Lightning performance and overvoltage surgestudies on a rural distribution line

A.J. Eriksson, Pr. Eng., Ph.D. (Eng.), Sen. Mem. I.E.E.E., M.S.A.I.E.E.,and D.V. Meal, M.S.A.I.E.E.

Abstract: In an attempt to arrive at an improved understanding of the disturbing influence of lightning uponpower distribution lines, a unique project is described involving the construction of a 10 km test distributionline together with an automated lightning and surge recording station. Preliminary data are presented frommeasurements carried out over two full thunderstorm seasons, in respect of direct strokes to the line andinduced overvoltage surge characteristics. The majority of lightning disturbances are caused by inductionfrom nearby discharges to ground, are of positive polarity, rarely exceed 100 kV, and are generally account-able in terms of classic electromagnetic field theories of induction. Several bipolar surge records have alsobeen obtained, however, whose characteristics deviate considerably from theoretical prediction, and it isconsidered that this might be due to the influence of the leader mechanism. It is also found that the directstroke incidence to the line is considerably greater than anticipated from traditional theory, as is also thefrequency of multiple pole flashovers, and it is concluded that further research is required on the attractiveeffects of lines and of the final stages of the striking process.

1 Introduction

This paper is intended to introduce a comprehensive researchprogramme covering the influence of lightning upon ruraldistribution lines in the Transvaal highveld region of SouthAfrica, and serves also to present preliminary data emergingfrom this project.

The principal motivations for this programme concern thepoor performance of rural distribution lines and theunacceptably high rates of failure of components; such asdistribution transformers and surge arresters, which have beenobserved during service experience in this region [1] which issubject to a moderately high annual incidence of lightning.(The regional annual ground flash density is about 7 flasheskm"2 [2]).

An examination of the technical literature in this fieldwould suggest that such problems are not confined to thisregion of the world. A series of articles in 1973 [3,4]illustrated American utility experience, and expressed concernregarding surge arrester design characteristics and test require-ments. Preliminary planning for an American researchprogramme commenced in 1977 [5] and a major collaborativeexercise was initiated in Florida [6] involving participation ofvarious research institutions, as well as utilities and manufac-turing industries.

The lightning protection of distribution networks has alsobeen a matter of some concern in Britain [7 ,8] , despite thecomparatively low incidence of lightning. British experience,over the period 1965 — 1970 [9], in common with that in theUSA, indicates that lightning was the single most importantcause of faults in the distribution system. A co-ordinatedapproach, involving the participation of various research andutility organisations, was initiated in 1969 and included severalfield studies [7]. Various modifications in system-protectionpolicy have been introduced, but it is still evident, some 9years later, that this remained an active area of ongoingresearch [10].

A general consensus of opinion in the literature [6,11], inview of the considerable capital investment in utility distri-bution networks and the substantial disruption to consumersthrough lightning outages, is that more practical research needsto be directed toward the lightning protection of distributionsystems and equipment.

The principal goals of the South African research pro-gramme, therefore, are to arrive at a better understanding of

Paper 1784C (Pll, P9), first received 23rd December 1980 and inrevised form 11th December 1981Dr. Eriksson is the Head, and Mr. Meal is a Chief Technical Officer, ofthe Electric Power Department, National Electrical EngineeringResearch Institute of the Council for Scientific & Industrial Research,PO Box 395, Pretoria 0001, South Africa

the influence of lightning upon distribution lines and, thereby,to develop optimised approaches to system design andimproved performance.

This programme commenced in 1975, with a two yearmeasurement project which was aimed at studying thecharacteristics of lightning surges on a short length (1.5 km) ofinstrumented l l k V distribution line, which itself was anelement of a complex and fully operational mixed urban/ruraldistribution network (about 100 km in extent).

A considerable body of data on surge amplitudes andvoltage rate of rise characteristics were accumulated [12]. Theprimary conclusion from this exercise, however, was that therecorded waveforms had been so modified by the complexnature of the distribution system itself (which included manyterminal and spur transformers, together with surge arresters)that they were not representative of the fundamental nature ofthe originating overvoltages. As a consequence, these dataprovided little guidance to an improved understanding of themechanisms whereby lightning disturbances arise in suchsystems, and thus did not readily facilitate the resolution ofthe problems being experienced with the lightning protectionof distribution systems.

The authors considered that such a fundamental understan-ing was a basic prerequisite to an optimised approach toprotection problems. It was noted from a study of recentliterature, for example, although there was a dearth ofcomprehensive field data, that there were a number oftheoretical analyses on the interaction between lightning anddistribution systems [5, 8—10,13—20] ;and, in some instances,these had already formed the bases for modified protectivepolicies and/or planning strategies. In view of the evidentdivergence among some of these theoretical approaches, how-ever, it was considered essential that direct measurements ofthe characteristics of lightning disturbances be carried out toclarify the validity of the fundamental bases for thesetheoretical calculations.

A collaborative research project has thus been initiated, inconjunction with the Electricity Supply Commission (ESCOM)involving the construction of a test distribution line ofrepresentative design and adequate length. The primaryobjectives of this programme are to study the fundamentalcharacteristics of lightning overvoltage surges and originatingmechanisms; initially unmodified by the presence of powertransformers, surge arresters, spurs or overhead shield wires,etc. Thereafter, in a graded programme, and building upon thefundamental knowledge as it develops, the influences ofsystem modification will be studied, in an attempt todetermine the effectiveness of such protective measures asoverhead shield wires, for example, and to arrive at optimisedapproaches to improved distribution-line lightning perform-ance.

IEEPROC, Vol. 129, Pt. C, No. 2, MARCH 1982 0143-7046/82/020059 +11 $01.50/0 59

The purpose of this paper is to describe the experimentalmethods being followed and to present the preliminary resultsobtained during the first few years of operation.

To evaluate the significance of this data effectively, it ispertinent to briefly review contemporary thinking on certaineffects of lightning upon distribution lines.

2 The interaction between lightning and distribution lines

In the main, lightning ground flashes can influence an over-head power line in two ways:

(a) through direct strikes to the poles, or conductors(b) through electromagnetic induction of overvoltage surges

in the event of flashes to the ground nearby.

