IEEE TRANSACTIONS ON POWER APPARATUS AND SYSTEMS, VOL. PAS-89, NO. 7, SEPTEMBER/OCTOBER 1970

measured can have a profound influence on the behavior of ameasuring system.

3) If the output voltage of a measuring system is smooth, thetest voltage will likewise be smooth and the errors in measure-ment of the test voltage will be those due to the response time ofthe measuring system.

4) The response obtained by connecting a zero-impedance stepgenerator to the measuring system through a resistance approx-imating the internal impedance of the impulse generating systemgives a good indication of the time it will take for the testvoltage and measured voltage to come together. Under nocircumstances will the test voltage and measured voltage be thesame in a practical system until after a period of twice the traveltime of the measuring system lead.

REFERENCES[1] F. C. Creed and M. M. C. Collins, "The measurement of short-

duration impulse voltages," IEEE Trans. Communication andElectronics, vol. 82, pp. 621-629, November 1963.

[2] F. C. Creed, T. Kawamura, and G. Newi, "The step response ofmeasuring systems for high-impulse voltages," IEEE Trans.Power Apparatus and Systems, vol. PAS-86, pp. 1408-1420,November 1967.

[3] F. C. Creed and M. M. C. Collins, "Transient impedance ofhigh-voltage impulse generating systems," this issue, pp. 1387-1393.

[41 Handbook of Analog Computation. Electronic Associates Inc.,Publ. L 800 0001 OA, p. 228, 1964.

For Combined Discussion see pp. 1391-1393.

Transient Impedance of High-Voltage Impulse

Generating SystemsFRANK C. CREED, SENIOR MEMBER, IEEE, AND MALCOLM M. C. COLLINS, MEMBER, IEEE

Abstract-The interaction between an impulse generating systemand an impulse measuring system is dependent on the reflectionsthat occur at the generator and divider ends of the measuring systemlead. In this paper the impedances that cause these reflections arestudied and discussed. The effects of the interactions are illustratedwith sample oscillograms.

INTRODUCTION

T HE interaction of the various components of an impulsegenerating system (including the impulse voltage divider)

was discussed in [1], where the effect of the generator impedancewas shown using simplified mathematical models. Since an actualimpulse generator is a very complex collection of resistors andcapacitors plus stray inductances and stray capacitances, it isnot practical to calculate the terminating impedance that agenerator presents to the measuring system lead. Furthermore,the physical size of the generator and the waveshaping com-ponents precludes their representation by their lumped com-ponent values and necessitates the use of traveling wave conceptsto explain the observed results. Because of these difficulties in

Paper 70 TP 27-PWR, recommended and approved by the PowerSystem Instrumentation and Measurements Committee of the IEEEPower Group for presentation at the IEEE Winter Power Meeting,New York, N. Y., January 25-30, 1970. Manuscript submittedSeptember 12, 1969; made available for printing November 3, 1969.The authors are with the Radio and Electrical Engineering Division

of the National Research Council of Cainada, Ottawa 7, Ont.,Canada.

Leod withSurge Impedonce Zo

toImpedane o osilloscopebeing tested Zo

Reedrelay I

Fig. 1. Test setup for impedance measurements.

the theoretical approach, an experimental technique was used todetermine the impedance of the generator as seen by waves re-

flected from the voltage divider. A side benefit of this approachis that the study of the interactions, as indicated by the reflec-tions, leads to various methods that may be used to improve thequality of the impulse voltages generated.

IMPEDANCE MEASURING TECHNIQUE

The impedance in which we are interested is the lead terminat-ing impedance that causes the reflection of an incoming voltagewave. The most direct approach to the measurement problem isto send a step voltage along the lead to the impedance understudy and to observe the reflection that returns. Fig. 1 illustratesthe experimental setup for doing this. With the relay open thesystem is charged to some steady-state condition by the battery.When the relay is closed (by a coil surrounding the relay andenergized from an audio oscillator), a step is applied to the leadwhich has a surge impedance Z0, through the resistor Zo. (Inthis particular test arrangement Zo = 470 ohms.) Any reflection

1387

IEEE TRANSACTIONS ON POWER APPARATUS AND SYSTEMS, SEPTEMBER/OCTOBER 1970

coming back to the input from the test object is absorbed by theresistor Zo to prevent further reflections and is also recorded onthe oscilloscope. The oscillogram will thus show the input stepfor twice the travel time of the lead length, after which it willshow the step plus the reflection. The accuracy of this system islimited since the surge impedance of the lead changes as itsheight is varied from that of the relay to that of the test object,but this limitation was considered to be negligible in the presentwork.

