IEEE TRANSACTIONS ON POWER APPARATUS AND SYSTEMS, VOL. PAS-89, NO. 5/6, MAY/JUNE 1970

Referring to the discussions of Mr. Chartier and Mr. Pakala, andMr. Perz, we can state that the equivalence between RN tests withsmooth tubes and with standard conductors under rain was checkedonly for single cylinders of 2.3-inch diameter. Tests with tubes wereused mainly for getting an idea of what bundles should be tested inthe cages. With the exception of 6.6-inch diameter, all curves of Fig. 4have been checked with, or are based on, tests with stranded conduc-tors in the cages. Cage tests are being used exclusively at present. Inanswer to Mr. Nigol and Mr. Reichman, we do not have an explana-tion for the relatively high steepness of the 6.5-inch curve.The described approach to line RN depends essentially on a clear

separation of the various effects and parameters as far as possible.The contribution of hardware mentioned by Mr. Perz is not includedin this study. It is also considered negligible during heavy rain.

Full-scale test lines may be advantageous as far as long-term all-weather RN statistics are concerned. If, however, the influence ofthe conductor type on RN has to be examined, cage tests under(artificial) rain are in the authors' opinion the fastest, cheapest, andmost reliable method.A linear correlation between audible noise and the excitation func-

tion, rather than the relation reported in Fig. 12, was found with cagetests performed after the presentation of this paper. The audiblenoise-excitation function relation is indeed different for differentnumbers of subconductors, a fact covered in Fig. 9, partly becauseRIV is used as abscissa instead of the excitation function. Investiga-tions on this subject, based exclusively on cage tests, are being con-tinued. The audible noise frequency spectrum in Fig. 13 is, above2000 Hz, affected slightly by weighting network C, which was used.

Lightning Performance of Double-CircuitTransmission Lines

MICHAEL A. SARGENT AND MAT DARVENIZA, MEMBER, IEEE

Abstract-This paper presents further studies of lightning outagerates on double-circuit transmission lines. Satisfactory agreementwith field data has been demonstrated for calculations of total anddouble-circuit outage rates and for prediction of outage types onhigh-voltage lines. An analysis of the parameters influencing double-circuit outage rates has enabled proposal of several line designsthat should achieve a low proportion of lightning faults involvingboth circuits. Two configurations are of particular interest-adouble-horizontal arrangement with one circuit above the other,and a double-triangular arrangement utilizing a conventionallattice tower with three crossarms.

INTRODUCTIONM ODERN EHV transmission developments have utilized

double circuit transmission lines extensively, as indicatedin Table I. These lines are generally used on the intuitive as-sumption that there is little possibility of an outage involvingboth circuits simultaneously. However, observed performances(Table II) indicate that the proportion of double-circuit outagesdue to lightning is much greater than might be expected, and it isnot sufficient to assume that the double-circuit outage per-formance of a line will be acceptable. This characteristic shouldbe estimated in design studies, particularly when such an outagecauses critical conditions in the system.

Paper 69 TP 702-PWR, recommended and approved by theTransmission and Distribution Committee of the IEEE PowerGroup for presentation at the IEEE Summer Power Meeting,Dallas, Tex., June 22-27, 1969. Manuscript submitted December19, 1968; made available for printing April 14, 1969.M. A. Sargent is with the State Electricity Commission of Queens-

land, Brisbane, Australia, on leave with the High-Voltage Lab-oratory, General Electric Company, Pittsfield, Mass. 01203.M. Darveniza is with the University of Queensland, St. Lucia,

Brisbane, Australia.

A technique for predicting the double-circuit outage rate oftransmission lines was described previously [1]. In the presentpaper, results of further investigations using this technique arepresented, together with conclusions regarding improved double-circuit line design.

COMPARISONS WITH FIELD DATAThe technique developed to predict double-circuit outage

rates utilizes a dynamic traveling wave method to calculatelightning voltages stressing line insulation and a statisticalmethod of manipulating the data required for calculation.A brief summary of the technique follows.The required lightning stroke and line parameters are selected

by M\onte Carlo methods from the statistical distributions ofthese quantities. Utilizing the step-by-step procedure of thetraveling wave method, the absolute magnitudes of voltages ateach increment of time are calculated at significant points on theline. With these voltages, effective corona radii of conductorsare calculated, and new values of conductor surge impedance,coupling factor, and reflection coefficients are estimated for use incalculations at the next increment of time. In this way anaccurate estimate of voltages stressing line insulation, in whichthe important nonlinearities are incorporated, can be obtainedfor each instant of time. If the corrected sparkover voltage of theinsulation is exceeded at any point at any instant of time then aflashover is considered to occur, and changes in self- and mutualsurge impedance are evaluated, to be incorporated in calculationsof voltages at subsequent increments of time. When determiningif flashover will occur, account is taken of the magnitude andpolarity of the instantaneous power frequency voltages of thephases. This iterative calculation is continued until the pos-sibility of further insulation flashover is negligible. The type ofoutage to be expected for the assumed conditions is derived by

913

IEEE TRANSACTIONS ON POWER APPARATUS AND SYSTEMS, MAY/JUNE 1970

TABLE IUSAGE OF DOUBLE-CIRCUIT LINE CONSTRUCTION (1966)

PercentageNominal Double-

Country Voltage Circuit Circuitof Origin (kV) Miles Construction

Australia 110 810 64132 1100 62220 1390 25330 840 3

USA 220 20600 47345 3500 52

Great Britain 275 3604 99400 1530 100

TABLE IILIGHTNING OUTAGE RATE OF TYPICAL DOUBLE-CIRCUIT LINES

Nominal Outage Ratet PercentageVoltage Double Circuit

Utility* (kV) Total Circuit OutagesNEA (Australia) 132 3.0 1.1 37ECNSW (Australia) 132 7.2 2.9 40Com. Edison (USA) 138 8.0 2.8 35TVA (USA) 161 3.2 1.77 56SECV (Australia) 220 1.64 0.61 37OVEC (USA) 345 7.6 0.4 5ECNSW (Australia) 330 1.5 0.7 46Tokyo, Area (Japan) 140 3.6 1.3 36

250 1.8 0.73 40Ontario-Hydro 115 9.2 6.3 68

(Canada)* Abbreviations as follows:ECNSW Electricity Commission of New South WalesNEA Northern Electrical AuthorityOVEC Ohio Valley Electric CompanySECV State Electricity Commission of VictoriaTVA Tennessee Valley Authority.

t Outages per 100 route miile-years.

inspection of the location of phases (if any) for which the analysishas predicted flashover.The above dynamic traveling wave calculation of outage

form is repeated for the previously selected stroke and lineparameters until a statistically meaningful analysis of the re-sults can be made to estimate the line outage rate. Normally,a five- to ten-year period of study is sufficient.

Following development of the technique, a series of investi-gations was undertaken to establish the validity of the methodsand data used. These studies consisted of duplication of specificoutage incidents for which detailed data were available and theprediction of lightning outage rates for lines of known per-formance.

Duplication of Outage Incidents, SECV 220-kV LineDetailed data concerning the actual outage forms resulting

from lightning strokes to a 220-kV transmission line wereavailable as the result of an extensive field investigation bythe State Electricity Commission of Victoria, Australia (SECV).An 80-mile section of line was fully instrumented with magneticlinks and surge-indicating cartridges, so that the peak magnitudeof current impressed on the line by a stroke could be determined.

