x= coordinate; distance along loop from clamp 2. Sproule, J. E., and Edwards, A. T., Progress toward optimumy = coordinate; deflection at point x damping of transmission conductors, Trans. AIEE (PowerYa = deflection of cable at end of deflection arm, differential dis- Apparatus and Systems), vol 78, pt III-A, Oct 1959, pp 844-847

placement for fixed clamp3. Group 23-24, Overhead lines, vibrations, CIGRE, Paris, France,

Yd=slopeflctontofor poixd clap vol 1, 1963, pp 582-585 (discussion by H. Wolf)yd=slope-deflection of point A 4. Edwards, A. T., and J. M. Boyd, Ontario Hydro live-liney0=deflection at x=L/2 vibration recorder for transmission conductors, IEEE Trans.y'= derivative of y with respect to x on Power Apparatus and Systems, vol 82, Jun 1963, pp 269-270y"=second derivative of y, curvature 5. Burgdorf, V. V., A. Yaliberman, and V. K. Meshkov, ConductorE= strain vibration and dancing on EHV lines employing bundle con-0=slope ductors, Rept. 219, CIGRE, Paris, France, Jun 1964Oo = end slope 6. Hard, A. R., Studies of conductor vibration in laboratory span,

x =L/j outdoor test span, and actual transmission lines, Rept. 404,Sconstant, 3.1416CIGRE, Paris, France, Jun 1958

p=rconstant, 3.1416 7. Zobel, E. S., A. H. Shealy, F. W. DeMoney, and R. R. Ruege-p=radius of curvature mer, Aeolian vibration tests on the 345-kv Muskingum-Tidd= normal stress line, Trans. AIEE (Power Apparatus and Systems), vol 78, pt

ek=x/j III-A, Oct 1959, pp 852-8618. Steidel, R. F., Jr., Factors affecting vibratory stresses in cables

near the point of support, ibid., pt III-B, Dec 1959, pp 1207-1212Acknowledgment 9. Ruhlman, J. R., J. C. Poffenberger, and S. Grosshandler, A

mobile vibration laboratory unit for monitoring dynamicThe authors wish to thank G. H. Ault, E. Pedde, and R. A. characteristics of overhead transmission lines (Dynalab), ibid.,

Wilhelms for their assistance in carrying out tests on single pt III-A, Aug 1959, pp 624-63710. Ruhlman, J. R., J. C. Poffenberger, and S. Grosshandler,

wire conductors and A. T. Edwards and J. M. Boyd of The closure of [9], pp 638-639Hydro-Electric Power Commission of Ontario for granting 11. Ruhlman, J. R., and J. C. Poffenberger, discussion of [2],permission to use the data from their tests on stranded cable. pp 847-850

12. Rawlins, C. B., discussion of [4], pp 270-27113. Poffenberger, J. C., discussion of [8], pp 1212-1213

References 14. Sturm, R. G., Vibration of cables and dampers-I, Elec. Engrg.,vol 55, May 1936, pp 455-465; -II, Jun 1936, pp 673-688

1. Tebo, G. B., Measurement and control of conductor vibration, 15. Morrison, F. R., On support stresses in a vibrating conductor,Trans. AIEE, vol 60, 1941, pp 1188-1193 M.S. thesis, Case Inst. of Tech., Cleveland, Ohio, Jun 1962

Ferroresonance During Single-Phase Switchingof 3-Phase Distribution Transformer BanksR. H. Hopkinson, Member IEEE

Abstract: It is possible to obtain ferroresonant overvoltages Fig. 1. Possible 17171 c,-coduring single-phase switching of distribution transformer banks. ferroresonant cir-When energizing the bank with single-pole devices at higher distri- cuitbution voltage, the internal winding capacitances of ungrounded 1*X

wye-delta banks can react with the bank saturation characteristicsproducing high sustained overvoltages. Transient network lcanalyzer (TNA) studies have determined the necessary condi- r T T T Ctions for the occurrence of the phenomena and have investigatedremedies. One promising solution is the substitution of wye-wye grounded banks for the ungrounded wye-delta banks on19.9/34.5-ky distribution circuits.