Interpretation of the results is quite straightforward whenconsidering lumped components in that a positive reflection (i.e.,in the same direction as the incident wave) indicates an imped-ance greater than Zo and a negative reflection indicates an im-pedance less than Zo. In addition, a reflection that increases in-dicates a capacitive impedance and a reflection that decreasesindicates an inductive impedance.When a test object is physically large, as is the case for most

high-voltage equipmenit, the time it takes for the voltage waveto travel through the equipment complicates the interpretation.In this case the surge impedance of the test object is the signifi-cant impedance until the wave has traveled from the input of thetest object to ground and back again. The importance of thiswill be seen in the next section where actual results will be dis-cussed.

IMPEDANCE MEASUREMENTS

Impulse Shaping Capacitor

The reflection from the capacitor used for shaping the impulsefront on a 1.2-MV 15-kJ impulse generator was studied. Thecapacitor is a series stack of eight 5600-pF capacitors having atotal height of 4.5 meters. The result for this capacitor is shownin Fig. 2(a) where the step ABC is the step applied to the lead.The lead is 9.1 meters long, so that the reflection from the capaci-tor returns to the input after approximately 60 ns (the velocity ofpropagation is approximately 0.3 m/ns for a conductor in an airdielectric) and is recorded starting at C. The initial reflection isnegative, showing an impedance less than the surge impedanceof the lead. At D, which is approximately 30 ns after the start ofthe reflection or twice the travel time associated with the height ofthe capacitor, the reflection starts a second decrease due to thereturn of a reflection from the ground end of the capacitor.The voltage does not decrease immediately to a minimum becausethere are several electrical paths through the capacitor and thewave will travel along each of these and be reflected from theground back to the top of the capacitor. At E the voltage finallyreaches a minimum of 0.15 times the height AB and then in-creases exponentially toward a final value of 2 AB, which corre-sponds to a reflection coefficient of +1. The initial surge imped-ance of the capacitor is determined from the portion CD. This isat a level of 0.84 AB, which gives a reflection coefficient of -0.16and corresponds to a surge impedance of approximately 350 ohms-the reflection coefficient is equal to (Zt.-mination - Zo)/(Zterminstioi + ZO).Analysis of Fig. 2(a) shows that the capacitor looks almost like

a short circuit after 90 ns, which is, in effect, a reflection of - 1returning from ground to the top of the capacitor at that time.If a parallel impedance that gives a reflection of +1 after 90 nswere added to the capacitor the two would tend to cancel, givinga reflection with less overall variation. Since the reflection fromthe capacitor starts to increase negatively after 30 ns the reflec-tion from the added lead should start to increase positively at

(a) (D)

Fig. 2. Reflections from impulse shaping capacitor: input stepalone ABC; step plus reflection from C onward. (a) Capacitoralone. (b) Capacitor with added lead.

the same time. With this in mind, a 13.5-meter open-circuitedlead was attached to the capacitor. This lead was made up ofthree sections, each approximately 4.5 meters long, the first beinga tube of 7.4-cm OD, the second a tube of 2.5-cm OD, and thethird a wire of 0.16-cm diameter. The tubes were chosen becausethey were available in the laboratory and were not cut specially.Each of these sections has a higher surge impedance than theprevious one, so that the positive-going reflection returned to thecapacitor increased in three stages to approximate the gradualincrease of the negative reflection from the base of the capacitor.Analysis of the reflection when this lead was added to the capaci-tor indicated that a resistor between the last two sections wouldbe beneficial, so 330 ohms was added and the resultant reflectionis shown in Fig. 2(b). This shows that two things have occurred:the initial impedance has been lowered due to the lead impedancebeing added in parallel, and the minimum impedanlce has in-creased so that the variation in impedance has been reduced. Theusefulness of this will be illustrated in a later section.