OUTAGE OUTAGE FORM PARAMETERS FOR CALCULATED FORMINCIDENT OBSERVED CALCULATED STROKE FOOTISNG CREST POWER FREGUENCY VOLTAGES

CGRREN RESIST. TIME A Pha. Phase IC Phase120kA o 0 (kA) (11) (jyS) (kVp)

L o o o o 120 4-0 ALL VALUES

STROKE TO NC OUTAGE NO OUTAGE 6-0TOWER

4lkA 00 0 0 ~~~~~~~90 -180 90AlGA 0 0 0 0 20~~~~~~~~~~~~~~9 -0 0 0 0~ ~~___ 9 -0 8

2 ; F.d0.*'X F OOSSX 41 3 4-0 NO VALUESSINGLE CIRCUIT SINGLE CIRCUIT 0 -156 156

STROKE TO OUTAGE OUTAGE 6- 0 -/5 /56MIDSPAN (FO. FLASHOVER) -90 -90 180

140k ISGA0 9X X 0 2-0 180 90-90

3 AA 0_O. 0 0EVF..\0 1o / - 80 -90 -9034 2144),. XX 0 /56140 10 40 -156 0STROKE TO DOUBLE CIRCUIT DOUBLE CIRCUIT 6-0 180 -90 -90TOWER OUTAGE OUTAGE 156 -156 0

5SkA 0 X 0 X 2-0 180 -90 -90oFO 0 o O. o

4x o x o 50 70-100 4-0 180 -90 -90

STROKE TO DOUGLE CIRCUIT DOUBLE CIRCUIT 0 VALUESSUARTERSPAN OUTAGE OUTAGE ___ ______

55kA 00 0 0 2-0 NO VALUES

5 L S F.Oo XEO 55 12-25 40 0 /56 -/5XW 0 0 0

-90 /80 -90STROKE TO DOUBLE CIRCUIT DOUBLE CIRCUIT 6-0 -90 /80 -90MIDSPAN OUTAGE OUTAGE

lA 0 X 0 XX 2-0 NO VALUES0 FO. 0 0F0O

6 X/\Xf X /Xx 91 60-80 4-0 180 -90 -90STROKE TO DOUBLE CIRCUIT DOUBLE CIRCUIT 6-0 0 -156 156

MIDSPAN OUTAGE OUTAGE 90-180 90

GOOkA 0.Xo ,X 2-0 NO VALUESES. ES 90 90 -/80

0X ~~~~~~ /00~~~lo 80 4-0 0 /56 -/567 0 0 0 0

-90 /80 -90STROKE TV DOUBLE CIRCUIT DOUBLE CIRCUIT 6-0 NO VALUESMIDSPAN OUTAGE OUTAGE

Fig. 1. Duplication of lightning incidents on SECV 220-kV system;phasing-ABC, CBA top to bottom. Crest times and powerfrequency voltages treated as variables; values tabulated foreach incident yield calculated outage form.

If a particular stroke had caused an outage, then additionalinformation, such as termination point of the stroke, phasesinvolved in the outage, and footing resistances of nearby towers,was obtained where possible. Seven of the available incidentswere analyzed in detail to test the accuracy of the dynamictraveling wave program. As described in [1], the only unknownsfor each incident were the stroke current time to crest and theinstantaneous power frequency voltages, and hence these weretreated as variables.

Details concerning the outage incidents analyzed and com-parisons of calculated and observed outage forms are shown inFig. 1. A tabulation of values of crest times and instantaneouspower frequency voltages is included in the figure. These values,when used in the calculations for the particular incident, willresult in the calculated outage form shown. Agreement betweenpredicted and observed outage forms is noted to be satisfactory.

Comparison of Predicted and Observed Outage RatesThe performances of several existing transmission systems

were analyzed to determine the over-all accuracy of the de-veloped technique comprising both the dynamic traveling waveand Monte Carlo techniques. For these systems, field dataconcerning both single circuit and double circuit outage rateswere available, and these are given in Table IIT, together withrelevant line parameters.

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915SARGENT AND DARVENIZA: LIGHTNING PERFORMANCE OF DOUBLE-CIRCUIT TRANSMISSION LINES

TABLE IIICOMPARISON OF CALCULATED AND OBSERVED LIGHTNING OUTAGE RATES

Average Footing Shielding Average Outage Rate/Height Resistance (ohms) Earth- Angle Span 100 mile-years Observed/

Line (feet) Range Average wires (degrees) (feet) Observed Calculated CalculatedSEAQ* 110 kV

(single circuit, woodpole)Single-circuit outages 48 0-15 10 2 30 800 0.35 0.45 1.3

SECV 220 kV 100 0-90 28 1 40 1200Total circuit outages 3.0 4.2 1.4Double-circuit outages 1.35 2.0 1.5Percent Double Circuit 45 47 1.05

OVEC 345 kV 148 0-10 5 1 35 1300Total circuit outages 7.2 3.4 0.47Double-circuit outages 0.35 0.2 0.57Percent double circuit 5 6 1.2

TVA 161 kV 110 0-100+ 30 2 0 1240Total circuit outages 4.27 4.14 0.97Double-circuit outages 2.45 2.66 1.06Percent double circuit 55 64 1.16

NEA 132 kV 100 10 1 30 1150Total circuit outages 3.0 2.2 0.73Double-circuit outages 1.1 0.7 0.64Percent double circuit 37 32 0.87

Ontario-Hydro 115 kV 109 5-300+t 200 1 30 1130Total circuit outages 9.2 10.1 1.1Double-circuit outages 6.3 7.5 1.19Percent double circuit 68 74 1.09

* Southern Electricity Authority of Queensland.t Continuous counterpoise.

100- A-SEAQ 110kV D-TVA 161kV- B-SECV 220kV E-NEA 132kV- C-OVEC 345kV F-ONTARIO-HYDRO

115kV

10-_C F

D

EB

10 - /,___._ _Io TOTAL OUTAGE RATE

C~ S/e OD-C OUTAGE RATE

A

0-1 / I,,, ,,I1-0 10

CALCULATED OUTAGE RATE100

Fig. 2. Comparison between predicted and observed outage rates(outages per 100 route mile-years).

The outage rates predicted for these lines are compared withthe observed outage rates in Fig. 2 and Table III. The comparisonbetween predicted and observed proportion of double-circuitoutages is shown in Fig. 3. It is noted that the agreement achievedin these analyses is again satisfactory. In particular, the propor-tion of double-circuit outages predicted by the technique for thevarious lines is in close agreement with the proportion observed.

In these calculations, the probability of shielding failures wasestimated from data given in [2] or [3]. Shielding failures con-

0

w

w

U,inm0

1001

80

/ ONTARIO-60 - HYDRO 115kV

0 "TVA 161kV

NEA 132kV 0 EV20V

20

OVEC 345kV

0 20 AO 60

CALCULATED /o80 100

Fig. 3. Comparison between predicted and observed percentages ofdouble-circuit outages.

tributed significantly to the calculated outage rates for two of thelines, viz., all the single-circuit outages for the OVEC 345-kVline and 29 percent of the single-circuit and 3 percent of thedouble-circuit outages for the SECV 220-kV line. Apart fromthese cases, the remaining outages listed in Table III werepredicted to result from back flashover.For two systems (Ontario Hydro 115-kV line K4W/K5W

and TVA 161-kV line Gallatin-Rockwood nos. 1 and 2), detailedinformation was available concerning the types of outages due to

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IEEE TRANSACTIONS ON POWER APPARATUS AND SYSTEMS, MAY/JUNE 1970

TABLE IVCOMPARISON OF PREDICTED AND OBSERVED PERCENTAGES OF OUTAGE TYPES

Percentage Outage Type OccurringOntario-Hydro 115 kV TVA 161 kV

Outage Type Observed Calculated Observed CalculatedSingle-Circuit OutagesL-G 83 80 79 86LL-G 17 20 14 14LLL-G 0 0 7 0

Double-Circuit OutagesL-G both circuits 62 64 64 79LL-G on at least one circuit 23 23 27 14LLL-G on at least one circuit 15 13 9 7LLL-G both circuits 8 7 0 0

TABLE VINFLUENCE OF LINE PARAMETERS ON LIGHTNING PERFORMANCE, SECV 220-KV LINE

Percentage Reduction inNominal Footing Insulation Outage Rate

Effect Voltage Resistance (10- by 5-inch disks) DoubleAnalyzed (kV) (ohms) Earthwires Circuit 1 Circuit 2 Total Circuit

220 Normal 1 15 15Power frequency voltage 0 Normal 1 15 15 12 10Footing resistance 220 10 1 15 15 60 75Addition of earthwire* 220 Normal 2 15 15 30 42Addition of coupling wiret 220 Normal 2 15 15 24 42

* Two overhead earthwires, separation distance 26 feet.t One overhead earthwire and one coupling wire at level of bottom-phase conductors.

lightning. A comparison between predicted and observed pro-portions of these fault types is shown in Table IV. The satis-factory agreement indicates that the technique can be used forpredicting the likelihood of multiphase faults and hence isapplicable to lines equipped with single-pole reclosing breakersor with arc-suppression coils.