2.1 Direct flashThe effects of a direct flash to an overhead line are consideredto be well known, and have frequently been described in theliterature [21].

The final stages of a direct strike to a line are considered toinvolve the initiation of an upward leader from the line duringthe approach nearby of a downward progressing chargedleader, and the subsequent interception of these two leaders.The attractive influence of the line will therefore vary,depending upon its height and the magnitude and distributionof charge upon the downward leader.

Most modern engineering studies consider that this attractiverange normally varies between about 1 and 4 times the heightof the line [9], with an average value approximately equal totwice the line height. More recent work by Eriksson, however,[22] indicates that the attractive influence of a structure may,in fact, be greater than previously thought, as outlined furtherin Section 6.1.

2.2 Nearby flashesOver the years, a wide variety of theoretical approaches havebeen postulated, whereby the induced voltage mechanismsmay be represented. These theoretical approaches vary in theirassumptions regarding the physics of the lightning dischargeprocess and yield varying conclusions concerning theanticipated induced voltage-surge characteristics.

This work has been adequately reviewed by several authors[13—20, 23] and it is sufficient here merely to record a fewsalient points.

The early approaches were mainly quasistatic in concept,involving the electrostatic effects of either the cloud chargesand/or the stepped leader charges followed by the suddenrelease of the bound charge upon the line at the instant of thereturn stroke. These theories predicted monopolar inducedvoltage waveshapes of positive polarity, in the event of themore common negative ground flash.

More recent analyses evaluated the electromagnetic fieldsassociated with the return stroke, using classical field theoryand retarded potentials. In certain instances, these theoriesyielded bipolar induced voltage waveshapes, comprising a shortbut substantial negative excursion of voltage followed by alonger duration positive component. Several variations of thesereturn stroke theories have also been postulated, which takeaccount of the roles of either the stepped leader and/or theupward rising connecting leader, and these generally also yieldsimilar bipolar waveshapes.

Although field data are extremely scarce, the claim isfrequently encountered in the literature that the available fielddata are supportive of a bipolar induced voltage waveshape,and that this usually comprises an initial short negativecomponent followed by a smaller positive component[14-17] .

60

It was against this background of anticipated overvoltage-surge characteristics that the test-line measurement programmewas initiated.

3 The 11 kV test-line project

A suitable area for the construction of the test-line was chosenapproximately 30 km east of Pretoria, in a region alreadycovered by an existing photographic lightning ground flashlocation network [2].

The terrain has an average altitude of about 1500 m, and isgently undulating in character. The test-line has a total lengthof about 9.9 km and generally follows a straight course.

The line was constructed by ESCOM during the first halfof 1978. The design adopted involves a treated wood-poleconstruction, as shown in Fig. 1, and is representative of aform widely employed for rural 11 kV power distribution.Porcelain pin-type insulators are used, and the three-phaseconductor configuration is horizontal, with approximately 1 mspacings. The cross-arm supporting brackets are of steel. Apole construction was chosen which would allow the erectionof a single overhead earthed shield wire at a later stage of theprogramme, because one of the project objectives is toestablish the technical implications (beneficial or otherwise) ofa shielded construction. Accordingly, each pole has alreadybeen fitted with a shield-wire bracket mounted at the top, asshown in Fig. 1, and a stranded earthing conductor was buriedaround the pole footing during erection, according to commonpractice. At this stage, however, no overhead shield wire hasbeen strung and the earth conductors from the footingconnection have been terminated in a coil some 2 m below thecross-arm support brackets, thereby providing a dischargeflashover point in the event of direct strikes. Throughoutconstruction, attempts were made to maintain a highinsulation strength, and all angle and strain pole configurations

Fox conductor :22.6mm2 copperequivalent 6-strand aluminium1-strand steel conductor 7/2.79mm

class B 95kVwithstand

porcelain-pininsulator

wooden crossarm•• 1 5 3 m mdiameter

earth wire to be extended totop of pole in next phase of

^project

-7/2.65 mm diametergalvanized steel earth wire

7450 mm

1800 mm

ground level

earth wire coiled at-bottom of pole

Fig. 1 Typical pole structure for the test line

IEEPROC, Vol. 129, Pt. C, No. 2, MARCH 1982

have been suitably insulated to maintain this level; nominallychosen as 500 kV impulse withstand. The three-phaseconductors at the one end of the line have been bonded andare earthed directly to a double counterpoise buried electrodesystem.

At the other end of the line, the three-phase conductorswere terminated in individual spark gaps, which were adjustedto have an impulse flashover voltage of about 200 kV. (Noteboth sets of termination arrangements are peculiar to thisphase of the programme only, and it is anticipated thatalternative arrangements will be used in future lightningseasons; such as matching line impedances, for example.)

Two recording stations have been located along the line.The first of these (termed NEERI) is positioned at a pointabout 4.3 km along the line from the short-circuit end, and hasbeen in operation since September 1978 (i.e. through threefull summer-thunderstorm seasons). The principal emphasis atthis station is upon the study of individual voltage wave shapesand their relationship to the positions of the orginating groundflashes.

A second station (termed ESCOM) is located at the easternopen-circuit end of the line. The objective at this station is themeasurement of line and arrester discharge currents. At thetime of writing, however, the recording instrumentation at thisstation had not been fully commissioned and all the surge datapresented in this report, therefore, were obtained from thefirst station.

4 The NEERI recording station

A general view of the station is shown in Fig. 2. The primaryequipment and instrumentation systems comprise the follow-ing:

(a) lightning-flash counters (both CIGRE 500 Hz and 10 kHztypes [24])

(b) diesel-powered generator, controlled via either the light-ning flash counter or a VHF-radio remote-control system

(c) an all-sky camera system, together with a radio-startsynchronised pulse-per-second clock unit

% „, 1

• * •

Fig. 2 General view of NEERI 11 kV test-line overvoltage recordingstation

(d) closed-circuit television camera and automatic video-taperecorder

(e) an automatic voltage-surge recording system, which isdescribed in more detail in the following Section.