Impulse Generator

Fig. 3(a) shows the reflection from a 1.2-MV impulse generatorwith the impulse shaping resistors included but the impulse shap-ing capacitor removed. The reflection coefficient varies from-0.2 to +0.15, which corresponds to a surge impedance of from0.67 to 1.35 Z0. Because of the generator capacitors the re-flection coefficient must eventually go to (RT - ZO)/(?T + ZO)where RT is the wave-tail resistance of approximately 5 Zo;but the time constant is in the order of 20 )s, so it does not haveany effect in the time scale under study.

Basically then it can be seen that the generator is a reasonablygood match for the measuring system lead. Some tests were donewith all the series resistance of the generator short circuited anda single variable resistor added between the bottom stage of thegenerator and ground. By varying this resistor until there was noreflection from the bottom of the generator, it was determinedthat thegenerator surge impedance was about 350 ohms. Two othergenerators were also tested in this manner and values of 250 and300 ohms were obtained. To an incoming wave the minimumimpedance for twice the travel time of the generator will be thegenerator surge impedance; after that the impedance will de-pend on the generator series resistance. The series resistance isdependent upon the value of the impulse shaping capacitor (in-cluding the load capacitance) and can probably vary in the rangeZo/4 to 2Z.

Fig. 3(b) shows the reflection when the impulse shaping capaci-tor is added. In this case the reflection is initially similar to thatof the capacitor alone, but as the capacitor is charged the reflec-tion becomes similar to that of the generator. The maximumnegatiVe reflection coefficient is about 0.8, which corresponds to

1388

CREED AND COLLINS: TRANSIENT IMPEDANCE OF IMPULSE GENERATING SYSTEMS

(a) (b) (c) (d)

Fig. 3. Reflections from impulse generator: arrow indicates start of reflection. (a) Without impulse shaping capacitor.(b) With impulse shaping capacitor. (c) and (d) With compensating lead added to impulse shaping capacitor.

Fig. 4. Reflection from impulse voltage divider: input step notshown; arrow indicates start of reflection.

an impedance of ZO/8. When the extra lead is added, as was donein the previous section, the reflection is as shown in Fig. 3(c).Here the maximum negative reflection coefficient has decreasedto about 0.7, which corresponds to an impedance of ZO/5. Fig.3(d) shows the same reflection as in Fig. 3(c) but on a ten timesslower sweep speed. This illustrates that the generator appears

briefly as a low surge impedance and then settles down to almosta matching impedance that gradually increases as the generatorcapacitors are charged.

Impulse Voltage Divider

The divider used, a shielded resistance type, is one describedby Creed [2]. The reflection is shown in Fig. 4 starting at thearrow. In this case the input step, which is five divisions in ampli-tude, is not shown. As the wave travels in the divider the apparentimpedance decreases to approximately ZO/3 (the reflection co-

efficient equals -1/2) after twice the travel time of the dividerheight. At this time the reflection starts to increase on an ex-

ponential toward a final reflection coefficient of (Rt - Zo)/(Rt +ZO) where Rt is the resistance of the divider (or terminator).It may at first be surprising that the reflection coefficient onlygoes to - 1/2, but this is due to the short time constant forcharging the capacitance of the shielding rings. In other words,the capacitance charges to one-half the value of the step in twicethe travel time of the divider height.

COMPARISON MEASUREMENTSIn order to illustrate the effects of the generator impedance

three different measuring systems have been used to make com-

parison measurements. The step responses of the three systemsare shown in Fig. 5. System 1 is a three-component system [3]having a resistor at the lead input that matches the surge im-pedance of the lead. The divider is a shielded resistance type withcompressed-gas insulation. This system is used as the referenceand its step response, as shown in Fig. 5(a), is nonoscillatory, andhas virtually settled down in 12 ns. The other two systems are

two-component systems and use the same divider as was tested in

the previous section. System 2 has a 2.6-meter lead, and its stepresponse, which is highly oscillatory, is shown in Fig. 5(b). Thisstep response settles in approximately 2 pus. System 3 has a 10-meter lead, and its step response, shown in Fig. 5(c), settles inapproximately 3.5 As.Comparison measurements made with different generators are

shown in Figs. 6-8. In each figure oscillogram (a) shows the out-put of measuring system 1 when it alone is connected to thegenerator, oscillogram (b) shows the output of measuring system1 when either measuring system 2 or 3 has been added in parallel,and oscillogram (c) shows the simultaneous output of the addedmeasuring system.