a 3-phase transformer. The overhead line is represented onlyby its capacitance. Connected to ground is C0 and con-nected phase-to-phase in a wye connection is C1-Co; C, and

A survey ofbte rcuits inwhc fro eso toervoltage Co are the positive and zero sequence capacitances, respec-

tively, of the overhead line or cable. All resistive and in-capacitance in series with some saturable magnetic device, ductive line impedances are small compared to the capaci-such as a transformer. tive and are considered negligible. A wye-delta transformerSome less obvious examples can occur under many 3-phase i ersne.Isnurlcnb ruddb lsn h

transformer bank connections. Figure 1 represents such a sic.We n rtoo h igepaesice r3-pas crcut.Th cicut ay epesnt sure, inle closed, it can be shown that the ferroresonant circuit is oh-

phase switching or interrupting devices, an overhead line, and tmd ihtetasomrgonigsic nete oi

Paper 31 TP 65-136, recommended and approved by the Trans- tion. For the case where C = Co, the transformer neutralmlission and Distribution Committee of the IEEE Power Group for must be ungrounded in order to obtain the ferroresonantpresentation at the IEEE Winter Power Meeting, New York, N. Y., circuit. The same statements are true for other transformerJanuary 31-February 5, 1965. Manuscript submitted November 2, akcnetos1964; made available for printing December 8, 1964.bakcnetosR. H. HOP:KIN5ON is with General Electric Company, Schenectady, These circuits have been studied in detail [1], [2], and theN. Y. conditions where ferroresonant overvoltages are possible have

TAPRIL 1965 Hopkinson-Ferroresonance During Single-Phase Switching 289

been further defined. In general, the method used to prevent pairs so that telephone interference from third harmonictheir occurrence has been to limit the length of overhead line sources is not expected.or cable which may be connected between the transformer For existing installations, the wye-delta bank can be re-bank and single-phase switching or interrupting devices, connected into the wye-wye and both neutrals grounded.which could result in the open conductor condition. To obtain the proper secondary voltage, the use of auto-

transformers has been reported to give good results. Or, if theRecent Experience bank is of sufficient size, one transformer may be removed andThe conductor lengths upon which ferroresonant overvolt- the remaining wye neutral grounded; thus, the open-wye-

ages begin to be a problem are shown in Table I for un- open-delta bank is obtained with the proper voltages.grounded wye-delta banks. Cables have much more capaci- Swamping out neutral capacitance by connecting capacitorstance to ground per unit length than do overhead lines and, from each primary phase to ground was unsuccessful. Thethus, the problem is found in cable systems more easily. In resulting circuit was also highly ferroresonant.Table I, it is seen that at the lower distribution voltages the In the study, it was determined that a neutral resistorproblem is moderate unless considerable lengths of cable are would limit overvoltages; however, because of the limited useinvolved. For higher distribution voltages, 14.4/24.9 kV, the of this voltage at the distribution level, the use of the neutraloverhead line capacitance becomes a problem [3], [4]. Fi- resistor was not pursued. Other methods of handling thenally, at 19.9/34.5 kY, even with no overhead line connecting problem have already been covered.the transformer and the single-phase switch or interrupter, A resistive load on the secondary of a 45-kVA bank at 34.5ferroresonant overvoltages can occur. In this instance, there kV on the order of 3 per cent full load transformer ratingis no capacitance from the line as shown in Fig. 1, so the was found to be sufficient to prevent the occurrence of ferro-necessary capacitance must be that of the transformer internal resonant overvoltages. However, the use of these highercapacitance network and that of the transformer bushings. distribution voltages suggests that, in some cases, there will beUntil recently, the amount of capacitance involved from this extended periods where the load is zero.source was considered too small to be of consequence.