4.1 Voltage-surge measurementsThe overall instrumentation arrangements at the NEERIstation are shown schematically in Fig. 3.

The primary measurement of voltage-surge-impulse shapeson the overhead-line conductors is achieved through thecombination of a three-phase set of wideband compensatedpotential dividers with automatic camera/oscilloscope systems.

The potential dividers have an impulse flashover voltage ofabout 250 kV (across external grading rings), and are adjustedto have a signal frequency bandwidth from DC to 15 MHz anda response rise-time capability of 0.2 fis. As shown in Fig. 3,the potential-divider output signals pass, via a system ofelectrostatically and magnetically shielded cables, to a wide-band matching amplifier, which allows signal gain adjustmentswithout affecting the divider compensation. From here, thesignals pass through a voltage trigger and camera control unit(which incorporates 2 jus delay lines and allows accuratebipolar adjustment of the system voltage trigger threshold level)before the signals are displayed and photographed by theoscilloscope/camera systems.

The earth reference for all voltage measurements is definedby the potential of a buried mesh electrode system underneaththe recording station. The potential dividers and associatedinstrumentation are connected back to this reference electrode,via low-impedance straps in a single-point groundingconfiguration. (The measured electrode resistance is about2 ft.) All station instrumentation has been designed forautomatic (i.e. unattended) operation and all the primaryfunctions are controlled remotely by the VHF-radio remotecontrol and monitor system. These functions include controlof the diesel, synchronous start of all sky-cameras and pulse-per-second clocks, as well as the TV video-recording systems.The radio system also incorporates a monitor link between theNEERI station and the main research base in Pretoria.

4.2 Complementary measurementsAll station operations, including flash counter and surge triggerregistrations, as well as all radio-control functions, are alsoregistered on a 10-channel master event recorder, as shown bythe example given in Fig. 4, and this provides a permanentrecord of station performance during each thunderstorm.

As noted earlier, the nominal impulse withstand level of theline insulation is about 500 kV; but the potential dividers atthe NEERI station have an impulse flashover level of about250 kV. This was intentionally chosen, because the maininterest was induced voltage surges, but means that theposition of the divider connection represents the lowestinsulation withstand level along the line route, and occasionalflashovers could therefore be expected to occur at this point.

An additional radio-controlled all-sky camera and televisionvideo-recording system is also located at the ESCOM station.Together with those at the NEERI station, these cameras are sopositioned as to have optimum visibility along the line route.Taking bearings off monitor video frame displays of anyground flashes within the common field of view of the TVcameras at each station again provides another means ofestimating ground-flash location. Time-to-thunder recordingsare also evaluated from the audio channels of the video taperecorders at each station, and these serve to supplemementthe flash-location determinations and, at the same time, tominimise the errors present in the triangulation method.

Toward the end of the first lightning season (i.e. in early1979), magnetic link brackets were mounted adjacent to the

IEEPROC, Vol. 129, Pt. C, No. 2, MARCH 1982 61

earth wires on every pole along the line route. The mainobjective was to obtain some basic data on the peak-currentmagnitudes and polarities involved during any direct strikes tothe line and/or in the event of pole flashovers. At present a4-link bracket configuration has been adopted and this allowsthe measurement of currents over the range 1 — 80 kA with a± 10% error.

5 Station performance during the 1978/79, 1979/80 and1980/81 thunderstorm seasons

The NEERI recording station was maintained in full operationthrough the above storm seasons. A summary of stationperformance over this period is given in Table 1, while Fig. 5depicts the combined monthly variations in activity.

At the end of the 1979/80 season, the line was modified by

11 kV lines

the inclusion of surge arresters mounted at every tenth polealong the line length. The main purpose of this change was toassess their influence upon the overvoltage-surge distributionon the line and to obtain direct evidence regarding theresponse of arresters under practical disturbance conditions.

The analysis of arrester performance is still proceeding andwill be the subject of a separate paper, currently in the courseof preparation.

Voltage surge data in the paper, therefore, relate to resultsobtained during the 1978/79 and 1979/80 seasons only.

As shown in Table 1, the station performed relativelyreliably over these three seasons; to the extent that successfuloperation was achieved on almost 80% of all storms. (Themain reason for occasional nonoperations was the fact that thestation did not always switch on for very weak storms in thearea.)

compensatedpotent icdividers

TV camera video-tape recorder

i i- , ii ii ii electromagnetic

EEL* = M ± ± ± i ± shading9

40 40 40matching amplifier

radio-control systemslow-impedanceearthing

flashcounters

triggerskycamera

± 1

trigger cameracontrol delaylines

oscilloscope camera

station event recorder

1 T

diesel control J-

Fig. 3 Instrumentation arrangements at the voltage-surge-recording station

16.00h 18.00hCIGRE 10 kHz flash counter

CIGRE 500Hz flash counter-

vertical-aerial 500Hz CIGRE prototypecomplementary flash counter

RSA 10 C prototype dual-circuit _flash counter (positive-field change) Illl III NIMBI BUI -n—i—r

RSA IOC (negative-field change )

surge trigger II li l I

diesel switch-on

all sky camera and one pulse per secondtiming switch-on

TV camera switch-on (10 hour speed)

TV camera (normal speed )

Fig. 4 Example of station performance during thunderstorm of 6th January 1980

62 IEEPROC, Vol. 129, Pt. C, No. 2, MARCH 1982

Table 1: Summary of NEERI station operational performance

Season Season1 /9/78-3113119 1 /9/79-30/4/80

Season1/9/80-30/4/81

3-yearresults

Total number of storms observed

Total number of storms duringwhich station was operated

Total number of storms duringwhich surges were recorded

Number of surges recorded inexcess of 12 kV trigger level

Total duration of all stormactivity, hours

Total duration of stationoperational hours

Total duration of those stormsin which surges occurred, hours

Total CIGRE 10 kHz countertotal and equivalent flashdensity, km" 2

Total CIGRE 10 kHz countsduring station operational time

Total CIGRE 10 kHz countsduring 'surge' storms

Mean CIGRE 10 kHz flash countsper storm — overall

Mean CIGRE 10 kHz flash countsper storm for those storms duringwhich surges were recorded