In Fig. 6, the reflection from the generator used is shown inFig. 3(b); the initial rise in voltage occurs when the generatorfires, putting a steeply rising voltage on the impulse shapingcapacitor which initially appears as a surge impedance. When thecapacitor reflection has reached its minimum, i.e., when the ca-pacitor looks like a capacitor rather than a surge impedance,the output voltage drops to a minimum and then charges up onan exponential. When measuring system 2 is added, a smallchange can be detected in Fig. 6(b) which is due to the reflec-tions from the added measuring system. Comparing oscillograms6(b) and 6(c) it is seen that they are in good agreement exceptfor the amplitude of the steeply rising and falling portions of theinitial oscillation.

Fig. 7 is similar to Fig. 6 except that the extra lead has beenadded to the impulse shaping capacitor, giving a reflection fromthe generator as shown in Fig. 3(c). Comparison of oscillograms6(a) and 7(a) shows how effectively the added lead cancels out thereflection from the bottom of the impulse shaping capacitor andalso reduces the amplitude of the initial steep rise, thus makingthe overall wavefront appreciably smoother. It is again seen thatadding measuring system 2 has a sinall effect on the output volt-age as measu,red by measuring system 1. Comparison ofoscillograms 7(b) and 7(c) shows good general agreement betweenthe two systems, but system 2 accentuates the initial steep riseand subsequent oscillation.

In Fig. 8 the generator used comprised one-half the generatorand impulse shaping circuit used in Fig. 6. In order to accentuatethe effect of adding a second measuring system, system 3 withthe 10-meter lead was added. Comparison of oscillograms 8(a)and 8(b) clearly shows the oscillation introduced on thetest voltage by the reflections from the second measuring system.It can be seen that this oscillation extends right to the crestof the impulse. Looking at the output of measuring system 3 onoscillogram 8(c), it is seen that this measuring system accentuatesthe oscillation but that the oscillation dies out at the same timethat it dies out on the actual test voltage as measured by system 1and shown in oscillogram 8(b).

1389

IEEE TRANSACTIONS ON POWER APPARATUS AND SYSTEMS, SEPTEMBER/OCTOBER 1970

(a) (b) (c)Fig. 5. Measuring system step responses. (a) Three-component system, 3.5-meter lead. (b) Two-component system,

2.6-meter lead. (c) Two-component system, 10-meter lead.

Fig. 6. Comparison of impulse measuring systems. (a) Three-component system alone. (b) Three-componentsystem output, two-component system in parallel. (c) Simultaneous output of parallel two component generators.

(a) (b) (c)Fig. 7. Comparison of impulse measuring systems. (a)-(c) Same as in Fig. 6.

(a) (b) (C)Fig. 8. Comparison of impulse measuring systems. (a)-(c) Same as in Fig. 6.

CONCLUSIONS

1) The measurement of the reflection of a step from a piece ofequipment can be used to determine the transient impedance ofthe equipment, which in turn is useful in analyzing the behaviorof the equipment in impulse work. An understanding of thetransient impedance based on traveling wave concepts can leadto a method for improving the waveshape of an impulse generat-ing system.

2) It is shown that the addition of a measuring system to an

impulse generator causes a change in the output voltage. Thischange is due to the interaction of the generator impedance andthe reflections from the impulse voltage divider.

3) Simultaneous measurements have shown that oscillations'appearing on the output of a measuring system are also presenton the input but not necessarily with as large an amplitude. It is

also shown that the oscillations at the input and output die outat the same time, so that if the measuring system output is freeof oscillations the test voltage will also be free of oscillations.

