Protection of Rotating A C Machines AgainstTraveling Wave Voltages Due to Lightning

BY W. J. RUDGE, JR.* R. W. WIESEMANt and W. W. LEWIStAssociate, A.I.E.E. Member, A.I.E.E Member, A.I.E.E

Synopsis-The problem of providing protection for rotating terminal to limit the voltage to the crest value of the machine higha-c machines against lightning overvoltages has two parts: potential test, and a capacitor at the machine terminal to slope the

1. The protection of insulation to ground, which is mainly a wave front, thus limiting the turn-to-turn stresses. The capacitorquestion of the magnitude of the overvoltage, and also serves to limit internal overvoltages due to reflection at the

2. The protection of turn insulation, which is primarily a matter neutral. For neutral grounded solidly or through resistance equalof wave front. to or less than the combined surge impedance of the machine wind-

To meet the first requirement, the overvoltage to ground must be ings 0.1 ,uf capacitance is sufficient; while for isolated neutral,limited to a safe value. This is assumed to be approximately equal capacitance up to 0.5 ,uf may be necessary. Ground wires over theto the crest value of the 60-cycle high potential test. To fulfill the line for the initial 2,000 feet are recommended to protect againstsecond requirement, the turn voltage gradient must be limited to direct strokes.conform to the turn insulation. For cases where the machine feeds the outgoing line through

To secure these results, a protective scheme is proposed, which transformers, a special arrester is recommended at the machineincludes arresters 2,000 feet and 500 feet out on the line, or other terminals, to be connected in multiple with a capacitor, which inmeans, to limit the incoming voltage to 2.5 times the crest value of most cases may be 0.1 ,f.the terminal arrester rating, a special arrester at the machine * * * * *

INTRODUCTION pears that in view of the aforementioned factors,THE behavior of traveling wave voltages in rotatingrotating machines should be considered to have an

a-c machines was discussed in two papers in impulse ratio only slightly greater than unity. Thus,1930." 2 Theprinciples of protection by means of for the purpose of selecting protective equipment,

arresters, capacitance and inductance were briefly safe practise will be adhered to by considering theoutlined. In the ensuing two and a half years many of crest value of the machine's one-minute high potentialthese principles have been studied further and some test voltage as the maximum value of impulse voltagehave been tried out in practise. It is the purpose of which should be allowed on the coil or major insulation.:the present paper to present the results of certain of Turn Insulation Impulse Strength. The develop-these studies and experiences and to offer a practical ment of turn insulation of machines has been based onmethod for providing protection for rotating a-c the normal voltage between turns and on operatingmachines against traveling wave voltages due to experience. Under normal conditions, the turn insula-lightning. tion is subjected to relatively low voltages, rangingThe coordination schemes discussed in various papers from 1 to 400 volts, but when a steep wave enters the

at this convention are not generally applicable to winding, 100 to 1,000 times normal voltage may berotating a-c machines, since the usual insulation used impressed across the turns. Since it is not economicalfor this type of machine is of an entirely different order to insulate armature coil turns to withstand such steepfrom the insulation of non-rotating apparatus, such as wave voltages, and because it is feasible to protect thetransformers, oil circuit breakers, bus bars, etc. A insulation normally in use, it appears rational to adherespecial treatment of the problem is necessary, therefore, to present insulation standards and to apply protectiveto obtain a proper protective scheme. measures.

Traveling Waves in Machine Windings. It has beenINSULATION CHARACTERISTICS OF ARMATURE WINDINGS shown previously (reference 2) that an armature wind-

Rotating a-c machine insulation strength is influenced ing behaves similarly to an equivalent transmissionby the limitations of the dry type of insulation used, line having a high surge impedance and a low propa-by space limitations, and by the types of insulation re- gation velocity. Fig. 1, based on tests on somequired to meet temperature and mechanical conditions. medium and large machines, shows how the surgeThese and other inherent characteristics of rotating impedance and propagation velocity vary with rated

machines make it difficult to incorporate high impulse terminal voltage. In the case of delta-connected orstrength in armature windings. Where transformers Y-ungrounded neutral machines, it iS important toand similar apparatus are considered to have an im- tArmature wTindings have a one-minute) test voltage whosepulse ratio in the general order of two or more, it ap- crest value is equal to V29 (2e + 1,000) where e is the rated

terminal-to-terminal rms voltage. The maximum allowable*General Electric Company, Pittsfield, Mass. impulse voltage therefore is equal approximately to 3e fortGeneral E,lectric Company, Schenectady, N. Y. machines whose ratings are aJbove 4,000 volts. If E is taken as1. For references see end of paper. the crest value of the machine's terminal-to-terminal rating,Presented at the Winter Convention of the A.I.E.E., New York, the maximum allowable impulse voltage for the macthine will