Discussion of Results of ComparisonsThe comparison between calculated results and observed

data has been satisfactory in all facets of the analyses under-taken-in the form of outages observed, in the overall lightningoutage rates of lines, and in the percentages of fault types ob-served. This correlation indicates that the data and methods[1], [4] utilized in the developed technique are adequate, and itdemonstrates the validity of the technique as a method forestimating the lightning performance of transmission lines.In particular, it is concluded that outages resulting from backflashover are adequately accounted for. It is of considerableinterest to note that the high outage rates of the TVA 161-kVand the Ontario Hydro 115-kV lines are demonstrated to ariseentirely as a result of back flashover.

PARAMETERS INFLUENCING DOUBLE-CIRCUIT OUTAGE RATE

Verification of the developed technique enabled investigationsto proceed into the factors influencing double-circuit outagerates of transmission lines. Apart from the provision of adequateshielding, which is the subject of detailed study elsewhere[5], the factors considered to be the most important were 1)tower footing resistance, 2) number and arrangement of earth-wires, 3) relative insulation level of the two circuits, and 4)the instantaneous power frequency voltages on the phases.

To investigate the effect of each of these factors, a sequence ofanalyses was undertaken using the SECV 220-kV system as aline of typical double-circuit construction. In each of theseanalyses, one of the parameters was altered in a known manneror to a known extent from the "normal" system. Any differencesin predicted lightning performance between the "normal" SECVline and each "modified" line could then be ascribed to theeffects of the particular parameter changed in the analysis.The sequence of analvses performed and the results obtained

are summarized in Table V. It should be noted that the valuesshown are only indicative of the effect of the parameter involved.

In other words the precise extent of the influence of aparticular factor is somewhat dependent on the system beinganalyzed.

Effect of Power Frequency Voltages and Phase Location on TowerThe conductors that will be involved in the outage and the

order in which they are involved depends primarily on threefactors: the potentials of the crossarms of the tower, the couplingfactors between the conductors and earthwire system, and theinstantaneous power frequency voltages of the phases. Analysesindicated that the difference in potentials between the lowercrossarm of a conventional double-circuit tower and the towertop is less than 15 percent for times greater than 2 ,us if noflashover has occurred previously, and almost negligible ifflashovers have occurred. In [1] it was demonstrated that theeffects of differences in crossarm potentials and coupling factorsare largely self-cancelling. Therefore, it is evident that the likeli-hood of back flashovers to any particular conductor locations isgreatly influenced by the relative magnitudes of the power fre-quency voltages.

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SARGENT AND DARVENIZA: LIGHTNING PERFORMANCE OF DOUBLE-CIRCUIT TRANSMISSION LINES

TABLE VIRESULTS OF ANALYSES-EFFECT OF POWER FREQUENCY

VOLTAGES AND CONDUCTOR LocATION ON TOWER

Power Frequency Voltageor Percentage

Conductor Location First FlashoversVoltage

Highest 83Median 17Lowest 0

Conductor LocationTop 10Middle 32Bottom 58

TABLE VIIMERITS OF DIFFERENTIAL INSULATION

Insulation Percentage Change(10- by 5-inch disks) in Outage Rate

Circuit Circuit DoubleLine 1 2 Total Circuit

SECV 220 kV 15 1514 16 + 5 -1813 17 +11 -42

TVA 161 kV 11 1110 12 +37 +1110 14 +37 -56

This effect was indicated in the performance analyses ofspecific transmission systems. For example, the data in TableVI show that the major proportion of first flashovers (excludingshielding failures) occurred on the phase with the highest powerfrequency voltage, whereas the effect of phase conductor loca-tion on the tower was not as pronounced. The effect that theinstantaneous power frequency voltages have on the sequence inwhich the phases are involved in the outage was also indicatedin these analyses by the prediction (considering double-circuitoutages only) that, after the first flashover, the next flashoverwould occur on the same phase in the second circuit in 55 percentof the cases, while in 30 percent it would occur on a differentphase of the second circuit. In 15 percent of the cases, the secondflashover involved another phase of the same circuit as the firstflashover. This general trend is also evident in the analyses ofavailable field data for the SECV 220-kV and the TVA 161-kVlines [1, Fig. 1 and discussion].Effect of Differential Insulation

Proposals have been made to reduce the double-circuitoutage rates of transmission lines by use of differential insulation.The design [6] of the differential required is based on the mag-nitude of the nominal line voltage, with safety margins, theprime objective being to overcome the effect that the instan-taneous power frequency voltages have on the sequence inwhich phases are involved in flashover. Analyses of the merit ofthe differential insulation were undertaken for the SECV 220-kVand TVA 161-kV lines. The results of these analyses, summarizedin Table VII, indicate that although differential insulation canreduce the double-circuit outage rate to some extent, the amountof differential required to eliminate completely double-circuitoutages would be unreasonable for economic tower and linedesign-at least for lines of these voltages and higher. Further,

TABLE VIIIEFFECT OF ADDITIONAL EARTHWIRE

PercentageReduction

in Outage RateEarthwire Double

Line Arrangement Total CircuitSECV 220 kV Horizontal 30 42

Vertical 24 42Ontario-Hydro Horizontal 35 43

115 kV Vertical 59 61

application of differential insulation normally necessitatesunderinsulation of one circuit, and as illustrated in Table VII,this results in an increase in the total number of outages experi-enced by the line. Also, it can be demonstrated theoreticallythat flashover to one conductor can in some cases cause anincrease in the voltage stressing the insulation of the remainingsound phases (rather than the much more usual decrease).Therefore, it can be expected that occasionally the use of under-insulation will also slightly increase the double-circuit outagerate. It is of interest that TVA recently converted a 161-kV linefrom the normal balanced insulation to differential insulation,with 10 disks on one circuit and 12 on the second. The results ofone years' service so far indicate a substantial increase in theoverall outage rate of the line with no reduction of outages onthe overinsulated circuit.Effect of Tower Footing ResistanceThe effect of reducing the footing resistances of the towers of

the SECV 220-kV line to a value of 10 ohms was analyzed toindicate the general influence of footing resistance on double-circuit outage rate. The results in Table V indicate that theproportion of double-circuit outages is not greatly influenced bytower footing resistances. However, both total and double-circuitoutage rates are significantly reduced, and to a similar extent,by decreased resistances.

Effect of Additional EarthwiresThe addition of a second earthwire to a line should improve

the lightning performance of the line for several reasons-improved shielding and coupling to phase conductors, and de-creased impedance presented to the lightning stroke. Theeffect of adding a second earthwire was analyzed for two lines(SECV 220-kV and Ontario-Hydro 115-kV). Two possible loca-tions were considered for this second earthwire-either at thesame level as the existing earthwire to provide a second shieldingwire, or at a point in the middle of the tower at a comparableheight to the phase conductors to act as a coupling wire. Theresults of these analyses, as summarized in Table VIII, indicatea significant improvement in both the total and double-circuitoutage rates of the lines. As the use of a coupling wire may involvecertain difficulties in maintaining clearances at midspan, theuse of the second shielding wire may be preferable, and wouldof course have the added benefit of improved shielding.

