IMPULSE VOLTAGE INSTALLATIONS - Research | … Bound...the impulse generator were demanded. ... we shall briefly recall the principle of the Marx ... OCTOBER 1938 IMPULSE VOLTAGE INSTALLATIONS

306 PHILIPS TECHNICAL REVIEW Vol. 3, No. 10

IMPULSE VOLTAGE INSTALLATIONS

by W. HOND lUS BOLDINGH.

By means of standardized components a series of generators may be build to give impulsevoltages of 800 kV to 4 million volts, and impulse energies of 3.2 to 80 kilowatt seconds.The base of the apparatus is 2 X 2 metres, its height about 2 m/million volt. The con-densers have an energy content of up to 80 wattseconds per cubic decimetre. A switchmechanism makes possible whole or partial series-parallel connection of the condensersin a simple way, so that impulses oflowervoltage with the full energy can also beproduced.Installations for impulse voltages up to 1200 kilovolts can be transported ready for use.

Introduetion

Since an impulse voltage installation was describedin the first volume of this periodical I] considerabletechnical progress has been made in this field, andwe shall discuss the result in the present article.This development coincided with the increasinginterest on the part of power engineers in impulsevoltage research, for which higher and higher volt-ages and especially larger and larger capacities ofthe impulse generator were demanded.

The knowledge that a large percentage of dis-turbances in high tension mains, power plants andtransformer stations is caused by brief excess volt-ages due to atmospheric discharges has emphasized •more and more the desirability of impulse voltagetests, of high tension installations ready for useas well as their separate elements. The greatcapacity toward earth of long transmission lines, ofthe high tension windings of large transformérunits and of other test objects explains the demandfor large impulse capacity. The charge of thegenerator must be high with respect to that necessaryto raise the test object to the required potential; sothat no excessif voltage loss will occur during theimpulse.

Since as a rule the insulation of the test objectto earth must be tested, standard generatorswill be constructed exclusively with one poleearthed, but in such a way that they can givepositive às well as negative voltage with respectto earth.

In practice it is furthermore important that thewhole impulse installation can be moved to.any pointof the high tension mains to he tested, without thenecessity of elaborate assembly dismantling, so thatthe investigation need not be confined to the factoryor laboratory.For the examination of outdoor lines it is some-

times desirable to be able to use the full energy at alow voltage, which can be attained by connectingthe' impulse condensers wholly or partially III

1) A. Kuntke: Philips techno Rev.l, 235,1936. •

621.319.5

parallel. Then with the maximum energy an impulseof low voltage and high current can be produced,in order to burn through with this so-called surgecurrent a defect caused with high tension, and thusbring it to light.The above considerations led to the design of

the impulse voltage generators to be described inthis article. Although they naturally differ in manyrespects from the generators for high constant volt-age which were developed for research on the,problems of nuclear physics 2), there are never-theless many common structural features. The chiefcommon characteristic is that by adequate designingof each composing part the main dimensions, evenfor very high voltages and energies could he keptwithin very narrow limits.

As for -the impulse generators, it was made pos-sible in this way to house an installation forseveral megavolts in a laboratory or factory hallof reasonable size, and on the other hand it wasmade' possible to transport installations for up to1'.2 megavolts in working order.

Scheme of the impulse generator. .

Before we go into the particulars of the circuit,we shall briefly recall the principle of the Marxcircuit which is the one' used here. A charginggenerator charges a number of condensers (the."impulse condensers") connected in parallel viasuitable resistances to a high voltage with the con-densers. As soon as a definite voltage is reacheda series of spark gaps break down and the impulsecondensers are suddenly connected in series. A volt-age is obtained ~hich is equal to the sum of thevoltages of the impulse condensers, which nowdischarge (again through suitable resistances}. Adamping resistance prevents the occurrence of anyoscillations on the voltage wave.Even .when we confine ourselves to unipolar'

2):A. Bo~wers 'and A. Kuntke: Z. techno Phys. 18, 209,1937; cf, Philips tec~. Rev. I, 6, 1936 and 2, 161, 1937.