Equivalent Circuit to Represent Transformer InternalCapacitance Network

Table 1. Switching Problem Occurrence in 3-Phase Banks The transformer internal capacitance network is associatedThrough Through with the distributed capacitance from each winding to ground

Circuit Voltage, At 200-Foot 200-FootkV Transformer Open Wire Cable (tank or core) and from each winding to each other winding,

2.4/4.16 No No Yes as well as between turns of each winding [5]-[7]. Although7.2/12.5 No No Yes it is possible to represent this capacitance in as much detail14.4/24.9 Yes [3] Yes Yes as desired in models as well as analytically, economy of time19.9/34.5 Yes Yes Yes

and model equipment requires that simple equivalent circuitsbe obtained which will give good results. In the presentcase, the equivalent circuits take the form of lumped- capacitors

Both the Pacific Power and Light Company and the Idaho connected to the transformer winding terminals and to groundPower Company have distribution voltages of 19.9/34.5 kV, (tank and core); i.e., nothing is connected to the internaland each company has experienced failures of ungrounded parts of the windings.wye-delta transformers because of ferroresonant overvoltages The values for these lumped equivalent circuits are a func-when switching at the transformer pole. tion of the voltage distribution within and along the windings.

In the fall of 1963, a field investigation was conducted by Therefore, the values are a function of this voltage distribu-the Pacific Power and Light Company in one of their Wyo- tion which, in turn, is a function of the applied voltage.ming division substations, and a transformer involved in the For switching surge and recovery voltage studies, it is assumedfield tests failed during the tests. Peak voltages as high as that each winding is an equipotential surface. Capacitance142 000 volts to ground were measured on an opened phase. values can be calculated by simple geometrical relations usingA TNA study was made to determine the nature of the field theory. Comparisons of measured values with calculated

phenomena involved and to explore the feasibility of possible values generally give good results.remedies. This paper presents the results. The transformer banks in question are composed of single-

cphase units. The approach will be to define capacitanceStudy Conclusions values for a single-phase transformer and then show how toHigh sustained voltages can exist where banks of single- connect three tranformers into a 3-phase bank.

phase transformers are connected ungrounded wye-primary- Figure 2(A) shows sueh a 2-winding single-phase trans-delta-secondary on 19.9/34.5-kV distribution systems (see former. Effective capacitance values from the high-voltageFigs. 7 and 8). The l)roblem is especially severe for the lower winding to ground and to the low-voltage winding and fromtransformer sizes. Therefore, the use of this bank connection the low-voltage winding to ground are shown in this figure.should be avoided at 19.9/34.5 and, perhaps, at 14.4/24.9 kV. The assump)tion of equipsotential winding surfaces is re-At the latter level, the transformer internal capacitance called.network has a relationship to the magnetizing reactance at Figure 2(B3) shows the cap)acitance values of Fig. 2(A0 con-rated voltag;e which is conducive to the production of over- centrated at the winding terminals. It is noted that half ofvoltages to ground as high as 4.5 times normal. each value is assigned to each end of the windings. This isFor new installations, wye-wye connections with both the most thorough equivalent circuit which will be consideredl

neutrals grounded are suggested, where the neutrals are con- in this paper.nected to the system multigrounded neutral. Also, where Figure 3 shows three of the single-p)hase trans;formers con-desired, open-wye-open-delta banks may be used. In most nected into an ungrounded wye-delta bank. Results fromlocations, telephone lines are in shielded cable with twisted this equivalent are considered to be correct in this study.