78

50 (64%)

14(18%)

120

148

233

30 (20%)

8 942(6.8)

5 689(64%)

3 797(43%)

114

271

96

91 (95%)

34 (35%)

158

143

290

71 (50%)

8 249(6.2)

7 943(96%)

6 489(79%)

86

191

86

59 (69%)

25 (29%)

112

146

238

62 (42%)

8 196(6.1)

7 089(86%)

4 787(58%)

95

191

260Mean 86.7

200Mean 66.7(77%)

73Mean 24.3(28%)

390Mean 130

437Mean 146

761Mean 254

163Mean 54.3(37%)

25 389(19.1)

Mean 8 462(6.3)

20 721Mean 6 907

(82%)

15 073Mean 5 024

(60%)

Mean 96

Mean 206

20-

in-

n

'/ // surge's orrns

1WA

CIGRE-10kHz counts

//'/

y//—

s* 2 0

in

O

2 10

otnc 0

Sept Oct Nov Dec Jan Febcumulative ;durations

'/I

Mar;torrr

Apr

Sept Oct Nov Dec Jan Feb Mar Apr

30

20

10

surges in excessof 12 kV

nSept Oct Nov Dec Jan Feb Mar Apr

Fig. 5 Combined monthly data over the 1978/79 and 1979/80seasons

At the commencement of this programme, an initial stationsurge trigger level of 5 kV was adopted, but this was soonraised to 12kV as data on impulse shape characteristics wereaccumulated. Only surges having a voltage amplitude in excessof 12 kV have thus been analysed.

As noted in Table 1, a total of 390 surges in excess of thistrigger level was recorded and these were distributed over lessthan 30% of the storms registered at the station. As might beexpected, a storm would have to be active in the immediatevicinity of the line to cause overvoltages on the line. This isborne out by the summarised data given in Table 1, whichshows that although only 28% of storms produced surges onthe line, these storms were responsible for about 60% of alllightning activity registered in the area.

The overall ground-flash density in the area over the threeseasons combined was 19.1 ground flashes km"2. Fig. 5indicates that the most lightning activity (over 20% of thetotal), and more than 30% of all surges, occurred duringNovember; with the months of October, December, Januaryand February being the next most active. Previous studies inthe Pretoria region over the past 12 years have shown thatpeak lightning activity usually occurs during these months,with November frequently dominating.

6 Analysis of results

6.1 Direct strike incidence and pole flashoversDirect strike incidence to the line was determined in severalmanners:

(a) by direct photography, or from CCTV video recordings(b) from examination of magnetic link registrations, in which

IEEPROC, Vol. 129, Pt. C, No. 2, MARCH 1982 63

Table 2: Summary of pole flashover analysis (October 1978 — April 1981)

Season

1978/791979/801980/81

Mean flashdensity

km" 2 p.a6.86.26.1

Number of polesdirectly struck

615

8

Number of polesdamaged throughflashover

103028

Number of polesdisplaying dischargecurrent> 1 kA

6 *6348

3-year mean 6.4 9.7 22.7

*Magnetic links were only installed along the line toward the end of the 1978/79 season. The 6 pole dis-charges recorded coincided with only the last 2 direct stroke events of this season

case the pole displaying the highest discharge current wasconsidered to have borne the direct strike

(c) from examination of pole flashover damage, in whichcase the pole displaying the most damage was considered tohave born the direct strike.

In many instances, these methods complemented each other.The total incidence of direct strikes is summarised in Table 2.On a number of occasions it was also found that several polesadjacent to each other, or in the immediate vicinity of astricken pole, also displayed evidence of flashover; hence thehigher total in this column of Table 2. In like manner, in mostinstances of direct stroke evidence, analysis of the links on theneighbouring poles showed that several more poles, in theimmediate vicinity, had also experienced discharge currents inexcess of 1 kA, as illustrated by the appropriate column inTable 2.

Direct stroke incidence Ns, to power lines over a particularperiod, is commonly expressed using the relationship

Ns = (2R + b)LNg x 1 x 10 - 3(1)

p.a.

where

R = attractive radius of the line, mb = structural width of the line, mL = line length, km

Ng = ground flash density in the area concerned, km"2

Substituting the three-year mean values Ns = 9.7 p.a. and Ng

= 6.4km"2 p.a., as recorded in Table 2, together with theline parameters b = 2.14 m and/, = 9.9 km, yields/? = 75.5 m.

In an analysis of lightning parameters [28], Anderson andEriksson have suggested * that values of Ng determined fromflash-counter registrations may require adjustment by 10% onaverage, to allow for root-branched flash incidence. If thiscorrection is made to the above value o£Ng, then the resultantvalue ofR reduces to 68.5 m.

In previous analyses [22], Eriksson has shown that theobserved average incidence of downward flashes to structuresof varying height could be accounted for in terms of astructure mean attractive radius Ra given by

Rn = 163H061(2)

where H is the average structure height in metres. Applying avalue H = 8.0m, for the test line, yields Ra = 58.0m; whichshows reasonable agreement with the above three-year valueioxR.

Clearly, the use of the more common estimate of lineattractive radius R = 2H [13, 23, 30] would yield a predicteddirect-stroke incidence substantially less than that actuallyobserved.

Owing to the stochastic nature of ground-flash incidence, itwill be necessary to continue these observations for severalmore years before fully meaningful conclusions, regardingexpected direct stroke incidence, may be presented; but thegeneral impression, so far, after three years of study, is that

distribution lines could attract a greater incidence of directstrikes than generally believed.

The data presented in Table 2 also indicate that the averageincidence of pole damage is approximately double the direct-strike incidence, because more than one pole is frequentlyinvolved. Normalising in terms of the line length and groundflash density yields an anticipated pole-damage rate Np equalto 35.8 poles/100 km linelength/flash km"2. As the averagepole spacing on this line is about 100 m, this corresponds to apercentage pole-damage rate of approximately 3% per flashkm"2.