- +r=: .~- ":-"--'~-~-'-'r'~---- (OCTOBER 1938 IMPULSE VOLTAGE INSTALLATIONS 307

impulse generators, there are in general two funda-mentally different possible circuits for the charginggenerator: the symmetrical and the uni-polar circuit.Fig. 1 a and b give diagrammatically the principle

Fig. 1. Fundamental scheme of a uni-polar impulse voltage in-stallation (four stages) a) with symmetrical charging generator,b) with uni-polar charging generator. T = .high tension trans-former, Cl = first impulse condenser, Cc = end condensersRc = discharging resistance. The test object is connected at P.

.of these two' cases. (For the sake of simplicity thedamping resistance is not drawn here). The sym-metrical circuit is more often used, and from thestand point of insulation it.is also thc simpler.If the high- tension transformer T gives a peak volt"age Es then with the symmetrical charging gener-atör (fig. la) each of the two supply connections isonly at the potential Es (either plus and minus)with respect to earth. With the uni-polar charginggenerator, on the other hand, where the chargingvoltage 2 Es is obtained by cascade arrangementof the two rectifier valves (fig. lbh the supplyconnection is at the full charging potential 2 Eswith respect to earth. Therefore with the uni-polarcharging generator more insulation is requiredespecially, since the .polarity must be able to bereversed, and thus both terminals must be insulatedfor the full voltage of 2 Es.

Nevertheless a uni-polar charging generator waschose~ fo~ the Philips impulse voltage installations,since it has various advantages. .1) Part of the charging resistances can also serve as

discharging resistance (thick lines in fig. lb).~part from a saving in m~terial by the omissionof the special discharging resistance Re (fig. la)this has the advantage that there is no largecurrent loop during the discharge: each conden-ser _is discharged through its own dischargingresistance. The external inductance which in-

• -creases with the area of the current loop is inthis way reduced to a minimum.

2) No spark gap to earth is necessary (cf, for con-trast fig. la.), since the first condenser Cl isalready uni-polarly earthed even during charg- 'ing. In' addition to the centre tap of Cl' thespecial arrangement for regulating the sparkgap to earth, which ;must always be adjusted tohalf of the sparking potentlal of the other sparkgaps, may also be omitted ..

By the abolition of the spark to earth an imp 01'- ,

tant source of oscillations on the voltage wave isavoided, since Cl is not first earthed by the breakdown of the spark earth. Both of the argumentsmentioned above contribute to obtain a smoothvoltage wave without using large damping resist-ances causing excessive voltage loss, or any othercomplication. ,, In designing an impulse generator the voltage ofthe charging generator, i.e. the voltage per stage, isvery, important, since it determines the number ofstages for the total voltage required. The charging"generator in the Philips impulse voltage instal-lations gives a maximum. charging voltage, of200 kilovolts. It is so generously designed thateven at the highest impulse energy the desiredrapid succession of impulses (for instance every10 seconds) can be obtained. The charging gener-ator, as well as the impulse generator itself, ismounted on a 'base plate, which may be providedwith wheels, so _that it can easily be transported,Because of the uni-polar circuit, in which the fulldirect voltage of 200 kilovolts with respect to earthis obtained, the charging generator by itself (se-parate f~om the impulse generator) may verywell heused for other purpose~, such for exampleas the testing of cables.

Main compónents of the installations

In the construction of the impulse' generatorsto be described here the use of "high-tensionPhilit~", 'which has already been described in thisperiodical 3), has been of great value. This insu-lating material has a break-down strength. of· 30kilovolts/mm, and may therefore be compared withthe best high-tension porcelain. It has thereby theadvantage of very small dimension tolerances sothat it, can be made with screw-threads. "Philite"is used in the Philip's impulse generators chieflyin the, form ofcylinders, with or without a bottomand with or without a flange, which may be screwedtogether and to other composing elements. The

3) L. L. C. Polis: Philips techno Rev. 3, 9, 1938.

308 PHILlPS TECHNICAL REVIEW Vol. 3, No. 10

cylinders are made in two sizes: with a diameter(not including the flange) of 18 and 30 cm and aheight of ll.l and 22.2 cm respectively, and usedfor insulating supports, condensers (with the ne-cessary number of condenser units connected inseries simply piled up in the "Philite" cylinder),for resistances, etc.