290 Hopkinson-Ferroresonance During Single-Phase Switching APRIL 1965

Any simplifications of the circuitry are tested, and the results Fig. 2. Two-wind- PRIMARYcompared to those obtained with this circuit. ing transformer CH- 2 G

Before proceeding with test results and comparisons, A - Effective ca- -TANK_ H

methods of measuring the values of capacitance and convert- pacitance values l2ing from the measured values to those used in the B - Concentrated V- SECONDARY V \diagrams should be outlined: capacitancevaluesA) (B)1. Connect both ends of the primary together to insure anequipotential surface.2. Connect both ends of the secondary together for the same PRIMARY SECONDARYreason. 23. Connect the core to the tank. 'CHG4. Connect 1 and 2 together and measure the capacitance from 2 I I CL-the primary and secondary to ground (core and tank) with a ICH-Lcapacitance bridge; the value obtained is CHL-G. 25. Connect2and3togetherandmeasurethecapacitancefrom Fig. 3-Wye-2elta 2the secondary core and tank to the primary; the value obtained Iako hreFg CH-Gs CCL-GiS CH-LG. bank of three Fig. 2~~~~~~~~~~~~~~-H-

is CH-l.G- ~~~~~~~~~~2(B)transformers ~ I t)6. Connect 1 and 3 together and measure the capacitance from

~~~~5 CHLG. 2~~~~~~~~~~~~~~~~~~~~~~~C-the primary; core, and tank to the secondary; the value obtained 2iS CL-IIG. ICHIL

Figure 4 shows what is being measured. In item 4, CHL is -CH-G TCL-Gshort circuited so that

3

CH-G, CH-L

CHL_G = CH_G+CL_G -

In item 5, CL-G is short circuited so that

CHLGG=CH-G+CH-L H CH-L LIn item 6, CHG is short ci cuited so that

CL-HG= CH-L+CL-G Fig. 4. Simplified capacitance cir-cuit of Fig. 2(A) transformer CH-G CL-G

Solving these three equations for CH-L, CH-G, and CLG interms of the measured values gives

GCH-G = -(CH-LG-CCL,HG+CHL-G)2

C'L-G = - CH-LG+CL-HG+ CHL-G)2 Fig. 5. Simplified circuitfor tests

CH_-L= 2(CHLG+CL-HG-CHL-G)

Thus, if the data are obtained in one form, they can easily beconverted to the other if desired. It is to be noted that the Table 11. Representative Characteristics for Pole-Typetransformer bushing cap)acitance will add directly to the value Distribution Transformersto glound with which it is associated (primary or secondary). Xag, CHLG, CHL, CLG, CHG,The effects may be included in the measured values and may kV kVA megohms pF pF pF pFhave to be included in calculated values. 15 0.0320 2020 1850 650 170

Since the capacitance network of Fig. 3 is fairly extensive, 25 0.0278 2060 1885 710 175Since the cal)acitancenetwork ~~~~~~~~~37.50.0244 2110 1930 800 180simIPlifications which would give the same results are desira- 2.4 50 0.0177 2160 1980 880 180ble. Figure 5 shows the simplified circuit desired. Extensive (100 0.09 15 2360 2160 1200 190tests have been made which show essentially the same results 167 0.003 92 2620 2410 1650 215as those in Fig. 3, l)rovided C in Fig. 5 is 1/4CH_LG. This 15 0.303 1580 1425 650 15'5'result can be obtained analytically if the capacitance network 25 0.238 16720 1510 800 160of Fig. 3 is reduced to an equivalent circuit by the usual tech- 7.2 50 0.150 1730 1560 880 165

75 0.0988 1830 1660 1040 170niques of wye-delta conversions, etc. Losing the identity 100 0.0774 1940 1760 1200 175of the secondary terminals will allow a simple equivalent with 167 0.0353 2220 2030 1650 185C)aiaCto ground from each l)hase, cap)acitance toneutral 15 1.1750 14260° 132950 6510 1350

fromz each l)hase, and cap)acitance to ground from the neutral. 37.5 0.664 1510 1375 800 140The cap)acitance from neutral to ground is three timnes that 144 750 0.6307 1560 154120 8840 1540from l)hase to ground. Tests show that the capacitance from 100 0.259 1760 1600 1200 160p)hase to neutral is not effective in p)roducing overvoltages. 167 0.141 2030 1850 1650 175Substituting values of cap)acitance from Table II into the l25 1.240 940 8300 6710 110exl)ressions for capacitance from l)hase to ground, and com- 19.9 )37.5 1.160 980 880 800 110

