a metalhigh-

86

capsule containing 0.2 mg of radium inside thevoltage electrode.



Table: 8.

87



The results in Table 5.8 show that for gaps greater than0.2the

cm long, irradiation produced practically no effect onbreakdown voltage.

88



Comparing the uniform-field geometry and thesphere gap as a standard method of measuringhigh voltages, it is readily seen that the formerhas certain advantages over the latter. Thebreakdown voltage of the uniform-field electrodecan be calculated accurately over a wide range ofgap lengths while there is no accurateexpression by means of which the sparkingvoltage is calculable for sphere gaps for allconditions to an accuracy better than several percent.

89


3. Uniform Field GapsThe air-density correction factor is a mean valuefor the whole range of spacing of the sphere gapwhile in the case of uniform-field electrodes it isa function of both the gas density and the gaplength.

The sphere gap calibration depends on whetherthe arrangement is symmetrical or with onesphere earthed. The uniform-field, however, doesnot show any such effect and the breakdownvoltage remains the same whether bothelectrodes are insulated or one electrode isearthed.

90



For uniform-field gaps the clearance to nearbyobjects is much smaller than is the case of spheregaps. The field in the central portion of the uniform-field gap is well shielded against the effect of nearbyobjects as compared with sphere gap.

91


clearanceFor uniform-field gaps the to nearbyobjects is much smaller than is the case of spheregaps. The field in the central portion of the uniform-field gap is well shielded against the effect of nearbyobjects as compared with sphere gap.

In spite of a superior performance of the uniform-fieldgap, it has not yet replaced the sphere gap as astandard method of voltage measurement.

are the need for

Thevery

and

serious practical disadvantagesaccurate mechanical finish of the electrodesextremely careful parallel alignment.

92


4. Ammeter in SeriesImpedance

with High

The effective value of high voltages can be obtained bymeasuring the current flowing through a circuit containinga high series resistance. Neglecting the impedance of theinstrument, the product of the current and the seriesresistance gives the unknown voltage.

93

H.V

I

R V =IR

A μA


4. Ammeter in Series with High Impedance

A milliammeter of the dynamometer or thermal type iscommonly used in such measurements; however,moving-coil instruments can be used for direct voltagesprovided the superimposed a.c. ripple is less than 10 %. Theaccuracy of measurements depends upon the design of the high-voltage resistor.These resistors should have negligible resistancetemperature coefficient and should be free from coronadischarges. Also leakage along the supporting structures

the stray has to be

should be small. With alternating voltages,Capacitance of the resistor sections to earthconsidered.)

94



Wire-wound resistances have often been used for high-voltageTaylor.

measurements and one such design is due to

One hundred units ( MΩ, 1W) made of Ni-Cr wire werearrangedmounted insulating

inin lid

twenty sets of 5 units each. Each set wasa spun aluminium corona shield with anmade of hard rubber or pyrex glass.provided between each resistance

A springset andcontact was

the aluminium cover ofthe next set.

95



The resistance units were wound noninductively and thetemperaturetemperature assembly

coefficient was 0.0152 %/C. Thetheand

wereresistancedetermine

d

characteristics ofunder the operating

conditions for voltages upin

to 1.3 kV / resistor and theofresults are reproduced Fig. 19. The accuracy

measurement was claimed to be 0.01%.

96



Fig: 19.

97



The length of the wire required becomes veryconsiderablefor the finest

for resistances of 100 MΩ and above-evengauge of the coil remained constant within

0.1 % over long periods. The coil formers are mounted onan ebonite rod to form a stack shown in Fig.20, andplaced in a container 18 in. high X10½ in. diameter.

98


4.The was The

Ammeter in Series with High Impedanceunit was tested at 125 kV when the power dissipation200 W and the oil temperature increased by 40°C.shape of the coil former assists in maintaining a

steady circulation of oil. The leakage resistance through1012the oil and the container was 3 X Ω which has a

negligible effect on the total resistance of the unit.

The accuracy obtained on 300KV was better than ±0.3%.Owing to the high cost of this instrument, its application is limited to installations such as high-voltage X-ray therapy, where a very high accuracy is required.

99



Park has recently described a new designresistor capable of measuring high direct accurately. It is made up of a large number

of a standardvoltages veryof 1-MΩ wire-

wound resistor elements connected in series andarranged to form a vertical helix.

Each individual resistor elementshield which completely encloses

is shielded by a metalthe element.

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Coronathereby working

formation on the surface of the resistance coil isprevented. A 100-MΩ unit suitable for 100 kVvoltage is shown in Fig.21. The high-voltage end

of the resistor is fitted with a large "hat" which, togetherwith the vertical helical configuration of the

fieldresistor unit,

preventsformation.

concentration of electric and corona

101



Fig: 21

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5. Series Capacitance Voltmeters

A method suitable for determining the peak and r.m.S.value of an alternating voltage is to measure thecurrent flowing through a capacitor connected to thehigh-voltage source.