4) It has been shown that the time for the step response of ameasuring system to settle to a steady-state value does not in-dicate how long it will take the measurement of the output volt-age of an actual generator to settle to a steady-state condition.

REFERENCES[1] F. C. Creed and M. M. C. Collins, "The systems concept in

generating high-voltage impulses," this issue, pp. 1383-1387.12] F. C. Creed, "The measurement of impulse waves chopped on the

front," CIGRE, Paper 320, 1958.[3] F. C. Creed, T. Kawamura, and G. Newi, "Step response of

measuring systems for high impulse voltages," IEEE Trans.Power Apparatus and Systems, vol. PAS-86, pp. 1408-1420,November 1967.

1390

CREED AND COLLINS: COMBINED DISCUSSION

Combined Discussion'2

Nils Hylten-Cavallius (Hydro-Quebec Institute of Research,Montreal, P. Q., Canada): To my mind, the reports crown a seriesof publications in the same field. When I first encountered theseauthors' work, their ideas (unfortunately) were presented under thename of the infinite line response. Many persons, myself included,then got the impression that these new findings were merely relatedto some subtleties concerning the measurement of impulseschopped on the front at extremely short times. Now we understandthat it is actually a method of taking account of the damping effectby the lead and the impulse generator impedance. This prescription ismuch easier to swallow and I believe that a response measurementwith a resistance incorporated in the step generator will find generalacceptance.

Manuscript received February 9, 1970.

F. A. Fisher (General Electric Company, Pittsfield, Mass): Theauthors in this' and in some of their previous papers have clarifiedmany of the points that must be considered by those of us attemptingto develop criteria by which to judge the merits of different impulsevoltage divider circuits. Although they have clarified some points,the authors have made some elements of our task harder.A part of our problem is that we have attempted to specify mea-

suring circuits in terms of their response to simple impulse waves.The fundamental measure of a divider system's performance hasbeen how well it responds to a step function input. Knowing the re-sponse it is a simple mathematical exercise to determine, via theconvolution integral, how that system would respond to other ideal-ized impulse waves. The most commonly analyzed wave has beenthe triangular wave, one rising uniformly to a crest and then decayingabruptly to zero. Some simple criteria have been developed inthe International Electrotechnical Commission (IEC) TechnicalCommittee and elsewhere to say what the step function response mustbe if the triangular wave is to be measured within certain limitationson accuracy.Now both a step function and its mathematical cousin, the trian-

gular wave, can only be produced by a generator of zero impedance.If any kind of load (other than an unattainable pure resistance) isconnected to a generator with an internal resistance greater thanzero, the voltage developed at the generator terminals will be differ-ent from the idealized open-circuit voltage of the generator. Theauthors illustrate this nicely in Fig. 31 where the voltage Vt. is shownto be considerably different from the ramp function voltage Vg.

This leads to my first question: If it is unrealistic to discuss theresponse of a divider system to a driving voltage that cannot be ob-tained on a real system, even theoretically, what sort of a drivingvoltage should we consider?My second question is: What kind of test can be made on a divider

system to give a simple measure of the divider's capabilities? The re-sponse to a step function is intellectually elegant and reasonably easyto obtain in practice. It suffers from the disadvantage that it mayexcite oscillations in the divider system that would not be excitedby an actual impulse circuit.

I see a possible way out of this dilemma and would like the authors'comments. The problems of the divider response criteria consideredby IEC Technical Committee 42 all seem to revolve around an un-fortunate choice of impedance levels, both actually used in the step

Manuscript received February 13, 1970.1 F. C. Creed and M. M. C. Collins, "The systems concept in

generating high-voltage impulses," this issue, pp. 1383-1387.2 -X "Transient impedance of high-voltage impulses," this issue,

pp. 1387-'1390.

response test and implied when we discuss response to triangularwaves. The impedance level considered in either case is zero. Can wenot pick a more realistic impedance level? Suppose in Fig. 21 wepicked a standard value of Zg, perhaps 350 ohms resistive, and wroteour specifications on divider performance around that value. Itwould seem that we could apply a step function voltage at the genera-tor and measure both Vto and the response of the voltage dividersystem. The modified step voltage at Vt. and the divider response tothis modified step could then be integrated to determine the responseof the divider if the generator were to produce triangular waves.The voltage at the test object would, as the authors show, not be atriangular wave, but we would at least be able to determine theactual test object voltage Vt. and how well the divider responds tothat voltage.No one value of Zg could truly represent the impedance of an actual

generator circuit, but almost any resistance would be more realisticthan the zero impedance now implied in our existing or proposedcriteria.