N. Y., January 28-27, 1933. be approximately 2.12E.434

33-32

June 1933 PROTECTION OF ROTATING A-C MACHINES 435

know the time required for waves to travel from the which should be used at the machine, as it will be seenline terminal to the neutral or midpoint of the winding, later that for other reasons a larger capacitance may bedepending on whether the machine is connected Y required.or delta. This time may be expressed in micro-seconds This paper deals primarily with traveling waveand is referred to as microseconds length. Fig. 2, voltages due to lightning. In some cases switchingshows how the machine length varies with rating. near the machine terminals may appreciably stress

the turn insulation. This fact should be taken intoFUNDAMENTALS OF PROTECTION consideration in applying the protection recommended.

The problem of protecting rotating a-c machinesmay be divided into two major parts:

(A) The protection of turn insulation. 50(B) The protection of major or coil insulation. t 1 X

(A) Protection of Turn Insulation. LWhen a voltage wave enters an armature winding, it L'E 25

requires a small but definite time to travel around a_ e_

coil. The voltage, Fig. 3, which occurs between adja- - - H,[:Chin5cent turns depends upon the steepness of the entering 4> 0 - -wave and the length of the coil. This voltage appears L 100 1Q00 10000 100000Machtnc kv-anotonly across the turn insulation, but across the tier

FIG. 2-EQUIVALENT ELECTRICAL LENGTH OF ARMATUREinsulation as well if the coil is wound in tiers. Strand OF MEDIUM VOLTAGE A-C MACHINES

(B) The Protection of Major or Coil Insulation.In general, the installations of machines in need of

8E protection may be classified into two groups: first,vi ao>11| |||i[ |]||| machines directly connected to outgoing exposed3 ol 51 11 1 11 11 11 11 I feeders, and second, machines connected to exposed

lines through transformers.1. Machines Directly Connected to Exposed Lines.

If a steep front long-tailed wave is allowed to entervzil oct in armatur indlfl9m the winding from directly connected exposed outgoing

70,o210500 lines, the voltage-to-ground throughout the winding10000 will depend on the machine connections. Fig. 4, shows

32 9500O 2 4 6 810O12 14 16 18 20 22 24 26 28 voltages measured to ground on a 6,600-volt Y-con-Machins terminal to terminal kilovolts (rms.)

FIG. 1-IMPULSE VOLTAGE WAVE CHARACTERISTICS OF MEDIUMAND LARGE A-C ROTATING MACHINES 'bI (a)

b) ' \

insulation and one-turn coils are not affected by this Z=surgr inpedanc of linevoltage. The maximum rate of voltage change (wave Tfront) which a coil will withstand, depends upon theturn insulation and the turn length. Movxiurtaqlo(abIe rate

It is obvious that by placing lumped capacitanceto ground at the machine terminal, the capacitance Strand Coil Turnwill be charged through the surge impedance of the n insus tion Strjrldline by all waves arriving over the line (Fig. 3). The mnsCtoncharging rate will depend upon the crest value of the L_= developed coilOna turn coils Multi-turn coilsincoming wave, the surge impedances of the line and FIG. 3-DIAGRAM ILLUSTRATING MANNER IN WHICH TURNSmachine, also the value of capacitance in microfarads. ARE SUBJECTED TO STRESS FROM STEEP WAVE FRONT VOLTAGESKnowing the characteristics of the machine and line,it becomes a relatively easy matter to determine the nected machine when a -1250 microsecond wave wasmaximum rate of change (wave front) which can occur applied to the machine over a line of 300 ohms surgein the winding. Examination of a large number of impedance. It is to be noted that voltage to groundmachines indicates that in general 0.1 ,uf (as a mini- was measured for three conditions, namely; neutralmum) placed at the machine and with a limit placed grounded through a resistance equal to surge impedanceon the applied wave, as will be given later, is sufficient of winding, neutral solidly grounded and neutralto prevent excessive turn stresses. This criterion does isolated. For the purpose of this study, the first twonotnecessarily establish the total amount of capacitance conditions may be grouped together. It follows, there-