The current is measured by a rectifier milliammetercircuit. The circuit of a peak voltmeter

Fig. 22.described by

Chubb and Fortescue is given in

Neglecting the rectifierC(de/

dt)

impedance, thetheThe

instantaneousinstantaneouscurrent

voltagethrough

is: i =C

where e isandeach

is the capacitance. total chargerectifier per cycle is:

103



Fig: 22.

104

t



dt dt

Q idtC de2 cos2 3 fdtfCE 2CE4

f

105

3/4 f

1/4 f

i dQ ,i c dv ,eEsin wt,de wECos wt

1 f

4



Where e = E sin 2πft.

The charge per second or the mean current throughinstrument is then 2CEf, where f is the frequency oftest voltage. The peak voltmeter, therefore, measures

thethethe

total voltage swing from positive to negative peak of analternating voltage. The chief source of error is theimperfect rectifier characteristic.

106



In the above treatment, the integration is taken betweenthe time limits when de/dt has one sign only (positive for one rectifier and negative for the other), the peak voltage cannot be measured if there are subsidiary peaks in the voltage wave.

This method is only applicable when the + ve and - vepeak heights are equal.

This method is used extensively for measuring the peakvalues of power frequency voltages up to about 1000 kV (r.m.s.), and accuracies of the order of 1 or 2 parts in1000 can be achieved.

107



The Chubb-Fortescue peak voltmeter described earliersuffers from two serious drawbacks. These are frequencydependence of the indication and the error introduced byheavy discharges on the h.v. system; a less importantshortcoming is the errorthe voltage waveform.

introduced by multiple peaks in

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The maximum voltage range of such a voltmeter is limitedby, the peak inverse voltage of the rectifier. Baker haspresented the design of an

loop,

instrument which includesrectifiers in a feedback and is to a great extentindependent of the rectifier characteristic.

The instrument is particularly applicable in themeasurement of h.v. in the presence of corona discharge.An outline of the method is shown in Fig. 23.

109


6. Potential Dividers

V

Dividers for Direct and Alternating Voltages.

Potential divider is, basically, a series combination ofhigh and low impedance. The voltage to be measured

ais

applied across the combination and the drop across thelow-impedance section is measured by means of anindicating instrument.

110

Low impedance

Z1

Z2 V2



The design of a potential divider is, essentially, the designof the high-impedance section and high-voltageresistances described in Section 5.4 (Ammeter in serieswith High-Impedance) can be used as a resistancepotential divider.

111



A resistance potential divider for the measurement of upto 1.25 million volts d.c. was described by Kunkte. Theresistance 1500 M,Q was made from 2000 units ofcarbon resistances 0.75 M,Q each. Across a tap on the

1500-V electrostaticlow voltage end was connected avoltmeter. The divider ratio was adjusted at 1:1000 andthe voltmeter scale was marked to read directly 1500 kV.The resistor was immersed in oil and a pump was used tomaintain a continuous circulation of oil. Under full-loadconditions the resistance value was within 1.5 %. Theaccuracy of measurement was about 1 %.

112


6. Potential DividersA very precise potential divider for measurement of directcurrent voltages of about 50 kV with an

and

accuracy of0.01%effect

has been designedcurrents

by Rymer Wright.partly

by

Thetheof leakage was eliminated

use of a comprehensive system of shielding circuitssupplied by an auxilliary network across the main h.t.supplycircuit

and partly by the use of a Wheatstone bridgewhich enables the potentiometer ratio to be

measured under operating conditions, while the highpotential is applied. The apparatus has been used forprecise measurement of the voltage in experiments onelectron diffraction.

113



The elaborations required in the proper design of an a.c.high-voltage resistor have led to

be used asthe

adevelopment of

capacitors which can high impedanceelement, either in a bridge circuit or in a potentialThe advantages of a standard air-capacitor over

divider.a high-

voltage resistor are the relative simplicity of itsconstruction, the ease with which it may be shielded fromstray capacitance effects, and the absence of heating.

114


7. Dividers for Impulse VoltagesA divider for recording high transient voltages may consistof resistors or capacitors or combinations of both. Theessential requirement is that the wave shape of thevoltage to by measured should be faithfully reproducedon the oscillograph with a reduction ratio which can beaccurately known.

115


7. Dividers for Impulse Voltages

The chief sources of error common to allare:

types of dividers

1) Residual inductance in any resistance orcapacitive element.

2) Stray capacitance: (a) from any section of theanydivider to the high voltage lead, (b) from

section of the divider to ground and (c)sections of the divider.Impedance drop in the connecting leadthe divider and the test object.

between

3) between

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4) Impedance drop in the ground return lead fromthe divider resulting from extraneous groundcurrents flowing in this lead.