APPLICATION TO LINE DESIGNThe problem of designing a double-circuit line with acceptable

double-circuit outage rate has assumed increasing importance inmodern EHV transmission systems. The results of the analysesdescribed previously indicate that there are only limited possi-

917

IEEE TRANSACTrIONS ON POWER APPARATUS AND SYSTEMS, MAY/JUNE 1970

bilities of improving the double-circuit outage rate of conven-tional lines without improvement of overall line outage rate bythe normal techniques of lowering tower footing resistance andusing additional earthwires. The limited economic usefulness ofdifferential insulation for the conventional double-circuit tower isan inherent consequence of the vertical circuit arrangement-the flashover of one phase of one circuit normally improves thecoupling factors of the remaining phases of this circuit to agreater extent than those of the phases of the second circuit.This suggests the possibility of rearranging the phasing of thecircuits to reduce double circuit faults. However, detailed ex-aminations of vertical configurations have failed to yield anyphasing arrangement providing significant improvement indouble-circuit outage rate.

Several design studies undertaken have confirmed these con-clusions and have indicated an improved double-circuit lineconfiguration, having acceptable double-circuit performance.One study providing significant results was the design of a 132-kV line that was also required to support a 22-kV circuit in anarea of high soil resistivities. Two alternative designs wereinvestigated for this line-in one, each circuit was to be mountedvertically on opposite sides of the tower, whereas the seconddesign considered an H-frame structure with the 22-kV circuitstrung below the horizontally disposed 132-kV circuit. It isnoted that these lines have a large degree of differential in-sulation. Analyses showed, however, that with the verticalcircuit arrangement, this differential was not sufficient toeliminate double-circuit outages; i.e., some outages still involvedthe 132-kY circuit. In contrast, for the H-frame line with hori-zontal circuit arrangements, double-circuit outages were elim-inated. The low sparkover voltage of the 22-kV insulation re-sulted in a large number of flashovers on the 22-kV circuit.However, these flashovers greatly improved the coupling of allphases of the 132-kV circuit located between the 22-kV circuitand the earthwires.

Fig. 4. Conventiornal 220-kV double-circuit tower.

I'9

rt18

87'

50'

EART HWIRES19/ 080 STEEL,~

N

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2'-4-12'-

=I- INSULATION\ IO'5"DISCS

I 16 FOR UPPER CCT.-14 FOR LOWER CCT.

12 CONDUCTORSSCA 54/7/-1291AVERAGE SPAN

1050'

Suggested Line Designs Having Low Double-Circuit Outage RatesThe previous studies suggested that a double-circuit line

construction using circuits disposed horizonitally, one above theother, would provide a situation in which most flashovers couldbe confined to one circuit. Because of smaller coupling factors,back flashovers are first expected to conductors on the lowercircuit. The preferential involvement of the lower circuit canbe enhanced by providing it with less insulation than the uppercircuit. Once flashover occurs to one or more conductors of thelower circuit, the conductors of the upper circuit are virtuallyenclosed by stricken conductors (overhead earthwires andfaulted lower circuit conductors). This results in relatively largecoupling factors, and hence creates conditioins most favorablefor avoiding the possibility of a flashover to conductors of theupper circuit.Most of the above features can also be achieved with a double-

triangular configuration of circuit conductors mounted on aconventional lattice tower. In this arrangement, conductors ofcircuit 1 form a triangle with two conductors on the top cross-arm and one on the center arm, and circuit 2 conductors areplaced on the other center arm position and on the bottomcrossarm. As before, the triangular arrangement promotesconfinement of flashover to circuit 2, first because phasing can bearranged so that conductors of this circuit are always belowcircuit 1 conductors of the same phase, and second as a resultof the use of differential insulation.

Fig. 5. 220-kV double-circuit line, horizontal conifiguiration.

These considerations were tested by a comparative analysisof the conventional vertical arrangement and the suggestedhorizontal and triangular configurations. Two line classes wereexamined:

1) the SECV 220-kV dc line of Fig. 4, the triaingular con-figuration arrainged on the same towxer, and its equivalenthorizontal modification as shown in Fig. 5;

2) the basic Ontario-Hydro 115-kV line of nornmal verticalconfiguration and the triangular and horizontal miiodifications.

In both instances the calculations were petforiiied for linesfitted with two overhead earthwires. Detailed considerationwas not giveni to mechanical aspects of the modified designs,but generally similar line parameters were retained (conductorsizes, sags, air clearances, etc.). The concductor phasings areshown in the caption to Fig. 6.The outage rates predicted for the three conifigurations are

given in Fig. 6. It can readily be seen for both the 115-kV and the220-kV lines that the horizontal and the trianigular configura-tions greatly reduce the likelihood of double-circuit outages,even in the presence of high footing resistances which result inhigh total outage rates. It is of particular signiificance that thereduction occurs mainly because flashovers are largely confinedto the conductors of one circuit, the lower circuit. This is illus-trated by an analysis of the flashover types predicted for the

I

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918

SARGENT AND DARVENIZA: LIGHTNING PERFORMANCIE OF DOUBLE-CIRCUIT TRANSMISSION LINES

DOUBLE CIRCUIT TOTALOUTAGE RATE OUTAGE RATE

VERT. 200SI+C'POISES 8/8DISCS

1l5kV `IsIc1, } -_ f f i _ z ZA~~~1

22LLINES

TRIANG. 20011+CPOISES

_~~~~~~/ DISCS/////g9 DSSVR.15115 DISCSKE

10S ///// ~~~~~SINGLE CIRCUIT OUTAGES

S DISCS5 ,,+E DOUBLE CIRCUIT OUTAGES

TR ANG. 16/ IA DISCS

0 2 3 4 5 6 7 8OUTAGES/ 100 MILE-YEARS

Fig. 6. Predicted total and double-circuit outage rates of vertical,horizontal, and triangular lines with two overhead earthwires.Data include mean of footinig resistance distributions, number of10- by 5-inch insulators on circuit 1/circuit 2, and conductor phas-ings as follows:

Vertical Triangular HorizontalA1A2 A, B1 At B1 C,B, B2 C1 A2 A2 B2 C2Cl C2 B2 C2

115-kV lines insulated with eight disks on each circuit. For boththe horizontal and triangular designs, all the single-circuitflashovers involved circuit 2 (the lower circuit) only. Whendouble-circuit outages were predicted, normally two and oftenthree coniductors of the lower circuit were involved in flashoversbefore conductors of the upper circuit became involved. Asexpected from purely geometrical considerations of coupling,the horizontal configuration is somewhat superior to the tri-angular configuration in this regard. The further improvementpredicted for the differential insulation (e.g., 9/8 disks on the115-kV line) is due first to enhanced confinement of flashoversto the lower circuit, and second to the increased insulationstrength of the upper circuit.The results in Fig. 6 also demonstrate that a comparable

improvemiient in the double-circuit outage rate of the verticalconfiguration can only be achieved by a reduction in the totalrate-see, for example, the result for the 220-kV line having afooting resistance of 10 ohms. Of course the total outage ratesfor the horizontal and vertical designs are also reduced to a lowvalue with this footing resistance. It is to be noted that nodouble-circuit outages were predicted for the 220-kV horizontaldesign with 10 ohms, and this result holds true for footingresistainees up to 50 ohms. (The predicted double-circuit outagesfor the footing resistance distribution having a mean of 28 ohmswere associated with individual towers having footing resistancesgreater than 50 ohms.)

These results show that the horizontal design is somewhatsuperior to the trianigular arrangement both in total and double-circuit outage rates, and that both have markedly better double-circuit outage rate performances than the conventional verticalconfiguration. It is concluded, therefore, that the horizontal con-figuration offers important economies to line designers in thosesituations where low footing resistances are diffitult to obtainand where a low double-circuit outage rate is still required. It is

recognized, however, that there are some difficulties in usingthis form of line (e.g., operationial nmaintenance, transpositions,novelty of tower design, etc.) that may mitigate against thesuperior double-circuit lightniing outage rate. Although the pre-dicted performance of the triangular configuration is not asgood, it does represent a substantial improvement over that ofthe conventional vertical line. Further, the triangular designhas the great advantage that an existing conventional verticalline can be converted readily to the suggested configuration. Theauthors believe that this facility provides a ready means forconfirming the validity of the foregoing predictions by serviceexperience.