6.2 Discharge curren t amplitudesAllowing for the few storms in November 1978 when mag-netic links were not yet installed in the test line, the datain Table 2 indicate that more than 100 poles experienceddischarge currents in excess of 1 kA (the magnetic linkthreshold of sensitivity). This is equivalent to about 4 polesdischarged per direct flash. All observed peak-currentamplitudes were of negative polarity.

On some occasions, the distribution of discharge currentsin the proximity of a stricken pole is a complex process.Important factors in this process, as suggested by Schei andHuse, for example [26], are the pole footing impedances andreflection effects. The writers consider it probable, however,that the processes of upward leaders, initiated from adjacentpoles, could also contribute to this distribution of currents;particularly when these intercept further branches of thedownward leader and give rise to the phenomenon of rootbranching. This process has been discussed elsewhere [22, 27],and recent observations suggest an incidence varying fromabout 10-20% amongst different storms [28].

A graphic example which was recorded on the test line bythe CCTV system is shown in Fig. 6. The right-hand branchstruck a pole some 10 bays away from the NEERI station,while the left-hand branch struck the ground a few hundredmetres away from the line; all within a period of less than20 ms (one video frame).

The overall cumulative frequency distribution of measureddischarge current amplitudes comprises a mixture of direct-stroke data and related associated pole flashover currents. Theresultant cumulative frequency distribution of peak-currentamplitudes is summarised in Table 3. The distribution, there-fore, indicates the potential range of surge arrester currents.Taking a log-normal approximation to this distribution yields:

7 = 6.4 kA

ffiogio = 0.41

Alternatively, taking the directly stricken pole data onlyyields

Td = 19.3 kA

= 0.25

64 IEEPROC, Vol. 129, Pt. C, No. 2, MARCH 1982

kV

F ig. 6 Example of a root-branched ground flash

One channel (right-hand) has struck the test-line

Table 3: Distribution of pole flashover dischargecurrent amplitudes —based upon 117 measurements

Current ranges Cumulative percentage ofcurrents in excess of ranges

kA< 22-55-1010-2020-3030-4040-50

%12.840.468.885.391.797.399.1

The latter values are not particularly meaningful, however,owing to the distribution of discharge current in adjacentpoles. An alternative analysis may be applied, taking thealgebraic sums of discharge currents for those poles adjacent tothose directly struck. This yields:

TD = 36.1 kA

ffiogio = 0-27

These values agree well with the CIGRE reference distribution[28], for negative downward ground flashes ( / = 34kA,aiog io = 0.30), and is, therefore, consistent also with theconcept of a linear superposition of discharge currents throughadjacent poles.

6.3 Overvoltage surge characteristicsA total of 281 voltage oscillograms was obtained over the twoseasons 1978/79 and 1979/80, i.e. comprising surges havingamplitudes in excess of the trigger level of 12 kV. It was foundthat these records could be classified, tentatively, into threecategories. By far the majority (about 93%) were of the so-called 'classic-induced' type, being always of positive polarity,generally impulsive in wave shape, and displaying identicalcharacteristics on all three phases. Two examples are shown inFig. 7. Fig. la illustrates the similarities amongst the three-phase records, while Fig. 1b resolves typical impulse front-shape characteristics, and also shows the positive reflectionfrom the open-circuited end of the line.

Most of the remaining surge records arose through directstrikes to the line (comprising about 4% of the total). Thesewere invariably negative in polarity, and always led to flash-over to earth across the line insulation; usually also across thepotential-divider grading rings. The absolute crest magnitudecould not normally be measured, therefore, but only the rateof rise of voltage prior to flashover. A typical example isshown in Fig. 8, which also shows the inverse reflection fromthe short-circuited end of the line. A common feature of these'direct' surge records, as shown by the example, is the initial

0 50 100

Fig. 7 Example of voltage-surge shapes

a Three-phase recording of classic induced surgeb Single-phase recording of classic surge, showing impulse frontcharacteristic

Fig. 8 Example of voltage-surge shapes

Single-phase recording of direct-stroke surge

Fig. 9 Example of voltage-surge shapesThree-phase recording of bipolar surge (nearby induced type)

slow development of a negative voltage on the line prior to theimpulsive component. On records taken at slower sweepspeeds, this development period is frequently of the order of50— 100/us and includes short negative pulses at intervals ofabout 20—40jus; which are very reminiscent of the electricfields recorded during high-speed studies of negative steppedleader processes [29].

The few remaining surge records have been arbitrarilytermed 'nearby induced' and comprise about 2—3% of thetotal. An example is shown in Fig. 9. In their early stages theseimpulses appear similar to the direct type, in that a similarpulsed negative voltage is present on each phase. The rapidtransient that follows, however, is a positive-going impulse,similar in character to those of the classic induced type. Noconfirmed instances of direct strokes to the line, or of poleflashovers coinciding with these events could be found; but onseveral occasions ground flashes were recorded within about200 m of the line.

IEEPROC, Vol. 129, Pt. C, No. 2, MARCH 1982 65

Apart from the above three main categories of event, in afew instances voltage surges in excess of the 12kV thresholdwere induced upon the line by the occurrence of overheadcloud flashes as confirmed by the photographic systems.

As the great majority of impulse shapes were of the onetype, all records have been grouped together for statisticalanalysis of their wave-shape characteristics; although positiveand negative polarity peak-amplitude components have beenidentified separately in the resultant histograms.

These histograms are shown in Figs. 10a and b for peakamplitudes and for maximum rates of rise of voltage on thefront of the impulse, respectively; whereas Figs, llfl and bshow the associated probability distributions for theseparameters. The latter distributions have been drawn on log-normal probability scales, as this distribution is frequentlyemployed in analysis of lightning parameters; but no attempthas been made at this stage to establish best-fit distributionsthrough the data, because the confidence limits around these

80r

° 30AE 20

10

positive polarities

J LJ 100negative polarities

200 300 400peak surge amplitude, kV

a

Fig. 10A Histogram of recorded surge amplitudes

Based on 281 records (shaded areas denote direct strikes)

110-

100-

negative rates of riseb

kV/us 500

Fig. 10B Distribution of surge voltage maximum rates of rise

Based on 2 56 records (shaded areas denote direct strikes)

66

samples are still comparatively large. The results of thisanalysis are summarised in Table 4.