* * ,,5, * > ' . ~~~~~~~~~~~500.762 1040 930 880 120l)aring With 0H_LG results in an average value of cap)acitance (75 0.600 1140 1020 1040 120from phase to ground of 1/40H-LG. 100 0.272 1240 1110 1200 130

APRIL 1965 Hopkinson-Ferroresonance During Single-Phase Switching 291

System Data 1. -In this instance, the system data are totally transformer -__ -__._

data. In anticipation of a general study involving trans- '. - . t -formers of various kilovoltampere sizes and kilovolt ratings, /-______the representative data in Table II were obtained for pole- -J - - - - - - .1type distribution transformers. Figure 6 shows a saturation > -| -i - - - - -Xcurve for times normal voltage vs times normal exciting __current which is representative of pole-type transformers. z APPROXIMATE SATURATION CURVE

UJ 0.4 ~~~~~POLE TYPE DISTRIBUTION TRANSFORMERSTable II gives magnetizing reactance at rated voltage forsingle-phase kilovoltampere ratings from 15 through 167 and 0_2kilovolt ratings from 2.4 through 19.9, as well as capacitance J E 2E Idata. TIMESNORMAL EXCIT'ING CURRENT|

o 2 3 4 5 6 8 9

Transient Network Analyzer Study Fig. 6. Approximate saturation curve for distribution trans-formers

The circuit studied consisted of a 3-phase source represent-ing an infinite bus, knife switches in each phase representingsingle-phase interrupters, and a 3-phase transformer bankconsisting of three single-phase transformers usually in un- IIKVA 144KV IIIKVAgrounded wye-delta connections. To this transformer bank 15KVA IOOKVA

5 1~~~~~~~~~~~9.9KVa capacitance network is connected to represent its internalcapacitance. The four component parts of the study willbe discussed in the following sections. UPPER CURVE - PHASE TO GROUNDLOWER CURVE - NEUTRAL TO GROUJND

V 4l X m,RX MAG

DETERMINATION OF PROPER METHOD OF TRANSFORMER 3 2T1fCH-LG @NORMAL VOLTAGE

MODELING (MAGNETIC CHARACTERISTICS)Extensive tests were made to determine the method of 2 2

modeling the transformer magnetic characteristics. It willsuffice to note that the air core reactance is not an important ,parameter in these tests, where only small amounts of OVERVOLTAGE Vs lc|mcapacitance are encountered. Transformer modeling thus O 1

I

0. 07E

170 0 G 0 400. 1 0.2 0.4 0.7 1.0 2 4 7 10 20 40 70 100 200 400 700 1000involves matching saturation curves on a times-normal basis xc /Xm

near the knee of the curve and determining an impedance base Fig. 7. Overvoltage vs X,/X20 with one energized phasechange by the ratio of magnetizing reactances at rated voltage.

DETERMINATION OF ACCURATE SIMPLE CAPACITANCENETWORK TO REPRESENT INTERNAL CAPACITANCE 15 K 167KVA "A14.4 KV 24KVHere again, extensive tests were made to determine the |15KVA 1KVO KVA

method of representing the internal capacitance network. 15KVA 167KVAThe section "Equivalent Circuit to Represent Internal 4Capacitance Network" gives the details of the capacitance 4 UPPER CURVE-PNEUTRALTO GROUNDnetwork. 2XfCH-L

3 Xmn. X MAG -NORMAL VCLTAGE

GENERALIZATION OF RESULTS TO INCLUDE ALL TRANSFORMERS I 1T OVERVOLTAGE VNGE/xmIN TABLE II Z WITH TWO ENERGIZED PHASES

With the modeling techniques just established, tests were _ Imade to finid overvoltages for the complete range of trans- Lformers shown in Table II. Figure 7 shows the results with -one energized phase, and Fig. 8 with two energized phases. LShown in these figures are maximum voltage from open phase 0l 0.2 0.4 0.7 1.0 2 4 7 10 20 40 70 100 200 400 7001000to ground and maximum voltage from neutral to ground. Fig. 8. Overvoltage vs Xc/Xm with two energized phasesXc is the capacitive reactance connected phase to ground inFig. 5, and Xm is the magnetizing reactance at rated voltage.