CONCLUSIONS1) Results of further anialyses using a previously described

technique have confirmed its validity for predicting lightningoutage rates on single- and double-circuit lines. Satisfactoryagreement with field data has been demonstrated for calcula-tions of total and double-circuit outage rates .of 110-345-kYlines, and for the prediction of outage types typically encounteredon such lines.

2) Analyses have been iilade of the various factors thatinfluence the double-circuit outage rate. The results of thesestudies have been applied to the question of designing lines withacceptably low double-circuit outage rates, and it is concludedthat the only satisfactory method of achieving the requiredperformance with conventional lines of vertical configurationis to ensure a low total outage rate. The use of differentialinsulation was examined in detail, and it was found that thisalone did not provide a practical method of reducing the propor-tion of double-circuit outages on lines of vertical configuration.

3) A double-circuit line design with circuits horizontallydisposed one above the other appears to provide an economicaldesign in which double-circuit outages are minimized even forhigh values of footing resistance. Because of the relative locationof the circuits, differential insulation can be used with optimumeffect on this construction.

4) Similar improvements in double-circuit outage rate per-formance are expected for a double-triangular configuration ofcircuit conductors, as can be arranged on a conventional latticetower having three crossarms.

ACKNOWLEDGMENTThe work reported in this paper was mostly carried out in the

Department of Electrical Engineering, University of Queens-land, and was supported by the Electrical Research Board ofthe Electricity Supply Association of Australia. The authorsgratefully acknowledge the cooperation of the Tennessee ValleyAuthority, the Hydro-Electric Power Commission of Ontario,and the following Supply Authorities in Australia: the StateElectricity Commissions of Victoria, New South Wales, andQueensland. The authors wish to thank Mr. H. Linck of Ontario-Hydro for helpful discussions and for suggesting study of thedouble-triangular configuration.

REFERENCES[1] M. A. Sargent and M. Darveniza, "The calculation of the

double-circuit outage rate of transmission lines," IEEE Trans.Power Apparatus and Systems, vol. PAS-86, pp. 665-678, June1967.

[2] V. V. Burgsdorf, "Lightniing protection of overhead lines andoperating experience in the USSR," CIGRE, Rept.,326, Paris,1958.

919

IEEE TRANSACTIONS ON POWER APPARATUS AND SYSTEMS, MAY/JUNE 1970

[3] M. V. Kostenko, I. F. Polovoie, and A. N. Rozenfeld, "Effect oflightning flashes on electrical conductors bypassing the shieldingof overhead groundwire for lightning protection of EHV trans-mission lines" (in Russian), Etektrichestvo, vol. 81, pp. 20-26,April 1961.

[4] M. A. Sargent and M. Darveniza, "Tower surge impedance,"IEEE Trans. Power Apparatus and Systems, vol. PAS-88,pp. 680-687, May 1969.

[5] G. W. Brown and E. R. Whitehead, "Field and analyticalstudies of transmission line shielding-II," and referencedpapers, IEEE Trans. Power Apparatus and Systems, vol. PAS-88, pp. 617-626, May 1969.

[6] M. Kawai and H. Azuma, "Design and performance of un-balanced insulation of double-circuit transmission lines," IEEETrans. Power Apparatus and Systems, vol. PAS-84, pp. 839-846,September, 1965.

Discussion

D. H. A. Tufnell (Central Electricity Generating Board, London,England): The authors describe valuable work in assessing whichelectrical parameters influence the double-circuit outage rate of anoverhead line.A recent survey of the lightning faults on the CEGB system gave

the results shown in Table IX. It will be noted from the table thatlightning performance of the CEGB system is better than thosequoted in this paper. This is probably due to the low isocerauniclevel in Britain (the number of thunderstorm days only exceeded 21in very few places during 1967) and due to low tower footingresistances. It is rare for this resistance to exceed 10 ohms, and in thecase of the two higher voltages, 5 ohms is a normal maximum.However, in certain mountainous areas resistances exceeding 200ohms have been measured.

TABLE IX

System Voltage(kV)

Year 400 275 1321965-1966 Outage rate/100 miles 0.76 0.67

Percentage double-circuit faults 0 431966-1967 Outage rate/100 miles 0.62 0.91

Percentage double-circuit faults 25 201967-1968 Outage rate/100 miles 1.1 1.22 1.49

Percentage double-circuit faults 4* 12 21* This is represented by one double-circuit fault. It should be

noted that this survey only counted the total number of simul-taneous faults on a double circuit due to all causes; but it is con-sidered that most, if not all, of these faults were caused by lightning.

It will also be noted from the table that the double-circuit outagerate improves for the higher voltages whereas the single-circuitoutage rate remains disappointingly high. It would be interestingto know the lightning performance of other systems with workingvoltages exceeding 300 kV.The authors go on to describe two line configurations that would

reduce the double-circuit fault rate. They point out the short-comings of the twin horizontal configuration, but I think the double-triangular configurations has serious operational safety problems.It would probably be necessary to have a double-circuit outage formaintenance on the "top" circuit unless "live line" working tech-niques are employed. Also line galloping could cause a double-circuit fault whereas the standard configuration only causes a single-circuit fault.

E. R. Whitehead (Illinois Institute of Technology, Chicago, Ill.):This paper and its predecessor paper [1] by the same authors con-stitute an extremely important contribution to our quantitativeknowledge of the lightning performance of transmission lines.The observation that the principal results agree with our accumu-

lated qualitative judgment should be somewhat reassuring withrespect to our deductions from more circumscribed analyticalprocesses. The power of the computer to carry out the enormousdetail of the traveling wave solutions, including variation of non-linear parameters, is responsible for the confidence one may feel inthe numerical results, but an equally important aspect of this workis the apparent adequacy of the analytical model and the resultantprogram.The discouraging results from the analytical prediction of lightning

performance in the past resulted chiefly from the inclusion of largenumbers of shielding failures in lightning tripout data. Although theestimation of the proportion of shielding failures from partiallyeffective shielding is not now, and is really unlikely to be in thefuture, a very accurate process, the effort is well worthwhile as thepresent paper clearly demonstrates. With improved estimation ofshielding failure rates, the study of "backflash" failure rates shouldbe placed on a firm foundation so that greater resolution of theinfluences of grounding methods and other design variables can beconfidently expected.

Manuscript received July 17, 1969.

M. Kawai (General Electric Company, Project UHV, Pittsfield,Mass.): The authors are to be congratulated on undertaking such avery interesting and comprehensive study. They have pointed outthat reduction of tower footing resistance is a very effective meansof improving lightning performance on transmission lines, and inthis observation I think almost everyone would concur. However,data on several lines collected by this discusser both in Japan andin the United States frequently show little if any correlation betweenoutage rate and tower footing resistance. Based on the sum of allfield data and on theoretical calculations, it can be easily concludedthat an improvement in footing resistance reduces the lightningoutage rate, but an effective reduction is often masked by a greatmany other parameters.

In a previous paper [7] involving a comprehensive study of trans-mission lines in Japan, it was noted that the yearly variation ofoutages was quite extreme and that flashovers were often concen-trated on particular sections of the lines, thereby strongly influenc-ing the line performance. Also, magnetic link studies made at thistime and covering several geological areas suggested strongly thatthe probability of occurrence of given values of stroke currentchanged appreciably from one area to another [6], [8]. It is hard tosee how these factors can be considered in the calculation of outagerates, yet nevertheless play an important role in the process. In fact,many other factors affecting lightning flashover are vague in theextreme and may not be clarified for many years to come. At thisstage of knowledge, it seems to this discusser that it is extremelydifficult to claim much accuracy in predicting the lightning per-formance of transmission lines. Therefore, this discusser does notconcur with the authors' conclusion 1) in spite of the indicated goodagreement.Examining Fig. 6(c), one notes a great improvement expected in

the double-circuit flashover rate of the triangular circuit 3. In thecalculation of the performance of phases Al and C, of the triangularcircuit, the improvement in performance is about the same as thatof the differential circuit 2, largely because the influences of cross-arm potentials and coupling factors tend to cancel each other out(Fig. 7). Thus, it is only natural that most of the double-circuitflashover in the differential insulation system should occur onthe bottom phase of the high-insulation circuit. However, investi-gating the performance of unbalanced (differential) insulation linesand normally insulated lines in Japan, I could find no such tendency.