As noted earlier, in Table 2, a total of 12 direct-typeoscillograms was recorded. In each instance, owing to lineflashovers, the crest amplitudes could not be determined, andonly the flashover voltage and initial rate of rise of voltagecould be estimated. The histogram of maximum rates of rise,in Fig. 10b, shows that the highest rates of rise of voltage wererecorded during direct strikes. The resultant values variedbetween about 17 kV//xs and 500kV/jus, with a mean value ofabout 140 kV//is.

Taking all surge events into account (which is relevant as far. as line insulation performance is concerned) the cumulativefrequency distribution in Fig. lib indicates a probability ofabout 1% of an initial rate of rise of surge voltage occurring inexcess of 500 kV//xs.

Jo

f !o

0.21

2

5

10

20

3040506070

80

90

95

9899

99.9

99.99

-

-

-

N

- ^"vA

* \ \ ^""""^ \\ \

-

, , , , ,,10 20 40 60 80100 200 400 600 800

voltage surge amplitude, kV peaka

0.1 0.2 04 0.8 2 4 6810 20 4060100200400800 2000 60000.6 80 600 4000 10000

maximum rate of rise on voltage surge wavefront,kV/usb

Fig. 11 Cumulative frequency distributions

a Voltage-surge amplitudesmeasured data95% confidence limits to distribution (based upon 281 records)

b Surge wavefront maximum rates of rise of voltagemeasured data95% confidence limits to distribution (based upon 256 records)

IEEPROC, Vol. 129, Pt. C, No. 2, MARCH 1982

Table 4: Summary of induced voltage-surge shape characteristics

Parameter

(a) Maximum peak amplitude u:Number of samplesData 10-percentile valueData medianData 90-percentile valueLog-normal approximation:

ua log u

(b) Maximum voltage rate of rise s:Number of samplesData 10-percentile valueData medianData 90-percentile valueLog-normal approximation:

a log u

All data

281—

26 kV100kV

32.3 kV0.33

2560.5kV/MS4.8 kV/jus

42 kV/jis

4.5 kV//iS0.73

Positiveimpulseonly

247—_—

30.0 kV0.31

231—_—

3.8 kV/MS0.68

6.4 The in fluence o f flash positionAn important objective of the measurement programme is torelate the recorded overvoltage-surge characteristics to thedistances of ground flashes from the line; as determined usingthe combined photographic CCTV and time-to-thunderlocation systems. While various problems were initiallyexperienced in accurately implementing these techniques,some 30 observations successfully correlating voltage surge andground-flash location were recorded during the first twoseasons of operation. (The mean triangulation errors weretypically of the order of about 100 m.)

The maximum observed distance from the line, at which aground flash still produced a measurable voltage surge, was2 500m; resulting, in that case, in a 24 kV surge. The closestdistance of approach was about 70 m.

Fig. 12 depicts a plot of the observed relationship betweenmeasured surge amplitudes and flash distances. As might beexpected, considerable scatter is evident, as the induced surgevoltage depends not only upon flash distance but also uponthe lightning current magnitude; as well as upon propagationand attenuation characteristics.

The classical theories of electromagnetic induction, asoutlined for example by Rusck [23], predict that themaximum voltage is induced on the line at the point of closest

300

-*J00en 70

1 50V 30

10

• \

• \

•v:\.VN100kA

"35kA

x10kAi0

10 30 50 100 300 500 1000 3000distance from line, m

Fig. 12 Relationship between recorded voltage-surge amplitudes andground-flash locations

Based on 31 measurements

approach to the flash location. Assuming a rectangular-return-stroke current /0 and a return stroke velocity v = (5v0 (wherev0 = speed of light), Rusck shows that the maximum inducedvoltage may be expressed by:

ZoIoh1 + r>2 \ l / 2 (3)

where

Io = peak-current amplitudeh = height of line conductors

Yo = closest distance of approach between the line and flashlocation

<3 = v/v0 = ratio of the return-stroke velocity to the speed oflight.

The velocity ratio /3 is normally considered to be a function ofreturn stroke current Io, with a typical relationship given byRusck as

j3 = 1 +4.5 x 105 -1/2

(4)

The above two equations may be used to estimate thetheoretical dependency of surge amplitude Umax in relation toflash distance Yo, for various values of Io, and several suchresults are included in Fig. 12. The majority of the observeddata points lie between the /0 curves for 10 and 100 kA, whichis consistent with the fact that about 80% of all ground flasheswould be expected to discharge peak currents in this range[28]. Although the sample size is too small to be meaningful,the scatter in the recorded points is also consistent with amedian anticipated ground-flash peak-current amplitude ofabout 35 kA.

To this extent, therefore, these prelimianry data supportthe above theoretical expression for maximum induced surge-voltage amplitude, and also constitute an indirect means forassessing the distribution of stroke crest amplitudes duringflashes to flat ground.

7 Comments on the recorded surge voltage wave shapes

Two main features are of interest here: first, the recordedsurge amplitudes and, secondly, the impulse shape character-istics. Comparisons with other field measurements are difficultbecause the latter are relatively scant, and few comprehensivesets of data have been published. The general form of the over-voltage amplitude distribution is comparatively similar tothose obtained by several other workers, however [9, 31], tothe extent also of a median amplitude in the region of 25—30 kV. The measured amplitude distribution is also consistentwith that generally predicted using classical theoreticalconcepts.

A greater divergence from theoretical predictions is evidentin respect of the recorded impulse shape characteristics. Incontrast to the expectations of most modern theoreticalstudies, the recorded voltage surges generally do not displaybipolar characteristics in the initial part of the wave. As notedin Section 6.3, more than 90% of the surges were positiveand monopolar in character and only about 2—3% of recordsdisplayed a bipolar form. In that comparatively rare event,also, the negative component differs from theoreticalprediction in that the negative 'loop' is always of smalleramplitude than the subsequent positive component, and isgenerally of long duration.