Condensers

The impulse condensers are naturally the mostimportant parts of the impulse generator. Theirshape and size determine the design of the wholeinstallation. The fact that great progress has beenmade in this field is obvious when the apparatusdescribed here is compared with that of two yearsago. Then condensers for 150kilovolts were used witha capacity of 0.01 [LF;the energy content (1/2 C.V2)per stage therefore amounted to 112.5 W sec. Thelength was 30 cm, while the diameter was 18 cm,so that 15 W sec per dm'' is taken up. This wasthen a very high figure.At present condensers for 200 kilovolts maximum

working voltage are used with a capacity of 0.125[LF,which can therefore take up 2500 W sec. witha length of 48 cm and a diameter of 30 cm. With acondenser length of 70 cm these values rise to0.2 [LF and 4 kilowatt sec., which corresponds to80 W secldmr, These condensers, whose lengthand therefore energy content can be made stillgreater, can easily meet present requirements as toimpulse energy at high tensions, withont it beingnecessary to connect several of them in parallel.

For impulse generators with smaller energy, forinstance for educational purposes, smaller conden-sers are used (see in table I at 800 and 1200 kilo-volts). They are built up out of the previouslymentioned "Philite" rings 18 cm in diameter. Afive-ring condenser of this type for 200 kilovolts

b caFig. 2. Impulse condensers for 200 kV. a) 0.04 fLF, 800 W sec;b) 0.125 fLF, 2.5 kW sec; c) 0.2 fLF, 4 kW sec.

has a capacity of 0.04 [LF,i,e. an energy content of800 W sec. Such condensers are used especiallyin constant voltage cascade generators.

The three types of condensers for 200 kilovolts,of 0.8, 2.5 and 4 kW sec respectively, are shown infig. 2.

A remarkable characteristic of these condensersis that they are short circuit proof. They canbe short circuited in the fully charged statewithout any series resistance. If this is done witha condenser as in fig. 2b in which 2500 W sec arethereby transformed into heat (600 cal) in a fewmicroseconds, and if the discharge voltage is re-corded, it will he seen from the damped oscillationwhich appears that the internal resistance of thecondensers is less than 0.5 Ohm, and the induct-ance at the most 0.7 [LHat a frequency of about106 els.In order to be able to obtain the required steep-

ness of the wave front, it is desirable to employa so-called end condenser. The duration of thewave front, ts, which the voltage at the test objectneeds to reach its maximum value, is determinedapproximately by the relation:

(1)

It will be smaller the smaller the total capacity Cpin the circuit, and the smaller the total dampingresistance Rd, In order to test all objects withabout the same standardized duration of the wavefront ts, the latter could be regulated only bychanging the damping resistance. With test objectswith small capacities a large damping resistancewould then be necessary. An upper limit is, how-ever, prescribed for the damping resistance in con-nection with the permissible voltage drop: Rdmay not amount to more than 10 per cent of thedischarging resistance Re. The latter is alreadylimited: its value is determined mainly by thestandardized value of the time to half value of thewave tail th, and by the capacity Cs+ Cp of thegenerator (impulse capacity and load capacity)according to the formula:

In generators with a high impulse capacity thereforethe discharging resistance as well as the dampingresistance must be small. The smaller the latterresistance can be made the better, from the stand-point of voltage loss.It is therefore preferable to regulate the time ts

by the addition of the end condenser already men-tioned. This may then have so large a value thatthe capacity of the test object has little influence

,OCTOBER 1938 IMPULSE VOLTAGE INSTALLATIONS 309

within wide limits on the time tso The t.otall.oadingcapacity, however, may also not be t.oo great sincethis also causes v.oltage drop. 20 per cent of theimpulse capacity is accepted as a maximum value.The end condenser is then also always considerablysmaller than the impulse condenser. In the Philipsimpulse voltage installatione it consists of units ofsmall diameter, for instance for 400 kilovolt.s,which may be connected in series or in parallelas desired. With test objects of large capacity theend condenser can of course be omitted.