S. D. Northrup and J. J. Brado (Westinghouse Electric Corporation,Trafford, Pa.): The systems concept presented' certainly merits moreattention than has been given to it prior to this time. This is especiallytrue in large outdoor UHV impulse systems, where high-voltagelead travel times are frequently in the order of tenths of microseconds.Although many large outdoor systems differ from that of the authorsin the placement of. the test piece relative to the impulse generatorand voltage divider, the same type of analysis may be used. We alsofeel that significant reflections may occur at the terminal of the testpiece, depending on its impedance.The output waveforms produced by the authors' analog computer

simulation are in excellent agreement with those observed using the6400-kV 160-kJ impulse generator of Westinghouse at Trafford, Pa.The time to crest of the impulse voltage is varied by changing thegenerator resistance. As the generator resistance is decreased belowthe surge impedance of the lead wire (about 500 ohms), causing thegenerator reflection coefficient Kg to become negative, reflections doindeed occur. These reflections are observed in the form of oscillationsin the output voltage of the divider, and the frequency of these oscilla-tions relates to the length of the lead between the generator and thedivider.A problem often encountered in large impulse systems is that the

test piece and divider are set up too far from the impulse generator,and the generator resistance necessary to obtain the required risetime of the impulse voltage is less than that for which K,g is zero, caus-ing oscillations in the circuit due to reflections. For a given test pieceand divider capacitance, a maximum high-voltage lead capacitanceand corresponding maximum lead length exist for which a smooth im-pulse of fast enough rise time can be generated. If the high-voltagelead length is below this limit, the generator resistance can be madehigh enough that reflections from the divider and test piece will bedamped and the resulting test piece voltage will be satisfactory.

Manuscript received February 9, 1970.

Richard A. Fitch (Maxwell Laboratories, Inc., San Diego, Calif.):Fundamentally, electrical energy is carried by electromagnetic waves(and the power transmitted is given by the area integral of Poynting'svector f A E X Hn dA); the conductors serve merely to confine thewaves to the desired region. It follows that quasi-steady-state condi-tions cannot be established in times shorter than the wave propa-gation time through the system in question; and where discontinuities-exist, the system can exhibit a transient response for several propaga-tion times. All this is well understood, and it is at first sight surprisingthat it has not hitherto been considered in high-voltage Marx gen-

Manuscript received February 11, 1970.

1391

IEEE TRANSACTIONS ON POWER APPARATUS AND SYSTEMS, SEPTEMBER/OCTOBER 1970

Z- 400a

nF= ye

k -) J

GENERATOR INTERCONNECTION VOLTAGEDIVIDER

Fig. 1.

=600O

1.33 1.67 2.00TIME WUS)

Fig. 2.

Rg a600a

erators, where the dimensions imply propagation times of many tensof nanoseconds. The answer probably is that the problem has notyet become serious in practice because the required rise time is stilllong compared with the propagation times of most existing systems:although a system can exhibit transient behavior for several propaga-tion times it will do so only if significant energy is injected at wave-lengths comparable to or less than the characteristic system dimen-sion.

In selecting their model for analysis the authors have made a numberof tacit assumptions that tend to exaggerate the transient problem.For example, it is assumed that the load (i.e., the test object) can beneglected, that both the generator and the voltage divider have zeroinductance, and that there is no radiation loss (conductors can onlyconfine electromagnetic waves effectively for wavelengths larger thantheir separation). Moreover, a worst case is considered wherein thetest object is exposed to the wave before the latter can be modified bythe capacitance of the divider. If the positions of test object anddivider are interchanged in Fig. 11 then the higher frequency com-ponents from the generator will tend to be blocked by the divider andso be prevented from exciting oscillations in the line between thetest object and the divider. (In his reply to the discussion Dr. Creedrejected my assumption that the divider formed part of the wave-shaping network; yet he states, "It is normally recognized that thecapacitance and/or resistance of the divider forms part of the shap-ing network."' This statement is borne out by the numbers intypical installations and it is, moreover, difficult to see how one couldprevent the divider from affecting the waveshape-except by mak-ing it of such high impedance as to be unmanageable for recording.)