Manuscript received July 16, 1969.

920

Manuscript received July 17, 1969.

SARGENT AND DARVENIZA: LIGHTNING PERFORMANCE OF DOUBLE-CIRCUIT TRANSMISSION LINES

I. V ERTICAL

Al A2Bl B2CI C2

(1 2) (12)

2. DIFFERENTIAL

A I A2B I B2C I C2(14) (12)

48

d-c OUTAGE00 %

32

(0LJ24

0

C)O 16U.tc..,

-82z

0

82%( FROM TABLEYl )

3. TRIANGULAR

A I CI(14) B A2

B2 C2 (12)330/h

Fig. 7. Various conductor phasings.

IJ195J7 1959 131 1963 1935 iSS7

Y(ARS(a)

TOTAL

YEARS

(b)Fig. 8. Lightning performance of unbalanced insulation lines. (a)

154-kV Kiso line. (b) 154-kV Subara-Oi line. *-unbalancedinsulation line; j-- normal insulation line; solid line-total out-age; dashed line-double-circuit flashover; *-start of unbalancedinsulation.

In closing, this discusser would like to present some curves showingthe performance of unbalanced insulation lines with that of normallines in Japan (Fig. 8). The design data for these circuits are pre-sented in [6].The differences in performance of the two lines are quite obvious.

The very considerable increase in flashover rate in 1963-1964 on thereduced insulation circuit after it was installed is not due to thereduced insulation, but to the fact that all lines in those years had amuch higher flashover rate due to an extremely severe lightningseason. This can be seen by comparing the total records on Fig. 8and 9.

d -c

1957 1359 1 I I93YEARS

(a)

I % ;

24

U-

1957 1959 196 1963 1965 1967YEARS

(b)Fig. 9. Lightning performance of unbalanced insulation lines.

(a) 154-kV Hida old line. (b) 154-kV Hida new line. Notation asin Fig. 8.

REFERENCES[7] M. Kawai, N. Kodama, and S. Minemura, "Lightning per-

formance of transmission lines in Tokyo Area," IEEE Trans.Power Apparatus and Systems, vol. PAS-87, pp. 13-23, January1968.

[81 M. Kawai, "Investigation on lightning performance of trans-mission lines" (in Japanese), CRIEPI, Rept. 65008, April 1965.

D. C. Smith and J. A. Downey (State Electricity Commission ofVictoria, Australia): The authors are to be congratulated on theiranalysis of a problem that can play an important part in the securityof supply and the solutions they have suggested. The paper analyzesone section of a 150-mile double-circuit 220-kV line used to transmitpower from Victorian hydro stations. The line passes through amountainous region characterized by high tower footing resistanceand high lightning incidence resulting in a high double-circuit outagerate. High-speed reclosure is used for both circuits. The Commissionhas within its system a large number of 220-kV lines of verticalconstruction used for both the transmission of power and the distri-bution of load. In general, these lines pass through areas of very lowtower footing resistances and low lightning incidence. These lineshave a very low lightning outage rate and they have had a total ofeight single-circuit outages but no double-circuit outages. Theconductor phasing is principally ABC, CBA, but on some lines thephasing is ABC, ABC, and one of the above single-circuit outagesoccurred on such a line. Table V shows that the double-circuitoutage rate for low tower footing resistances is less than 40 percentof the total outages, and Fig. 6 shows that all outages for a 10-ohmtower footing resistance are double circuit. Could the authors com-ment on whether this is due to the different phase rotation and

Manuscript received July 17, 1969.

C,,LO

se10

o

LOC_."=..

n

40

ui

921

IEEE TRANSACTIONS ON POWER APPARATUS AND SYSTEMS, MAY/JUNE 1970

whether lines with a low tower footing resistance are influencedmore than high tower footing resistance lines by the positioning of thephase conductors?The horizontal line arrangement suggested by the authors has

the disadvantage that tower costs will be higher and conductormaintenance on the top circuit would mean lengthy double-circuitoutages that may, in some cases, be unscheduled (e.g., broken pinon an insulator). This arrangement also relies on the flashover ofmultiple phases of the lower circuit and this could lead to an increasein the incidence of three-phase faults in the system, which mayadversely affect the stability of machines in the system (our par-ticular system is designed for a two-phase ground fault). Could theauthors give an indication of the increased incidence of a three-phasefault compared with the normal vertical configuration? The alter-native triangular configuration suggested is not favored because ofthe critical nature of the identification of circuits for maintenance;also, conductor maintenance on the upper circuit would necessitatedouble-circuit outages.

A. Beloff and R. X. French (Sargent and Lundy Engineers, Chicago,Ill.): When the authors presented a method for the computation ofdouble-circuit outage rates some two years ago [1], there were highhopes that they would continue their investigation in a manner thatwould satisfy the expectations of such a valuable and needed tool.Today it is most gratifying to find that the authors have furthervalidated the method by comparing actual and predicted per-formance of operating double-circuit lines, and interesting con-clusions have been drawn for the benefit of transmission line engineerspointing to areas of improvement in the design of double-circutittransmission lines.We would like to congratulate the authors for what appears to be a

good correlation between actual performance observed on severaloperating double-circuit lines and the calculated douLble-circuitoutage rates obtained with the analytical model. It would be in-teresting to know, however, whether the rather high total lightningoutage rates shown in Table II are typical, as stated in the paper,or whether they are examples of particular lines with unusuallyhigh tripout rates. If today's transmission lines are designed for 1outage per 100 mile-years or less, even a 50-percent chance of double-circuit outage may not be of practical significance to system planningor operation. Power system reliability criteria must always considerdouble-circuit outages, even if their probability is low; therefore, theactual rate is not of great importance. It is, of course, always desir-able to provide the lowest outage rate consistent with economy.

It would be beneficial to have the authors expand on the point ofthe effectiveness of tower footing resistance in reducing the propor-tion of double-circuit outages. Table V shows a modest benefitfrom reducing tower footing resistance on a particular line. How-ever, this appears inconsistent with Table III, which indicates somecorrelation between tower footing resistance and the percentage ofdouble-circuLit outage, although no definite conclusions can be drawnfrom such a small number of cases. Footing resistance is often thecrucial factor in a designer's decision; therefore, practical designguides based on the authors' new method would be a significantcontribution to the industry.We would like to comment on the practicality of suggested line

designs having low double-circuit outage rates. The use of double-circuit lines using circuits of the same voltage disposed horizontallyhad very low appeal in the past, mainly because of maintenanceproblems. As we move into EHV, and specifically speaking of 345-kVlines in the USA, the trend has been toward the design of circuitsthat can be maintained with the use of hot line tools and withconductors supported on offset crossarms in order to minimize theeffect of galloping conductors on line performance. The proposeddouble-triangular phasing arrangement for double-circuit linesdisposed in a vertical configuration, on the other hand, shouldcertainly merit the attention of transmission engineers since it willstill retain the physical advantages of the conventional vertical line.