It is noted that high ratios of XC/Xm will approach the con-dition of no capacitance in the limit. Thus, a check on the voltages. It is recalled that the winding connection involvedstudy values can be made. For one energized conductor, is wye-primary-delta-secondary with neutral ungrounded.under the condition of no capacitance, unity voltage to groundfrom each phase and from neutral would be expected since no SPECIFIC STUDY OF TRANSFORMER IN PACIFIC POWER ANDcurrent can flow. This condition is confirmed in Fig. 7 for LIGHT STUDIES WITH REMEDIES FOR OVERVOLTAGElarge Xc/Xm. Similarly, with two energized phases, the volt- CONDITIONage from open phase to ground and from neutral to ground will The tests conducted by Pacific Power and Light Companybe half that applied. Here again, Fig. 8 confirms these values, were repeated on. the TNA. The transformer bank testedThe transformer kilovoltampere ranges and kilovolt levels was composed of three single-phase transformers rated 15 kVA,

shown in Table II are also plotted in Figs. 7 and 8 as functions 19.9 kV and connected wye-delta with neutral un-of Xe/Xm. These show clearly that the lower kilovoltampere grounded. Special care was taken to determine the internalratings at the higher kilovolt ratings may encounter high over- capacitance. With average values of capacitance from Table

292 Hopkinson-Ferrbresonance Duringy Single-Phase Switching APRIL 1965

I1, Xc/Xm is 6.45; for values obtained from a special test, base was connected neutral to ground, and voltages were com-XC/Xm =5.5. For two energized phases, Fig. 8 shows pletely normal for the number of phases energized. In theoveroltaes o 3.6and 4.1 times normal. Note that these study, it was determined that a neutral resistor would limitovenearl evoltages

o overvoltages; however, because of the limited use of this voltageare nearly the highest voltages measured. at the distribuition level, the use of the neutral resistor was not

Using this transformer representation, several possible pursued. Other methods of handling the problem are coveredremedies were tested to see which might be practical ways of in the previouis section.preventing the overvoltages from developing: Using resistive load: It was fouind that 10 000 ohms (model)

connected across each delta winding were sufficient to preventarounding the neuetral: This resutlted in normal steady-state the occurrence of ferroresonance, but 12 500 ohms were not.valules with no overvoltages. That is, the voltages were normal Converted to the actual transformer, this amounts to 2.75for the open condtuctor conditioni with no capacitance represented per cent of full load on the transformer. If this model data is(large Xc/Xm). This method, althouigh satisfactory from an equated to a bank of 15-kVA 14.4-kV single-phase transformersovervoltage standpoint, wouild be likely to cause overheating of the resistive load is about 1.5 per cent of full load on the trans-the bank as it would act like a grotunding bank and, at the same former.time, deliver load power.Opening one corner of the delta: This is equivalent to a wye-wye Acknowledgmentbank connection with neutrals floating. However, this is not The author wishes to thank F. C. Van Wormer for his advicea practical bank connection. Maximum overvoltages of twice and assistance.normal were measured.Grounding the neutral of the above: No overvoltages were noted. ReferencesUsing delta-delta connection: Voltage as high as 1.65 times 1. Clarke, E., H. A. Peterson, and P. H. Light, Abnormal voltagenormal was measured with one phase energized. conditions in three-phase systems produced by single-phaseswitching, Trans. AIEE (Power Apparatus and Systems), vol 60,Connecting the bank open-wye-open-delta: This also resuLlted in 1941, pp 329-339completely satisfactory operation. 2. Hendrickson, P. E., I. B. Johnson, and N. R. Schultz, Abnormal