436 RUDGE, WIESEMAN AND LEWIS Transactions A.I.E.E.

fore, that machines may be classified into two groups: rating, it is apparent that for the higher voltages it is(a) machines operating with their neutrals grounded desirable to keep the required capacitance small.(where the resistance at the neutral is equal to or less Hence, for the higher voltage grounded neutralthan the combined surge impedance of all windings), machines, it will be more economical to use smalland (b) machines operating with their neutrals un- capacitance of the order of 0.1 4f (to prevent turngrounded (1R greater than Z), and delta-connected stress), in parallel with an arrester, which will limitmachines. the crest value of the entering wave below 2.12E, than

(a) Grounded Neutral Machines. The one-line dia- to use large capacitance without the arrester in parallel.gram given in Fig. 5A shows the plan of protection This type of protection can be used because no positiveproposed for grounded neutral machines. The selection voltage reflections occur at the neutral when the neutralof the capacitor and the arrester located at the terminal resistance is equal to or less than the combined surgeare made on the premise that the waves which arrive impedance of the windings. For low-voltage machines,over the 2,000-ft section of the line adjacent to themachine are limited in some manner to approximately -v rhad groundwiroX2.5 Ea (in which Ea is the crest value of the maximumname-plate voltage rating of the terminal arrester). DirectdonnecdIn Fig. 5A this limitation of voltage is indicated by line -1type arresters placed 2,000 ft and 500 ft from the gen- cerator. The selection of 2,000 ft was arrived at by arrs9t9r rgrestnrtheory and tests. This distance allows approximately4 microseconds for the line arrester to operate before -_reflections from the capacitor reduce the incoming 500ft-twave at the line arrester. This distance also makes it 2000 ftpossible to utilize the surge impedance of the line FIG. 5A-SCHEME OF PROTECTION FOR GROUNDED-NEUTRALbetween the line arrester and capacitor to obtain a MACHINEScharging rate for the capacitor.

Ovaehaad ground wiriz

1.08 l ; X 0 m 5] iDirect connzctodsolated nzutral exposed fe||dtr

Lightning-Neutral groundad throughtTT

C_ surqgzimpodanca qualtoth2sUrga mdace

-500f> 0.8 0 g q 3 i OOft-

E7 0).6ffl E E E E E w H i FIG. 5B-SCHEME OF PROTECTION FOR UNGROUNDED-NEUTRALXGr4 f oundad neutral~\ _MACHINES

0.2 where the capacitor costs are not so high, the use of a0 0 1 0.2 0.3 0.4 .5 0.6 07 0.8 0o9 1.O large capacitor dispenses with the need of an arrester

Terminal Armaturc winding Neutral in parallel.FIG. 4--MAXIMUM VOLTAGE-TO-GROUND ON A 6,600-VOLT (b) Non-Grounded Neutral or Delta-Connected Ma-

Y-CONNECTED ARMATURE WINDING WHEN A 0.5/150-MICRO- chines. The problem of protecting ungrounded neutralSECOND IMPULSE WAVE Is APPLIED (Fig. 5B), or delta-connected machines, is much more

difficult than that of protecting machines whoseThe value of placing a limitation on the crest of the neutrals are grounded. This is obvious when we con-

applied wave can be seen by examining the formula sider that a machine winding may have a length offor capacitor voltage, where the capacitor C is charged 50 microseconds and the reflection point for the wavesby a wave E1 over a line of surge impedance Z1. may be 50 microseconds removed from the point of

ec = 2E1 ( 1 - E-t/zlc) voltage limitation. It is also apparent that where theThe higher the crest of the applied wave, the higher waves which enter the winding may reflect approxi-

the voltage to which the capacitor will charge. It mately to double value at the neutral, the enteringshould also be noted that the higher the crest of the voltage must be held at the line terminal to one-halfapplied wave, the greater will be the rate of charge the allowable machine impulse voltage (2.12E). The(steepness of wave front) on the capacitor. It follows, difficulty of this problem can further be appreciatedtherefore, that placing a limitation on the applied wave when we consider the fact that the crest value of theis a matter of economics. Since the cost of capacitance operating voltage is E, and that in order to preventincreases approximately as the square of the voltage positive reflections from exceeding the allowable im-