Resistances

While formerly liquid or carbon resistances werecommonly used because of the high value of theresistance necessary to obtain the n.ormal waveshape with the small impulse capacities then inuse metal resistances can and must always be usednow with the so much greater impulse energies.The resistance necessary for charging, discharging,damping and measuring are all mounted in dust-proof insulating material, and are of such gener.ousdimensions that no excessive heating occurs, evenwith the highest impulse energies and short impulseintervals.While the resistances which serve exclusively for

charging may be wound in the ordinary way, thesmallest possible inductance must be attained forthe damping, discharging and measuring resist-ances. The damping resistance which, as wehave seen, must be small in connection with thevoltage drop, must on the other hand be largeenough to insure the non-periodicity of the circuitCs - Rd - Cp' For this the following conditionis valid as an approximation:

. . . (3)

where L is the total inductance of the circuit inquestion. It was possible to reduce self-inductionof the damping resistance to such a low value thatits resistance value can even be chosen smaller thanthe permissible limiting value of 10 per cent ofthe discharging resistance.The total damping resistance is subdivided III

such a way (see fig. 4) that each impulse condenser isconnected .in series between two equal sectionsof it. The damping is hereby so large that nohigh frequency oscillarions on the main wave canoccur in each separate stage due to parasitic extracapacities.

In order to be able to discharge the impulsecondensers under all circumstances even withoutan impulse on the test object, an earthingr esis tan ce is built into the charging generat.or

(not shown in fig. I). This resistance has suchdimensions that if necessary it can take up the fullenergy of all the impulse condensers together. Theearthing switch earths the whole installation auto-matically when charging is interrupted.

Construction of the impulse generator

The impulse generator consists of four columns, ineach of which condensers and insulators are mount-ed alternately one above the other. The conden-sers are connected through resistances in such away that the Marx circuit is built up in the formof a screw (seefig. 3). With this method of const.ruc-rion any number of stages may be completed, and

Fig. 3. Impulse generator with a impulse energyof 25 kW sec,commutable for 2000, 1000and 200 kV (with 0.0125,0.05 and1.25 [lF respectively). Height 4.5 m, base 2x2m. In this typethe large "Philite" cylinders are not yet used, so that thecondensers may easily be distinguished from the insulators.

310 PHILlPS TECHNICÀLREVIEW Vol. 3, No. 10

others added later, without increasing the neces-sary floor space, which is always 1x2 m. For reasonswhich will discussed later, the preferred number ofstages will be as a rule a multiple of four, and there-fore the impulse voltage will be a multiple of800 kilovolts. In table I (page 316) the data aregiven of a series of impulse generators for differentvoltages, assembled with the three types of con-densers shown in fig. 2.

The compact construction of the generators,which is illustrated by the main dimensions given-in the table, has besides to purely spacial advant-'ages the additional satisfactory' result that, asidefrom the above-mentioned reduced inductance ofthe damping resistances, the remaining inductancein the circuit in question is also small.

The damping resistances are mounted at thetop and bottorn of every impulse condenser, andeach has a switch arm. In this way every' conden-ser is provided with two switch arms mountèd atto .top and the lower one of which serves as sparkgap, while both together can connect the condenserin parallel with its predècessor. The spark gapscan be adjusted by remote controlled simultaneousmotion of all the spark gap arms by means offive insulating bars' (see in fig. 3 in the middlebetween the four columns). The middle one of thesebars always serves the last, highest spark gap, theother four serve the other spark gaps and at thesame time they serve to connect the impulse con-densers wholly or partially in parallel by means ofthe second series of switch arms which are coupledin pairs with the spark gap arms.