The pulsed nature and relatively slow development of thisnegative component, as well as the overall similarity to the

IEEPROC, Vol. 129, Pt. C, No. 2, MARCH 1982 67

early stages of direct-stroke records, is consistent with theexpected influence of the nearby approach of a negativelycharged stepped leader; and this is supported by the fact thaton several occasions the ground flashes responsible for thesesurges terminated within about 200 m of the line.

While the authors consider that upward-connecting leaderprocesses certainly participate in the final stages of a groundflash, and suspect that such leaders probably contribute to thecomplex pole discharge current sequences recorded adjacentto directly struck poles, no photographic evidence of theoccurrence of such leaders has yet been noted; althoughthis is one of the objectives of the photographic and CCTVtechniques currently being employed on the line.

Both Singarajah [16] and Stringfellow [17] have analyti-cally examined the role of downward and upward leaderprocesses in the induced voltage mechanism, but theiranticipated bipolar impulse shape characteristics are notconsistent with those recorded on the line.

There is, therefore, a clear need for further analyticalexamination of the induced voltage mechanism, as well asadditional field data, to clarify the above apparent divergencesbetween theory and observation.

8 Extension of the test-line project

Notwithstanding the questions that have been raised, it isconsidered that sufficient fundamental data have now beenobtained to characterise induced overvoltage-surge-impulseshapes on distribution lines, and to define the basis for mean-ingful theoretical interpretation of induced voltagemechanisms. The next phase of this experimental programmeis therefore being implemented, at the time of writing, in thecourse of the 1980/81 lightning season. This entails thejudicious location of distribution surge arresters at variouspositions along the line and a study of their performanceduring lightning disturbances. Arresters of differing manufac-ture are being used, and steps are also being taken to recordcertain of their discharge characteristics. The role of multiple-stroke-discharge operations is also being examined, because itis known that over 50% of ground flashes involve two or morestrokes of current [28].

It is expected that this phase of the project could be spreadacross two thunderstorm seasons. During the first seasons(1980/81) arrester performance on lightning surge dischargeoperations only is being monitored, while the line will beenergised at power frequency voltage in the second season, toexamine the influence of power following discharge currents.

Thereafter, subject to what is learnt during the above studyof surge-arrester performance, a further phase of the projectcould entail the erection of an earthed overhead shield wireand a study of the relative influence on system performance.

Throughout the following storm seasons ground-flashlocations will be monitored, as well as direct-stroke incidenceand characteristics, in an effort to resolve certain of thequestions that have been raised during this first phase of theproject; as well as to clarify the preliminary trends that haveemerged.

9 Conclusions

This paper has not set out to provide a comprehensivestatement on the lightning performance of rural distributionlines. Instead, in the light of a variety of theoretical studiesand assumptions relating to the influence of lightning on suchlines, the authors' purpose has been to record the implemen-tation of an experimental programme whose primary objectiveis to provide a meaningful background against which thelightning performance of distribution systems may both beunderstood and then improved.

The first few lightning seasons of study have served todemonstrate the successful operation of an automatic voltage-surge recording station, and a large amount of data has alreadybeen obtained. The authors consider that the first analyses ofthese data and the corresponding trends are already ofsufficient interest (and divergence in relation to theoreticalconcepts) to justify implementation of the experimentalprogramme. In particular, with regard to both induced over-voltages and direct stroke incidence, it would appear that agreater number of events may be anticipated (in this region ofthe world anyway) than previously estimated, and that sucha line may display an increased attractiveness to direct strokesthan normally believed.

A related paper on this project has already demonstratedthe application of these overvoltage-surge studies to the designchoice of a co-ordinated line insulation withstand level; witha view toward improved system performance [32].

In addition, the recorded overvoltage impulse shapecharacteristics have demonstrated the need for furtherexamination of theoretical models of the induced voltagemechanisms. The authors hope that the data presented in thispaper may facilitate such re-examination, and, in this context,consider that more attention should be paid to the extensivelybranched nature of the downward leader and to the finalstages of the downward and connecting upward leaderinterception processes.

10 Acknowledgments

The authors take this opportunity to record their appreciationto the Director of the National Electrical Engineering ResearchInstitute of the Council for Scientific and Industrial Researchfor the opportunity to carry out this work. The collaborationand assistance of the Electricity Supply Commission, and ofthe authors' colleagues in the National Electrical EngineeringResearch Institute, are also gratefully acknowledged.

11 References

1 ACKERMANN, R.H.: 'The lightning performance of distributionlines in the Rand and Orange Free State regions of ESCOM',/« 'Theoperating duty of surge arresters', Trans. S. Afr. Inst. Electr. Eng.,1981,72, pp. 113-115

2 ANDERSON, R.B.: 'Lightning research in Southern Africa', ibid.,1980, 71, Pt. 4, pp. 75-98

3 MAMBUCA, J.A.: 'Surge-arrester designs questioned', Electr. World,1973, 1, pp. 36-37

4 'Should surge-arrester standards be changed?', ibid., 1975, 15, pp.51-53

5 Jack Fawcett Associates, Inc. (Maryland): 'Sampling plans forlightning surges test program — final report for the period May1976 - January 1977'. Energy Research and DevelopmentAdministration, March, 1977

6 Florida Distribution Lightning Research Group: 'Research intolightning protection of distribution systems I - equipment for fieldtests'. IEEE Conf. Paper A 79 527-3, IEEE PES summer meeting,Vancouver, July 1979, pp. 1-9

7 'Lightning protection of distribution networks' (Electricity Council,London,1973)

8 'Lightning and the distribution system'. IEE Conf. Publ. 108, 19749 STRINGFELLOW, M.F.: 'The interaction between lightning and

overhead distribution lines'. Electricity Council Research ReportECRC/R602, April 1973

10 BAKER, W.P.: 'Lightning stress on 33 and l lkV overhead distri-bution networks'. Electricity Council Research Report ECRC/R1120, February 1978

11 Electric Power Research Institute (Palo Alto): 'Lightning relatedresearch at EPRI - an overview'. Power Distribution Conference,University of Texas, Austin, October 1978