June 1933 PROTECTION OF ROTATING A-C MACHINES 437

pulse voltage, any device which is placed at the termi- protective equipment located at the terminals of thenals to limit the incoming wave must hold the voltage machine is based on the assumption that the incomingto 1.06E. surges are limited to 2.5Ea for a distance of at least

Since our present day arresters cannot operate be-tween such narrow voltage limits, some other means VG 2/aof obtaining lower impulse voltages must be used. 6 Z2 VNIt is apparent that by placing a limit on the impulse C1voltage which could reach the machine from the line,and by placing a sufficiently large lumped capacitance 40Q 1 I1 Iat the machine terminals to ground, the capacitance Neutral voltagewill act to reduce the impulse voltage. -

For the purpose of calculating the value of this ^ l j-f/1,capacitance, it has been assumed that the average in- -300 -- _7 __stallation will consist of a double-circuit three-phase L - 'line, whose combined surge impedance is not less than c C250 ohms per phase. It is further assumed that the 7oJ_ Wa mach_n_L( X : _W 212% El-Allow able madchinevoltage which is allowed to pass the line arresters over 4 1 impulsevoltage_the 2,000-ft section adjacent to the machine is a 0/40 2oC0 rna volt agzmicrosecond wave, whose crest value is 2.5Eaand that d - - -

L Iwaves arrive simultaneously over each of the six , - - -21 - - - - iconductors. 1XThe 0/40 microsecond wave was selected as fairly Is , -/||| 1| | - -

representative of the most severe waves met in practise.The sheer front was chosen to simplify the calculations a Ilby eliminating one term from the equations.

II- I

Fig. 6 is a plot of the voltages which occur at the - -_ -machine terminal and also at the isolated neutral for 00 020 30 40 506I070 80 90 100 110210140150various values of capacitance at the machine terminals. MicrOsecOndsThese curves indicate that for the assumed set of condi- FIG. 6-Ations 0.5 ,f will, in general, offer sufficient reduction VOLTAGES WHIICH OCCUR AT MACHINE TERMINALS AND

AT NE-UTRAL OF AN ISOLATED-NEUTRAL MACHINE WITH Ain the voltage at the terminal to prevent overstressing 0/40-MICROSECOND IMPULSE WAVE OF 2.5Ea ARRIVING AT THEthe machine insulation throughout the winding. These MACHINE. MACHINE LENGTHS L VARYING FROM 10 TO 40assumed conditions apply to a single-circuit winding. MICROSECONDSIf the machine has a multiple circuit winding or there Z= 250 ohms, Z2 = 800 ohmsare multiple machines, the voltage will be lower than C = 0.124findicated by the curves.The exact determination of the capacitance required 3 l

is more important from an economic standpoint in the h I I-.S

_212%E=Allowable mncithne- ---case of machines which are in the 12- and 15-kv ratings, impuls voltage -0 -or higher, as these are the ratings at which capacitor 0 p Z 4costs are relatively high. It is obvious that the larger E200 I-Ithe capacitance in microfarads, the greater will be the t vltagR lmargin of protection to the machine, and where a large )capacitance can be obtained without substantial in-

-4, / |'crease in cost, it is advisable to use larger capacitance. _ I

It should be noted in this set of curves that the - 1 - - - - - - -machine length is given in microseconds time required fIfor voltage to traverse the windings, and that the - j +mi voltagecurves are plotted for machines whose lengths L vary p 4 Ifrom 10 to 40 microseconds. For a given value of oT

at themachine termials, the resulting 10 20 30 40 50 60 70 80 90 l00 luO 120 130 140 150capacitance ath ahn emnl,terstngMicroseconds.voltage at the neutral depends on the length of the FIG. 6-Bwinding up to a length where 100 per cent reflection

, . . Z1 ~~~~~~~~~~~~~~~~~=250 ohms. Z2 = 800 ohmsOf the entering wave occurs. Lengthsgraethnhsc=0.,uwoulld not increase the value of reflected voltage.