If one includes these factors in the analysis, the resulting trans-mission-line effects are markedly reduced. Nevertheless, the authors'point is both timely and relevant since the progress toward UHVtransmission systems is demanding ever bigger pulse generators whosedistributed behavior will become more and more marked if otherthings remain the same. But things need not remain the same: forthe past two years Maxwell Laboratories, Inc., have carried out anintensive development program for advanced high-voltage generatorswith the result that major improvements are now within the state ofthe art. In particular, significant reductions in size have been achievedwhich tend to offset the longer propagation times associated withhigher voltage pulsers, and a system of controlled spark-gap trigger-ing has been perfected which assists the generation of specific wave-forms.To illustrate the effect of including one of the factors neglected by

the authors (namely, finite generator inductance) we have analyzedthe circuit for a 2-MV generator as shown in Fig. 1 using a digitalcomputer. Analyses have been carried out for three idealized inputwaveforms: a step function, a ramp function, and a (1- cos) function.The resulting waveforms at the divider (the test object would beconnected beyond the divider) are shown in Figs. 2-4 with thegenerator impedance R0 as parameter. The circuit values have beenchosen to emphasize the transmission-line effects (i.e., L is less and ris more than in a typical 2-MV generator); nevertheless, even in thestep) function case, the waveform distortion is not severe and, withcontrolled generator waveforms, it is scarcely discernible.

Fig. 3.

Rg = 600Q

1.00

TIME (,US)2.00 2.33 2.67 3.00

Frank C. Creed and Malcolm M. C. Collins: The various discussershave significantly aided the authors in achieving their aim in present-ing these papers,1'2 namely, to get people thinking in terms of thecomplete impulse system rather than of the behavior of the in-dividual subsystems. For this the authors are highly grateful.Both Mr. Hylten-Cavallius and Mr. Fisher have indicated their

acceptance of the idea of taking a measuring system response with astep generator having an internal impedance of some value otherthan zero. Mr. Fisher has suggested that possibly a value of 350 ohmscould be used for this purpose. This matter needs more study, but atpresent it appears to us that it would be most desirable to obtainthe response with two values of impedance so that the limitations ofthe measuring system could be determined for the complete range ofgenerating systems with which it might be used. Two suitable values

Manuscript received March 27, 1970.---

.33 .67 1.00

VC

.50

.25~

1392

Fig. 4.

IEEE TRANSACTIONS ON POWER APPARATUS AND SYSTEMS, VOL. PAS-89, NO. 7, SEPTEMBER/OCTOBER 1970 1

for this purpose would be, say, 100 and 400 ohms. For any particulargenerating system, an appropriate interpolation could probably thenbe made, so that it would not be necessary to perform the test morethan once. This type of approach appears to be especially importantfor measuring systems having a large capacitance.Mr. Fisher's idea of recording both the input and output voltages

in this test is a good one. However, we do not think that it isnecessary to integrate the two voltages-it would be sufficient to deter-mine how long it takes for the two voltages to become and remainessentially similar. This could be for instance when the differencebetween the two voltages has settled down to less than 0.1 on thenormalized basis. It is then safe to say that the output voltage of themeasuring system is following the input, except for the response timeerror.Mr. Northrup and Mr. Brado and Mr. Fitch drew attention to the

ability of the test object to affect the shape of the impulse generated.This is quite true for any test object other than such simple ones asrod gaps. It then becomes necessary to take into account the testobject characteristics in the same manner as those of the generatorand measuring system. In this presentation it was tacitly assumed forthe sake of simplicity that the front shaping capacitor was the testobject.With regard to the comments of Mr. Northrup and Mr. Brado