Possibilities of commutation

Inftg. 4 the different possibilities of commutationare drawn for the case of a generator with 8 stages,i.e. for 1600 kilovolts. The switch arms with thesame numbers, see fig. 4a, are controlled by a com-inon bar (due to the screw-shaped construction.they lie vertically above each other). By means ofbar I condenser 1 is connected in parallel with 2,and 5 with 6, while by bar Ill, 3 is put in parallelwith 4, and 7 with 8, see fig. 4b. At half voltage a

, fourfold capacity is now obtain:ed. If now by meansof bar II condenser 2 is put in parallel with 3 and6 with 7 (fig. 4c) there are then two groups e'ach offour condensers in parallel, so that 1/4 of the totalvoltage' can be reached (400 kilovolts) with 16 timesthe original capacity. Finally by means of har IV4 and 5 can also be connected in parallel (fig. 4d)so that a surge .voltage of 200 kilovolts with 'acapacity 64 times the original capacity is obtai~ed.It is clear that all the possibilities of commuta-

tion can only be utilized at nominal voltages whichare a multiple of 800 kilovolts, i.e. when the numberof stages is a multiple of four. Table I gives an idealof the voltages and' capacities which may be ob-tained byicommutation.

Fig. 5 is a photograph of a commutable impulsegenerator for 16 kW sec in the present form with

1600kV

Ce

b)

d)

Fig. 4..Diagram ofan impulse generator which can be commutedfor voltages of 1600, 800 400 and 200 kV with the full surgeenergy in each, case. The circuit which is actually built up inthe form of a screw is shown here "unwound"; The switcharms I, 11, ... are operated by fivé bars "(visible in fig. 3). Thecharging' generator is connected at' L. Ra = dampiitg resist-ances, R. ~ discharging resistances, Rm = measuring resist-ances, Cc = end condenser, Ch = auxiliary condenser, Rh =auxiliary end resistance. The cathode ray oscillograph is con-nected at O. a) All condensers, 1,2, ... 8, in series. b) Switcharms I and 11 closed: four groups of two condensers inparallel, 800 kV. c) Switch arm II also closed: two groups offour condensers in 'parallel, 400 kV. d) Switch arm IV alsoclosed: all eight condensers in parallel, 200 kV. The ignitionspark gap is indicated in each case by a zigzag arrow.

OCTOBER 1938 IMPULSE VOLTAGE INSTALLATIONS 311

Table I Data of the Philips impulse generators

Nominal voltage 800 4,0003200 JI-4-00-/200_ ':--6-0-0/-2-00--;C-18-0-0/-4-00-/'2-0-01200/600/2001

1

1600/800/2002000/1000/200

4, 6 I 8 12 I 16 20

kWsec 1 ~.~ 1-4-'.-0-:1

;'-0-.8---,-2.-5----,-4-,.-0+--2.-5-.-4-'.-0-,1-2.-5-------,--4.0II~~I 2.5-

fLF__ 0_.040.125 0.2 I 0.04 0.125 0.2 0.125 0.2 0.125 0.2 0.125 0.2: _0_._12_5_

kV

Commutation voltagesStages of 200 kV

kV

Energy per stage .Capacity per stage.

1200 1600 2400

4.00.2

3.2 10 161650

4.8

3.4 I 3.74.91 5.72100 2200

kWsecIII 103 pF 10 30in 103 pF 40 125 200

III 103pF I I

III 103 pF 1 160 500 800 240 750 1200 1000

il 2.312.3 2.6 3.0 3.03.3 3.3 3.6 4.5 4.5

11200 1700 1800 1500 1950

Total energy .Total capacityCap. at half voltage.Cap. at quarter voltage.Cap. at 200 kV

kg

HeightMin. height of roomWeight

m

m

large "Philite" cylinders. The dimensions indicatedin table I refer to this type.

The switch arms are controlled electrically fromthe control desk. The desired series parallel circuitis obtained after moving a handle in the base plateof the generator which can only be operated whenthe installation is completely earthed.

In order always to ohtain the same form ofimpulse wave in agreement with the formulaementioned, even with impulse condensers in parallel,

Fig. 5. Impulse generator with an impulse energy of 16 kWsec, commutable for 800, 400 and 200 kV (with 0.05,0.2 and0.8 fLFrespectively) in the present form with large "Philite"cylinders. Height 2.6 m.

15 2433

20 32 30 48 40 6425 10 17 8 12.5100 40 70 30 50400 170 270 125 2001600 1500 2400 2000 3200

50 806.25 1025 40100 160

2500 4000

20727 85 130 62.5

250

7.4 I I 9.014.0

4.2 I1 5.16.2 8.1

1240012700 3700 4200

5.8 6.5 7.913.08.8 10.5 11.4

3000 I 3200 3600

the values of the discharging resistance Re, dampingresistance Rd and the load capacity Cp must beadapted to the larger impulse capacity Cs obtained.