A question arises, whether, in the authors' opinion, a change in thedouble-triangular phasing, such as A,, A2, B2 top-to-bottom phaseson one side of the tower and B,, Cl, C2 on the other side of the tower,would be as effective as the one suggested in the paper. We foundthat this phasing arrangement would lower the overall conductorsurface gradient by about 9 percent on an average as comnpared tothe more common vertical phase arrangement, whereas the oneproposed by the authors would give slightly higher values.At this juncture, we would like to ask the authors whether they

have investigated the use of double-circuit triangular configurationwhere each triangle occupies one side of the tower with one conductorsupported oni the top arm and the two lower conductors supportedon the lower arm. This arrangement is typical of a low-profile towerline with two shield wires, and it would be interesting to knowwhether it would provide better inherent dotuble-circuit otutage per-formance than the conventional tier-type tower of imiuch widerapplication.

Finally, it would be of much interest to transmission entgineers ifsome type of application curves were developed for predictingdouble-circuit outage rates for different line configurations, much inthe same way as probability curves have beein published by IEEEand more recently by others after numerous studies on lightningphenomena anid their effect on line performancee.

H. Linck, T. J. McClelland, and K. R. McClymont (The Hydro-Electric Power Commission of Ontario, Torotnto, Ont., Canada):Ontario Hydro is constructing two parallel 230-kV circuits as atransmission link between their East and West systems. The 1968peak demand for the East system exceeded 9000 MW and for theWest system 600 MW. This interconnection will be more than 500miles long and will have five intermediate stations for connectingload and generation along the line. Wheni completed in 1970, it willbe capable of transmitting 150 MW in each direction. Since this linewill from the only direct electrical link between the two systems,it is important to minimize simultaneous outages of both circuitsfor those parts of the line where double-circuit construction waschosen for economical reason.The terrain traversed by the line is part of the Canadian shield

and solid rock is seldom more than a few feet below the surface.Table X shows that more than 50 percent of the tower footingresistances are in excess of 100 ohms. These values were measuredbefore counterpoise was installed. Previous experience had shownthat two shield wires and two-wire counterpoise would be requiredfor satisfactory lightning performance. The isocerauniic level forthe area is 20 to 25. Although our experience with double-circuitoutages on 230-kV lines of the type contemplated had been good,this experience was gained largely with lines in terrain where towerfooting conditions are more favorable. In view of these considera-tions it was decided that an anialytical study of the lightning per-formance of the double-circuit section of the tie line was mostdesirable.

In the search for a facility to carry out this work we soon dis-covered that only the authors of the paper were able to offer adefinite proposal regarding immediate action. First, outage calcula-tions were performed for an existing double-circuit 115-kV linelocated in territory similar to that of the East-West tie line. Theresults of this investigation are given in the paper. We were quiteimpressed with the close agreement between computer resuilts andservice experienice and, therefore, embarked on the study of the230-kV tie line, employing the authors' expertise and computingfacilities.To avoid galloping outages on our douible-circuit 230-kV lines

when conductors are ice coated, we offset the center conductorsin the horizontal direction about 10 feet from the upper and lowerconductors and provide an 18-foot vertical spacing. This require-ment makes the type of horizontal configuration illustrated in Fig. 5impractical for our coniditions.

Manuscript received December 9, 1969.

922

Manitseript received Jttly 17, 1969.

SARGENT AND DAIYEVINIZA%: LIGHTNING PERFORMANClE' OF DOUIILE'-CIRCUIT TRANSMISSION LINES

TABLE XGROUND RESISTANCE DISTRIBUTION FOR 285 TOWERS OF

EAST-WEST TIE LINE3

Percenitage Towers withResistance Footing Resistance(ohms) Greater than Value

20 8140 7860 7280 67

100 6415( 49200 35250 27300 17

TABLE XISUMMARY OF ESTIMATED OUTAGIE RATES FROM LIGHTNING-EAST-WEST TIE LINE, 230-KV DOUBLE-CIRCUIT (BASED

ON FIVE-YEAR SERVICE PERIOD)

Outage Rate*(per 100 mile-

Insulation years)Circuit Circuit Double

Circuit 1 2 Total CircuitConventionalNo counterpoise 14 14 2.5 1 .9Counterpoise 14 14 1 .5 0.9Counterpoise 14 10 2.8 0. 8

Double-TriangularCounterpoise 14 10 3.3 0.4Counterpoise 14 12 2.2 0.4Counterpoise 15 13 2.1 0.3

* Outage rate assuinmed to be 50 percent of flashover rate.

Computer results for a variety of conditionis, based on the useof our conventional double-circuit tower, are shown in Table XI.On the basis of these results, the use of the triangular configurationwith differential insulation appeared promising.

Other considerations in comparing the coniventional and thetriangular configuration were:

1) With the triangular configuration, a brokeni conduictor canresult in a double-circuit ouLtage. Extended outages of two circuitsfrom this cause can be avoided by temporarily reconinecting thephases at the line terminials after the occurrence of this type of failure.

2) Since the triangular configuration is unconventional, mainte-nance personnel need to be warned, possibly by installing specialmarkers on the towers.

3) Differences in current balance, radio iinterference, and com-munication circuit initerference between the two arrangements werenot considered significant.

4) Ten insulators per string would be marginal, especially inpolluted areas. It was decided that a minimum of 12, or preferably 13units should be used in the bottom circuit.

5) The alternative of constructing both circuits with full insula-tion, and shorting out several units by jumpers on the lower circuit,was ruled out because of a lack of suitable hardware.

After weighing these considerations, it was decided to use thetriangular configuration, with 15 and 13 insulators on a 240-miledouble-circuit section of the East-West tie line and also on a 120-mile 230-kV line in the same area, which is not part of the inter-connection but for which double-circuit outages can result in inter-ruptions of customers' supply. A 40-mile section of the tie line,already strung with 14 insulators when the decision on the triangularcircuit scheme was made, will be operated with 14 units on the topcircuit and 12 units on the bottom circuit.

Michael Sargent and Mat Darveniza: The authors wish to thank thediscussers for their comments and interest. Because of the increasingneed to achieve maximum utilization of transmission rights of way,double-circuit lines are being used to an increasing extent at highervoltages, and their reliability has become a matter of considerableimportance. Except for areas of low lightning activity, lightningperformance is usually the main parameter controlling line insula-tion design, and this topic was the subject of a colloquium arrangedby CIGRE Study Committee 33 in Sydney, Australia, duringSeptember 1969. A report on double-circuit lines was presented tothe colloquium by one of the authors (M. Darveniza), and this wasbased on the present paper suitably broadened to include data anddevelopments from other countries.

It has been suggested that the lightning outage rates shown inTable II are not representative of the performance typically achievedby double-circuit lines. We have not made an extensive survey-further lightning outage statistics are given in Table XII, and theseindicate a wide variety in lightninig performance. As noted by Mr.Tufnell, the data suggest that both the double-circuit outage rateand the proportion of double-circuit outages decrease with increasingsystem voltage. A study of the available data on fault types andlocations suggests that the majority of single line-ground faultsmay be attributed to shielding failures and that the more complexoutage types such as double-circuit faults are associated withbackflash events.We agree with Prof. Whitehead that improved estimation of

shielding failure rates, when possible, will contribute to more accurateestimations of backflash failure rates. Similarly, advances in ourknowledge of other parameters involved in the analysis of trans-missioin line lightning performance, such as the correlations betweenmeteorological and geographical conditions referred to by Mr.Kawai, will contribute to the refinement and accuracy of predictiontechniques. However, until such data are available, somewhatimperfect methods based on somewhat imperfect data must be usedto estimate the approximate magnitude of these effects, to providean estimate of the linle performance. Therefore, it was concludedthat the technique proposed in this and previous papers [1], [4]provides a method of estimating the lightning performance of trans-mission lines with an accuracy consistent with the accuracy ofavailable data. As improved data become available, the technique isamenable to refinement, incorporating new data to improve theaccuracy of estimation.