5) Oscillations in the divider circuit causedterminal

bytocapacitance from divider high-voltage

ground and lead inductance.

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7.

effect

Dividers for Impulse Voltages

The of residual inductance becomes verywherepronounced

the voltagein the fractional microsecond range,applied to the test object is increasing at a

rapid rate at the instant when flashover occurs. Thecurrent through a resistance potential divider is likewiseincreasing at a corresponding rate and the residualinductance in each element of the divider will generate anL(dI/dt) voltage which is superimposed upon the IR drop.

118



The simplest type is a resistor divider which is often1/50acceptable for recording the standard impulse of

μsec wave. When the duration of the surge is less than Iμsec a resistor divider may give large errors due to straycapacitance. The error is a function of the product of the

earthresistance of the high-voltage arm and thecapacitance.

119



The layout of the test object and the divider needs specialattention to minimize the recording error. The test object is generally located not very close to the divider in order to avoid the possibility of the divider distorting the fieldnear the object. Under these circumstances the

theinductanceconnectionserrors.

ofto

thethe

looptest

formedobject

by the divider andmay cause considerable

120

Measurement of High-Voltages7. Dividers for Impulse Voltages

When the divider is constructed from pure capacitors,response is perfect on fast as well as slow transient;

thethe bedivider is independent of frequency. An error may

introduced due to the earth capacitance.The high cost of pure high-voltage capacitors makes aresistance divider preferable; the latter, however, isdifficult to shield and its frequency response is limited.Also the circuit arrangement incorporating a delay cableintroduces complications.

A mixed divider of parallel arrangement can be obtainedby adding a series of capacitors in parallel with resistor units.

121



This arrangement behaves as a resistor divider for slowtransients and as a capacitor divider for fast surges. Thedelay cable, with such a divider, can be terminated with aresistor.In the series arrangement of the mixed divider, the resistorunits are connected in series with capacitor units. Thisarrangement is suitable for tests involving superimposedpower frequency and surge voltage measurements.It has the advantage of having finite impedance at infinitefrequency. The response of such an arrangement on fastsurges corresponds to that of a pure resistor divider.

122



One of the simplest types of apotentiometer connected to aIf the high-voltage resistance

divider is a plain resistancecable as shown in Fig. 24.has no distributed capacity

and no self-inductance, the above arrangement willrespond satisfactorily to all transients.The possible sources of error in a cable type divider suchas shown in Fig. 24 are as follows: (1) the terminalresistor R ≠ Z (surge impedance), due both to errors inadjustment and to the fact that the concentric cable is nota true surge impedance invariant to changes of waveshape; (2) capacitance of the measuring circuit;

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Fig: 24.

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8. Mixed Divider

It can take the form of either parallel or seriesarrangement of capacitors and resistors. The equivalentnetwork of the parallel arrangement is that in which C: is infinite and its response can be obtained by neglectingthe self-inductance L’. From the response terms, too

the

fast

lengthy to be included here, it can be seen thatarrangement behaves as a capacitor divider withtransients and as a resistor divider with slow transients.

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8. Mixed Divider

Fig: 25.

126

Measurement of High-Voltages8. Mixed Divider

Fig: 26.

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9. Delay Cable

The low-voltage arm of a potential divider is connected tothe oscillograph deflection plates by means of a coaxialcable.

128

H.V

Z1

Delay Cable

Z2 CRO


9. Delay Cable

The length of the cable varies between a few meters and100-200 m, depending upon the time delay required

arrival of

between the operation of the generator and thethe signal to the plates.

The two main types are the air-cored cable and thepolythene cable. In air-cored cable, the surge voltagetravels approximately at the speed of light and a cable of100 m length gives a delay of, 0.333 μsec which issufficient for the C.R.O. time base to be triggered.

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9. Delay Cable

In a polythene cable, the velocity of propagation would beslower, being inversely proportional to the square root ofthe permittivity, and therefore. a shorter length of cable can be used to provide the same time delay.

The delay cable causes distortion and attenuation in therecorded wave and a study of the response of air-spacedand polythene cable shows that, in order to avoidappreciable errors, air-spaced cable should be used onfast surges.

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9. Delay Cable

An ideal cable is onesurge impedance does

which has no losses and whosenot vary with frequency. However,

all cables have some attenuation and their surgeimpedance varies with frequency. The sources of cablelosses are the resistance of the

andcentral conductor and

outer sheath and conductancethe insulating medium used.

dielectric hysteresis in

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9. Delay CableLosses are usually expressed as attenuation in deciblesper 100 ft at a fixed frequency and in general the

theand

logarithmlogarithm

of attenuation increases directly withvalu

eof frequency. The error in the peak

wave form caused by cable attenuation can be calculatedby using the known values of attenuation at variousfrequencies.

This method is, however, time consuming and has tobe repeated for each different wave shape of theapplied voltage.

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HV Engineering

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