2,000 ft from the machine. In regions where directOVERHEAD GROUND WVIRES strokes are frequent, overhead ground wires should

It has been pointed out that the selection of the be located overZthel,2,000-ft section of line. The effects

438 RUDGE, WIESEMAN AND LEWIS Transactions A.I.E.E.

of arrester ground resistance can be largely overcome side of the inductance to prevent the voltage resultingby!lconnecting the arrester grounds, the capacitor from oscillations between the lumped capacitance andgrounds and the machine frames together with the the inductance from exceeding 2.12E. This plan hasoverhead ground wires, and bonding all grounds to the not proved practicaJ up to the present time, and itcommon station ground system. Where a station ar- seems that the alternative plan proposed has certain

advantages which are applicable to the problem of300 _ _ _ _ protecting machines in general.

V PROTECTION OF MACHINES CONNECTED TO LINESTHROUGH TRANSFORMERS212'l%E= Allowable machine

200.71 + H < volt- 1 1 3Messrs. Palueff and Hagenguth have shown the- ii]0 0mechanism by which surges are transmitted through

Neutr'al voltag -_ - transformer windings.3 Examination of their solutiont- discloses that for YY-connected transformers with9l - lt-f-H -e - both neutrals grounded, the electromagnetic component10o- - d - 9 -., of surge voltage which may be transmitted through

- - - - -= - X to the machine windings should be dealt with by re-// / \L S- t -ducing the (upper) circuit shown in Fig. 7 to that showna Teralt e below where L in henrys is obtained from the trans-

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 ZM cro Seconds

FIG. 6-WCL Z2

Z= 500 ohms, Z2 = 800 ohmsC 0.3,pf C

rester is used at the machine terminal for the protection -of a grounded neutral machine, and the 2,000-ft section ,2(L2Lis protected by overhead ground wires, as suggested, Eit should not be necessary to install the line arrester at500 ft from the generator (Fig. 5A). C

300 -.

J | | | | | | | I I 1 T n FIG. 7-PROTECTION OF MACHINES CONNECTED TO LINEL 212%E=Allowable machine THROUGH TRANSFORMERS

,1impP Se voltageT.C - ::t1t:I:1:: Upper-Actual circuit200 -

-

-e- Lower-Equivalentcircuit

- - - - _ _ 1 former short-circuit reactance as measured from the-- - -,\| 4 oiW ' 1/\4 low-voltage side and the surge impedance of the line

-lool W PH A] L'.l ,1 \ 1\ is reduced to its proper value, that is, by multiplying'vI0I the surge impedance on the high voltage side by

-

[Tr in which r is the ratio of high-tension to low--~~~~ voltage r0 ti/iontu0s

o io 20 30 40 50 60 70 80 90 tn n110 1rn130 140 150Mi1cro0 00cond1 5 YY transformers with both neutrals grounded con-FIG. 6-D stitute the only connection for which the waves trans-

Z= 250 ohms, Z2 = 800 ohms mitted to the machine cause positive reflections ofC = 0.5 uf voltage at the machine neutral. Under these conditions

sufficient capacitance must be used to prevent reflec-INDUCTANCE AT MACHINE TERMINALS tions at the isolated generator neutral from exceeding

It would be possible to remove the necessity for 2.12E. The voltage whichmaybe transmitted throughlimiting the line voltage at approximately 2,000 ft out the transformner depends upon a number of factors,on the exposed lines, by inserting an inductance in such as kilovoltamperes, reactance, ratio of trans-series with the machine and placing a capacitor to formation, etc., and it istherefore necessary to examineground between the machine and the inductance. It each case to determine the amount of capacitanceis necessary, however, to place an arrester on each required.

June 1933 PROTECTION OF ROTATING A-C MACHINES 439

For all other transformer connections, the voltages first entering wave is completely reflected at the neutraldue to the waves which are transmitted into the machine and travels back to the terminal, where part passeswindings cancel when the waves arrive at the machine out and part is reflected back into the machine withneutral, hence, for protection to the machine it is only reversal of sign, and becomes what we may call thenecessary to limit the crest value of the entering waves second entering wave. This wave enters T secondsand to prevent sudden voltage changes (steep waves) after the first wave, where T is the time required for awhich would cause excessive turn stresses. Examina- wave to travel twice the length of the winding fromtion of a number of cases indicates that by placing terminal to neutral or point of complete reflection.0.1 .tf capacitance or more in parallel with the proper In the same way there is a succession of entering wavesarrester between transformer and generator, this pro- at intervals of T seconds, each being opposite in signtection will be accomplished. to the preceding one. Each wave produces double itsThe capacitor which is placed in parallel with the voltage at the neutral or point of complete positive