voltage conditions produced by open conductors on three-phaseC7onnecting shunt capacitors from each phase to ground: 1) A circuits using shunt capacitors, ibid., vol 72, Dec 1953, pp 1183-5-kVA capacitor was connected from each phase to ground, and 1193voltages to ground as high as 4.5 times normal were measured 3. Crann, L. B., and R. B. Flickinger, Overvoltages on 14.4/24.9-with two phases energized; and 2) a 1-kVA capacitor was con- kv rural distribution systems, ibid., vol 73, pt III-B, Oct 1954,nected from each phase to ground, and voltages as high as 4.9 pp 1208-1212times normal were measured with one phase energized. This 4. Auer, G. G., and, A. J. Schultz, An analysis of 14.4/24.9 kvmethod was therefore abandoned. Apparently, the extra grounded-wye distribution system overvoltages, ibid., Aug

capactanc swaped ut tht ofthe ransormerbut esuled ~ 1954, pp 1027-1032i apacitance swamped out that of the transformer but resulted 5. Abetti, P. A., I. B. Johnson, and A. J. Schultz, Surge phenomonain a highly ferroresonant circuit of its own. in large unit-connected steam turbine generators, ibid., vol 71,Using a neutral resistor: 1) 2.88 megohms on the system base Dec 1952, pp 1035-1047were connected neutral to ground, and voltages as high as 3.75 6. Abetti, P. A., Electrostatic voltage distribution and transfer in

times ormalwereeasure withtwo pases eergizd; 2)3-winding transformers, ibid., vol 73, Dec 1954, pp 1407-1416times normal were measured with two phases energized; 2) 7. AIEE Committee Rept., Simplified method for determining1.44 megohms on the system base were connected neutral to permissible separation between arresters and transformers,ground, and voltages as high as 2.25 times normal were measured presented at the IEEE Winter General Meeting, New York,with two phases energized; and 3) 0.288 megohm on the system N. Y., January 27-February 1, 1963

Switching Surge Tests on Simulated andFull-Scale EHV Tower-Insulator SystemsA. W. Atwood, Jr., Senior Member IEEE A. R. Hileman, Senior Member IEEE J. W. Skooglund, Senior Member IEEEJ. F. Wittibschlager, Member IEEE

Abstract: Switching surge tests performed on simulated tower- conditions should be 1065 kV. From the design-applicationinsulator systems representative of possible 345- and 500-kV curves, the center phase of the SCE 500-kV transmission tower-transmission line designs were performed so that line design insulator system was designed to have a 12-foot 8-inch minimumparameters could be estimated for a specific value of switching horizontal strike distance and 27 insulators used in a 90-degreesurge overvoltage. Switching surge studies of the Southern V-string configuration. Full-scale tower tests confirmed theCalifornia Edison Company (SCE) 500-kV transmission system switching surge design criteria.dictated that the switching surge withstand strength for wet

Paper 31 TP 65-163, recommended and approved by the Trans- In the study of switching surge overvoltages and their effectmission and Distribution Committee of the IEEE Power Group for on transmission line design, field studies and miniature systempresentation at the IEEE Winter Power Meeting, New York, N. Y,January 31-February 5, 1965. Manuscript submitted November 2, studies determine the magnitudes and their probability of oc-1964; made available for printing December 9, 1964. currence. It remains, therefore, to determine by laboratoryA. W. ATWOOD, JR., is with the Southern California Edison Company, tests the insulation strength of various tower-insulator com-Los Angeles, Calif.; A. R. HILEMAN and J. W. SKOOGLUND are both binations or systems. Switching surge tests on insulation sys-with the Westinghouse Electric Corporation, East Pittsburgh, Pa.;- eswr eotda al s 96[] hs eefloeand J. F. WITTIB5CHLAGER is with the Ohio Brass Company, Bar- tm eerpre serya 96[] hs eefloeherton, Ohio. by other tests in 1961 [2]. With the imminent step to 500-

APRIL 1965 Atwood, et at.-Tests on EHV Tower-Insulator Systems 293