Mr. Smith and Mr. 1)owney have noted differenees between theproportionis of double-circuit outages for the SECV 220-kV linewith 10 ohms footing resistance as shown in Table V, and for thevertical 220-kV line in Fig. 6. The former analysis was for a line withone overhead earthwire (OHEW), whereas a two-OHEW line wasanalyzed in Fig. 6 and permitted nio shielding failures. The SECVexperience of eight single-circuit outages on 220-kV lines with lowfooting resistances was of interest and further details were sought.Seven of the outages occurred on lines having one OHEW, andthese can be reasonably attributed to shielding failures. However,the sinigle-circuit outage on the lines with two OHEWs might havebeen a back fiashover anid, if so, this was in conflict with the resultof all double-circtit otutages given in Fig. 6. This prompted furtherstudies using the same computer program, but converted to FORTRANIv for the University's newly acquired PDP 10 computing system.As well as greatly reducing computation time compared to theoriginial GE 225 system (344 seconds compared with 227 minutesfor a five-year analysis), the larger memory core storage of the PDP10 made possible the incorporation of certain refinements in theprogram. The stroke magnitude distribution was originally describedby 250 values with the result that at the high-current end the currentsteps were rather larger than desirable. The number of values wasincreased to 1000, and a rerun of the 10-ohm 220-kV vertical case ofFig. 6 yielded one single-circuit outage for a ten-year service analysis.However, as Mr. Kawai suggests, the effect of footing resistance

on line performance may be masked by other factors. Design of aline in terms of average footing resistance can lead to erroneousconclusions. Lightning outages of well-designed lines are occurrencesof low probability, more dependent on extremal rather than average

Manuscript received December 9, 1969.

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IEEE TRANSACTIONS ON POWER APPARATUS AND SYSTEMS, MAY/JUNE 1970

TABLE XIILIGHTNING OUTAGE RATES OF DOUBLE-CIRCUIT LINES

PercentageVoltage Outage Rates Double-Circuit

Utility* (kV) Earthwires Total Double Circuit OutagesOntario-Hydro (Canada) 115 1 9.2 6.3 68NEA (Australia) 132 1 3.0 1.1 37ECNSW ((Australia) 132 2 7.2 2.9 40CEGB (Britain) 132 1 2.0 0.5 25Com. Edison (USA) 138 1 8.0 2.8 35Com. Edison (USA) 138 2 1.3 rare 0Tokyo Electric Power (Japan) 140 0 10.4 2.4 25Tokyo Electric Power (Japan) 140 1 3.0 1.3 44Tokyo Electric Power (Japan) 140 2 2.5 0.9 36Kansai Electric Power (Japan) 140 1 9.0 3.4 38Kansai Electic Power (Japan) 140 2 6.5 4.3 67TVA (USA) 161 2 3.3 2.1 63SECV (Australia) 220 1 1.6 0.6 37Ontario-Hydro (Canada) 230 1 3.4 1.2 35Ontario-Hydro (Canada) 230 2 0.86 0.04 4Tokyo Electric Power (Japan) 250 2 2.2 0.9 41Kansai Electric Power (Japan) 250 2 2.2 0 0CEGB (Britain) 275 1 1.7 0.2 12OVEC (USA) 345 1 7.6 0.4 5AEP (USA) 345 2 4.4 0 0CEGB (Britain) 400 1 1.4

IEEE TRANSACTIONS ON POWER APPARATUS AND SYSTEMS, VOL. PAS-89, NO. 5/6, MAY/JUNE 1970

vertical circuit configuration, double-circuit outages are not elimi-nated by the use of differential insulation. Double-circuit outageshave even been known to occur on double-circuit towers supportingtwo different voltage classes-Brown and Whitehead [5] reported adouble-circuit outage on a 69/138-kV line, and in Australia, theSnowy Mountains Hydro-Electric Authority observed a similaroutage on a 66/132-kV line. With the proposed double-triangularand double-horizontal designs, however, differential insulation can beused with optimum effect, as shown in the analyses undertaken.

In reply to Mr. Beloff and Mr. French, we have not performedany detailed analyses on the double-delta arrangement. It wouldappear that the application of differential insulation to this arrange-ment would give results similar to those achieved with the verticalcircuit arrangement. However, because the conductors of, say,circuit 2 cannot all be at a lower height than the conductors of thesame phase in circuit 1, the improvement in coupling to circuit 2following flashovers on circuit 1 would not be as effective as thatobtained with the double-horizontal and double-triangular con-figurations. We believe that the alternative phasing arrangement forthe double-triangular configuration would be as effective as the onesuggested in the paper. When considering phasing arrangements, theeffect on the series reactance of the double-circuit line must also beconsidered.Many discussers were concerned with operational and mechanical

difficulties of the proposed double-horizontal and double-triangular

designs. These problems can be overcome with prior planning, usingmodern operational techniques. Mr. Linck, Mr. McClelland, and Mr.McClymont have summarized the techniques used by Ontario-Hydro to overcome these difficulties in the application of the double-triangular design to an important 230-kV line.The proposed double-horizontal and double-triangular designs

provide improved double-circuit outage performance by restrainingoutages to only one circuit, normally without markedly affectingtotal outage rates. This means that there will be an increased num-ber of multiphase faults on one of the lines. However, this shouldnot adversely affect the stability of the system, except in those caseswhere multiphase single-circuit faults prove more onerous thansingle-phase or multiphase double-circuit outages. Field data forconventional lines indicate that a higher proportion of double-circuit outages involves more than one phase of each circuit than forsingle-circuit outages.We believe that the double-horizontal and double-triangular

configurations may find useful application in those circumstanceswhere a low double-circuit outage rate is required in the unavoidablepresence of a high total outage rate.

REFERENCES[9] R. M. Kimbrell, "Insulation differential cuts tripouts," Electron.

World, vol. 159, pp. 66-67, May 1963.

Boundary-Relaxation Analysis of RotationallySymmetric Electric Field ProblemsIVAN A. CERMAK, MEMBER, IEEE, AND P. SILVESTER, MEMBER, IEEE

Abstract-A new numerical method is described for the solutionof electric field problems where the field is not bounded in space,but extends infinitely far. This method belongs to the class of boun-dary-relaxation techniques in which an artificial finite boundarycondition is initially introduced to allow solution and then itera-tively removed as the solution proceeds. The method has beenimplemented in extensive computer programs, which produceequipotential plots of the electric field in the neighborhood ofdielectric and conductive bodies possessing rotational symmetry.Application to the analysis of suspension insulators and relateddevices is described, and several field plots exhibited for standardinsulator types. The method permits sufficiently short computationtimes to allow detailed investigation of the effects of insulatorcontamination by conductive substances, variation of dielectricconstant, or other factors.

Paper 69 TP 703-PWR, recommended and approved by theTransmission and Distribution Committee of the IEEE PowerGroup for presentation at the IEEE Summer Power Meeting,Dallas, Tex., June 22-27, 1969. Manuscript submitted February 14,1969; made available for printing April 10, 1969. This work wassupported by the National Research Council of Canada.

P. Silvester is with the Department of Electrical Engineering,McGill University, Montreal, P. Q., Canada.

I. A. Cermak was with the Department of Electrical Engineering,McGill University, Montreal, P. Q., Canada. He is now with BellTelephone Laboratories, Holmdel, N. J.

INTRODUCTIONT RANSMISSION voltages have risen remarkably rapidly

in recent years, and there is every indication that evenfurther substantial increases are to be expected. The very highvoltage levels now becoming common have posed many dif-ficult problems for the equipment designer, especially in con-nection with insulation. At the same time, extensive work hasbeen devoted to such factors as the effect of industrial pollution.A clear need exists for new methods for analyzing the electricfields surrounding high-voltage conductors, bushings, insulators,and hardware parts, and for evaluation of the effects on per-formance of contamination, mechanical damage, and otherfactors.Two fundamental difficulties generally beset attempts to

determine the electric field near a high-voltage device: thegeometrical shapes involved are very complicated, and the spacein which the problem is posed extends outward infinitely far.For example, Fig. 1 shows a simple insulator. Mathematically,the electric potential V is determined by solving Laplace'sequation

V2V = 0 (1)everywhere outside the conductive portions, subject to therestrictions that

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