arrester has two useful purposes: first, it aids in sloping reflection, and the total voltage at that point is the sumoff steep waves and, second, it reduces the crest value of the voltages due to the several entering waves addedof the entering wave where the impressed wave is not at the proper time intervals.of sufficient severity to charge the capacitor to the The voltage at the machine terminal is also thevoltage required for arrester operation. superposition of a number of voltage waves impressed

at intervals of T seconds. However, these voltageCONCLUSIONS waves are easily determined if we know the variousThis paper outlines a rational method of providing entering waves. The first terminal voltage wave is

protection for rotating a-c machines from surge voltages equal to the first entering wave. The second terminalcaused by lightning and originating on exposed trans- voltage wave is equal to the sum of the first and secondmission lines. entering waves (which are opposite in sign). The nthMachine characteristics are indicated which in- terminal voltage wave is the sum of the nth and the

fluence the proper selection of protective equipment. (n - 1) entering wave.The plan of protection involves a limitation of the Thus, for the present purposes it is only necessary

voltage of the applied wave to 2.5 times the maximum to obtain formulas for the various entering waves inname-plate rating of the terminal arrester for a distance order to determine the total voltage at the terminalof at least 2,000 ft from the machine, and the use of and the neutral at a given instant. It is not necessaryspecial arresters and lumped capacitance at the ma- to take space for the derivations here.chine. Capacitance of 0.1 to 0.5 ,uf may be necessary, To obtain the voltage waves for the several machinedepending on whether the machine neutral is grounded lengths examined, the various entering waves wereor isolated. first plotted and then added in their proper time rela-Overhead ground wires should be placed over the tionship, depending on the length of machine assumed.

exposed line to a point at least 2,000 ft out from the The formulas follow:station to protect this section of line against direct E = E, C-Bt = wave on linestrokes. B = 0.0173 for a 0/40 waveWhere the rotating machine is connected to the Z1 = surge impedance of incoming line

exposed line through a transformer, a special arrester Z2 = surge impedance of machineand a capacitance of 0.1 microfarad or greater in C = capacitance in microfarads at machine terminalparallel should be located between the transformer L = time in microseconds required for a wave toand machine. travel the length of the machine windingBreakdown of machines is probably a progressive VG = voltage at machine terminal

process and if the suggested protective means are not VN = voltage at machine neutralapplied until the machine has been in operation for V1e, V2e, V3e = first, second, third entering wavessome years, the anticipated results may not be fully t = microseconds from beginning of each wave.realized. Let

ACKNOWLEDGMENT Z1 Z2The authors wish to acknowledge the vahiable A z7 C,

assistance of Mr. H. G. Brinton in the mathematicalanalysis connected with this problem and in the prepa- K =_ 2ration of the appendix, which follows: B-A

Appendix M =Z1CThe voltage at the mnachine neutral was calculated D-2

by superimposing the original entering wave and the S ___K 1various reflections in their proper time relation. The 1)

440 RUDGE, WIESEMAN AND LEWIS Transactions A.I.E.E.

E1 = 100 per cent mended by the A.I.E.E. Transformer Subcommittee is of littlevalue, since the dielectric strength of the point gap which has

K-At -BtA been standardized increases more rapidly than that of solid insu-

M \ / lation for very short time lags.A careful consideration of the basis, on which coordination of

K-

2K t insulation has been worked out, shows that one cannot form aV2e = - -SteA _eB e- general opinion merely from these time lag curves. PrimarilyM M D coordination of insulation was started to form a basis for therelation of power transformer insulation to that of the rest of the

K 2-At cBt\ ircuit, for those transients which these transformers should beV3e= S E - e expected to withstand when connected to overhead circuits.The time lag curve shown by Mr. Montsinger indicates that an

2K / \ t 2K t2 insulation barrier, which has a 10 per cent margin of dielectric- ' 1 + S 5 -At + 2 _ E At strength over a point gap of a given setting for wave fronts ofM D M D2 2 microseconds or more, will fail if a wave with a front of less

than Y4 microsecond and of sufficient voltage to break the pointKS3 '

-A! \Bt 2K ' S \ t -At gap is impressed on it.V4.=- M (e -A t+ M (1+S+S% A Effective coordination implies a factor of safety in actual trans-former insulation with which the transformer has not been