In order to retain the original time of halve valueof the wave tail with impulse condensers inparallel, the value of the discharging, (end) resist-ance must be reduced according to formula (2).From fig. 4 it is seen that when the impulse con-densers are put in parallel, part of the dischargingresistance (which are also the charging resistances)is short circuited. However this is not sufficient, theremust also be an auxiliary end resistance (in fig. 4bindicated by Rh) in parallel with the tcst object.

Since the value of the damping resistance, asexplained above, may not he more than 10 per centof the end resistance, the damping resistance mustalso be reduced upon commutation. As may be seenfrom fig. 4 the parts of the damping resistance areautomatically put in parallel, together with thecorresponding impulse condensers, By this changein Rd, however, according to equation (1) the frontduration ts of the impulse wave is affected. In orderto retain the normal value of ts the end condensermight to he divided in the same way as the dampingresistance and the parts connected in parallel. How-ever it is often simpler to add an auxiliary conden-ser (indicated by Ch in fig. 4b) such that the originalvalue of Cs is recovered. The auxiliary condenser Chand the auxiliary resistance Rh are not combinedin a single unit, in order to be able to omit theauxiliary condenser if necessary with test objectswithlarge capacities, or to replace it by a smaller one.

The auxiliary end resistance which is used in thecircuit for 200 kilovolts must be able to take uppractically the entire impulse energy with therequired impulse intervals.

312 PHILlPS TECHNICAL REVIEW Vol. 3, No. 10

Measurement of the voltageFor the measurement of the course ofthe impulse

voltage the cathode ray oscillograph is now generallyrecognized as the only suitable instrument. It givesnot only the crest value, but makes it possible tofollow the entire course of the voltage wave. Theindication mast of cource be a faithful reproduetionof the phenomenon to be measured. Since we arehere concerned with a single non-periodic phenom-enon of very short duration it is preferable to usea resistance potentiometer above a capacitive one,such as is sometimes used.It is obvious that the measuring resistance may

only have a minimum inductance. The ordinarymethods of measuring this latter cannot be usedbecause of the high damping. For the determinationof self-inductance of the measuring resistance amodel is constructed in which the resistance wirewas replaced by copper wire, still retaining thegeometric form (which determines the inductancein the first instance).In how far a true reproduetion is obtained with

the measuring resistance used may be seen mostclearly from an oscillogram of a breakdown at thewave crest of an impulse, as is given in fig. 6together with an oscillogram of a normal wave.

Besides the impulse voltage itself the chargingvoltage mayalso be measured. Thanks to the uni-polar circuit of the chargiqg generator it is.£g,~b..!!:.,.by means of a very large- series resist.ance-to .indi- .cate the charging volta~:iof thc iP1}:mls~~e~se~~-directly with a voltmetJr' ()n_th~ control de's~·:This .

L"!"'::: -: ;,:., ;:.:, . " .'

gives a more complete and more accurate check onthe voltage during charging, than the measurementof the length of the spark, by which only the peakvalue of the charging voltage can be determined.

a)

b)

c)

d)28755

Fig. 6. a) Oscillogram of a normal1.0/50 wave (duration ofthefront ts = 1.0 IJ. sec, time to half value of the wavetail til50 IJ. sec). b) Oscillogram of the same wave with breakdownats-the-cresse -c~ .Like a). with larger time base. d) Like b) withlarger time base ..The time. base is obtained by the exponentially.d_~cr~~si~~ vo1ta,t;e during discharge ?f a cO~ldeJ?-ser,in c thetIl:n' 'IS'-reco ded by U1$ns of a sinusoidal oscillation of 106 cis,'in which -thcrefore one period indicates I I.L sec. '

Fig. 7. Transportable impulse voltage installation with an impulse energy of 10 kWsec for 800 kV com-mutable for 400 and 200 kV. Surface of the trailer 2.20,4 m", total height from the earth 3.30 m.