about the lead length, it is quite true that there will be a maximumpermissible lead length for nonoscillatory impulses when the generatorresistance is less than the surge impedance of the lead. However, ifthe generator does match the lead, the lead length has negligibleeffect on the shape of the impulse generated unless it is long enoughfor the generator to discharge appreciably during twice the leadtravel time. In our laboratory investigation we have been unable todetect any difference in the shape of the impulse when the lead length

is varied from less than 1 to 45 meters when the generator resistanceapproximately matches the surge impedance of the lead. In this casethe front capacitor was 700 pF and, although the capacitance of the45-meter lead was 300 pF, there was no increase in front time.We find the comments of Mr. Fitch somewhat confusing. He

accuses us of having ignored the inductance of the generator when aconsiderable portion of the papers deals precisely with the surge im-pedance nature of the generator. We have endeavored to show thatyou cannot treat the components of an impulse system as lumpedparameters-they must be treated as distributed ones. Thus the in-ductance of the generator has been treated as a distributed parameter,which it is. Consequently, we cannot accept his treatment of it as alumped parameter. In addition, he states that we "rejected his as-sumption that the divider forms part of the shaping network." Thisstatement can hardly be correct when one paper1 is devoted to show-ing how the measuring system affects the shape of the impulse gen-erated. Finally, he states that the voltage divider should be connectedahead of the test object, which is contrary to the specifications laiddown in both the international and American standards related to thismatter [1], [2]. In any event, the divider will not filter out the highfrequencies as he suggested since it does not appear as a capacitoruntil after twice its travel time. If the front shaping capacitor (whichis usually many times larger in value than the capacitance of adivider) will not eliminate the high frequencies as illustrated by Fig. 22it is difficult to see how the divider will.

REFERENCES[1] "High-voltage test techniques," International Electrotechnical

Commission Publ. 60, 2nd ed., 1962, p. 53.[2] "Techniques for dielectric tests," USASI Standard C68.1, 1968,

p. 21 (IEEE Standard 4, September 1953).

Calibration and Checking Methods of Rapid

High-Voltage Impulse Measuring CircuitsNILS R. HYLTLN-CAVALLIUS, MEMBER, IEEE, AND R. LEWIS VAUGHAN

Abstract-Errors in high-voltage impulse measurements areclassified as scale factor errors and errors due to limitations inresponse characteristics. Running work within the InternationalElectrotechnical Commission (IEC) gives tentative requirementsintended to keep the mentioned errors within reasonable limits.The theoretical basis of these requirements is presented and checkedagainst some practical cases.

INTRODUCTION

A DIFFICULT and interesting problem in the high-voltagetest technique is the measurement of amplitude and time

variation (shape) of high-impulse voltages. This study is con-centrated on some of the problems related to such measurements

Paper 70 TP 31-PWR, recommended and approved by the PowerSystem Instrumentation and Measurements Committee of the IEEEPower Group for presentation at the IEEE Winter Power Meeting,New York, N. Y., January 25-30, 1970. Manuscript submittedSeptember 11, 1969; made available for printing December 9, 1969.The authors are with the Hydro-Quebec Institute of Research,

Montreal 128, P. Q., Canada.

without any effort either to present a complete picture or togive reference to all the work that has been done on the matterthroughout the years. Instead, the report will give some principalaspects mainly related to the work of IEC Technical Committee42, especially the revision of [1], now in progress. The opinionsexpressed, however, are only those of the authors.

Usually, impulse measurements are made in a circuit com-prised of an impulse generator, a test object, a voltage divider,and a recording instrument such as an oscillograph. The con-ventional concept is that the voltage across the test objectshould be measured with a given accuracy when the nominalscale factor of the measuring system (that is, the divider ratioand the instrument sensitivity) and its response characteristicsare known. Obviously, the above concept is similar or identicalto the one that has been used for decades for moderate voltagemeasuring devices and in electronics; the reluctance sometimesfound against having it introduced in the high-voltage testtechnique is thus rather difficult to explain. However, there aresources of errors, some of them specific for high-voltage impulsemeasurements, which can be listed as follows:

1393