2K t2 -A 43 credited in this curve.(2 + S) e -At + -K f -At Secondly, for power transformers at least, it is very doubtful ifM D2M 3D3 they will be subjected to waves of less than 1 microsecond front.The coordination point gap settings for these were worked out on

KS4 1 A E 2K / A the basis that they would be placed close to the transformer butV6e = M-te t_ e Bt') M +S2+S3hDe interposed between it and the line over which the transients wereV~~e = M~~~~~' lvi '~~~~~ expectedto fow.Any stroke on the line, whether induced or direct, which has a

+ 2K /3 + 2S + S2 d e -A voltage of the order of the coordination gap breakdown is aboveM k + D2 the corona point of the line, and the wave front is therefore sloped

by losses as the wave is propagated toward the station. In addi-4K / X_t3 2K t4 tion to this, the wave front on arriving at the station is sloped by

_ _ t3 + SJ f_-At+___ At the lumped capacitance of the station apparatus and connections.M 3D3 M 3D4 It appears, therefore, that a wave front of 34 microsecond or

less, which the time lag curve shows might be dangerous toFormulas for further entering waves, if desired, transformer insulation can appear only at the transformer termi-may be written out from inspection of the preceding nals, not by reason of direct strokes on the line, but actually atformulas. transformer terminals unprotected from such strokes by grounded

metal such as overhead ground wires, steel work, or direct strokeReferences masts.1. Effects of Lightning Voltages on Rotating Machines and the Furthermore, it appears that the rate of voltage rise in the rare

Methods of Protecting Against Them, F. D. Fielder and Edward event of a direct stroke striking the transformer terminals, due toBeck, TRANS. A.I.E.E., Vol. 49, Oct. 1930. the enormous voltage of the lightning stroke, would be such that

2. Voltage Oscillations in Armature Windings under Lightning any gap, even a few feet away, might not receive anything likeImpulses, E. W. Boehne, TRANS. A.I.E.E., Vol. 49, Oct. 1930. the same voltage in a fraction of a microsecond as that received by

3. Effect of Transient Voltages on Power Transformer Design- the transformer terminal struck. The type of gap, therefore,IV, K. K. Palueff and J. H. Hagenguth, TRANS. A.I.E.E., Vol. might have little effect in preventing excessive voltage on the51, 1932. transformer in such cases. Good engineering practise would ob-

viously seem to demand that valuable apparatus such as powerDiscussion transformers shouild be shielded effectively from direct strokes

to their terminals.PROGRESS REPORT ON IMPULSE TESTING OF C. L. Fortescue: The title of the impulse testing paper is very

COMMERCIAL TRANSFORMERS satisfactory. The writer feels that on such an important matter(VOGEL AND MONTSINGER) as the impulse testing of apparatus it was advisable to proceed

FACTORS INFLUENCING THE INSULATION with caution for the reason that while a great deal of data haveCOORDINATION OF TRANSFORMERS been obtained during the past five years with regard to surges

(F. J. VOGEL) produced by lightning on transmission lines, there has not beenCOORDINATION OF INSULATION time enough yet fully to digest these data.

(MONTSINGER, LLOYD AND CLEM) There has been a tendency to treat the subject of coordinationIMPULSE VOLTAGE TESTING of insulation in apparatus and system as if it were an entirely new

(HARDING AND SPRAGIJE) approach to the solution of the economic protection of power sys-PROTECTINOF ROTTING A CMACHINESAGAINST tems. For this reason suspicion might very well be created in thePRTRAECTIONG OFVROLTATING AUCMAHIE AGAINST minds of some of our system engineers that the development ofTRAVELINGAVE VOLTAES DUE TOLIGHTNING the principles of coordination might lead to the throttling of(RUDGE, WIESEMAN AND LEWIS) initiative in the design and layout of electrical systems. It is very

F. F. Brand: The interesting time lag curves, given in Mr. vital that this impression be prevented.Vogel's paper and more completely in the paper by Messrs. In discussing the paper by Messrs. Montsinger, Lloyd andMontsinger, Lloyd and Clem, which show the relation of insula- Clem the writer points out that it is possible to protect the trans-tion and various gaps, are apt to give the impression that the mission and connected substation so that the line surges resultingwhole scheme of insulation coordination which has been recoin- from lightning due to direct or induced strokes will be reduced at