12 ERIKSSON, A.J., and MEAL, D.V.: 'Data on l lkV distribution-line lightning and overvoltage performance in relation to flash-counter registrations'. CSIR special report ELEK 123, May 1977

13 GOLDE, R.H.: 'Lightning surges on overhead distribution linescaused by indirect and direct lightning strokes', AIEE Trans., 1956,Pt. Ill A, pp. 437-446

68 IEEPROC, Vol. 129, Pt. C, No. 2, MARCH 1982

14 CHOWDHURI, P., and GROSS, E.T.B.: 'Voltage surges induced onoverhead lines by lightning strokes', Proc. IEE, 1967, 114, (12),pp. 1899-1907

15 CHOWDHURI, P., and GROSS, E.T.B.: 'Voltages induced on over-head multiconductor lines by lightning strokes', Proc. IEE, 1969,116, pp.561-565

16 SINGARAJAH, C: 'Surges induced on overhead transmission linesby indirect lightning strokes, with particular reference to Nigeria'.Ph.D. Thesis, University of London, September 1971

17 STRINGFELLOW, M.F.: 'Electric fields close to ground lightningflashes'. Electricity Council Research Report ECRC/R897, April1976

18 YOKOYAMA, A.: 'Numerical analysis of induced lightning surges'.IEEE Conf. Paper No. A 78 123-2, IEEE PES winter meeting, NewYork, January/February, 1978

19 KOGA, H., MOTOMITSU, T., and TAGUCHI, M.: 'Lightning surgewaves induced on overhead lines', Trans. Inst. Electron. & Commun.Eng. Jpn. Part E, 1979, E62, pp. 216-223

20 YOKOYAMA, S.: 'Experimental analysis of earth wires for inducedlightning surges', IEE Proc, C, Gen., Trans. & Distrib. 1980, 127,(1), pp. 33-40

21 ERIKSSON, A.J.: 'Lightning overvoltages on high-voltagetransmission lines - investigation of wave-shape characteristics',Electro, 1976,47, pp. 87-110

22 ERIKSSON, A.J.: 'The lightning ground flash - an engineeringstudy'. Ph.D. Thesis, University of Natal, Durban, December 1979

23 RUSCK, S.: 'Protection of distribution systems - Lightning Vol. 2'(Academic Press, London, 1977), Chap^23

'24 ANDERSON, R.B., VAN NIEKERK, HTR~ PRENTICE, S.A., andMACKERRAS, D.: 'Improved lightning-flash counters', Electra,1979,66, pp. 85-98

25 SCHNEIDER, H.M., and STILWELL, H.R.: 'Measurement oflightning current waveshapes on distribution systems'. IEEE Conf.Paper A 79 526-5, IEEE PES summer meeting, Vancouver, July1979

26 SCHEI, A., and HUSE, J.: 'Currents through surge arresters due tolightning with main reference to distribution systems', Electra,1978,58, pp. 41-79

27 ERIKSSON, A.J.: 'An unusual lightning flash?' Weather, 1977, 32,pp. 102-106

28 ANDERSON, R.B., and ERIKSSON, A.J.: 'Lightning parameters forengineering application',Electra, 1980, 60

29 KRIDER, E.P., WEIDMAN, CD., and NOGGLE, R.C.. The electricfields produced by lightning stepped leaders', /. Geophys. Res.,1977,82, pp. 951-960

30 GOLDE, R.H.: 'Lightning performance of high-voltage systems',Trans. S. Afr. Inst. Electr. Eng., 1969,60, pp. 269-278

31 HORNER, D.J., SALMAN, Y.E., and SINGARAJAH, C: 'Researchinto lightning at Ahamadu Bello University, Zaria', Nigerian Eng.,1971,pp. 13-18

32 ERIKSSON, A.J., STRINGFELLOW, M.F., and MEAL, D.V.:'Lightning-induced overvoltages on overhead distribution lines'. IEEConf. Paper 81 SM 361-5, IEEE PES summer meeting, Portland,July 1981

Abstracts of papers published in other Parts of the IEE PROCEEDINGSThe following papers of interest to readers of IEE Proceedings Part C: Generation, Transmission & Distribution have appeared inother Parts of the IEE Proceedings:

Calculation of 3-dimensional eddy currents at power fre-quenciesMX. BROWN

IEE Proc. A, 1982,129, (1), pp. 46-53

The forseeable increase in the speed of digital computers overthe next few years is, alone, unlikely to increase significantlythe range of 3-dimensional eddy current problems amenable tonumerical solution. Thus greater use of suitable approximationsand highly efficient numerical methods are required to attackthe general 3-dimensional problem. The paper reviews thecurrent techniques and approximations used and, from this,proposes the form of a general program for calculating 3-dimensional eddy currents suitable for present digitalcomputers.

DC power-supply system with inverting substations for tractionsystems using regenerative brakesT. SUZUKI

IEE Proc. B, Electr. Power Appi, 1982, 129, (1), pp. 18-26

Reversible DC power-supply systems with thyristor invertersare described. Design principles applicable to and technicaldifficulties encountered by the reversible DC power-supplysystem are discussed. To give an example of application, thepower-supply system of Kobe Metro is described in detail,together with principal designs, solutions adopted to overcomethe technical difficulties and some operational results. Somemore examples of application in Japan are referred to asfurther experiences of the system. The designs described aresupported by service experience.

Anomalous breakdown in synthetic insulating materialsimmersed in transformer oil and subjected to switching surgevoltagesTAHER D. EISH, C. VENKATASESHAIAH and C.N. REDDY

IEE Proc. A, 1982,129, (1), pp. 62-65

The occurrence of anomalous breakdown in synthetic insulatingmaterials has been reported in recent years under alternatingvoltages, direct voltages, and direct voltages with ripple. In thepaper, the authors report the results of their investigations onanomalous breakdown phenomena under positive and negativeswitching surge voltages, and discuss the mechanisms of break-down.

End effects in series-wound linear induction motorsK.J.R. WILKINSON

IEE Proc. B, Electr. Power Appi, 1982, 129, (1), pp. 35-42

In a moving linear induction motor, each magnetic-field poletravelling in the gap between stator and track, grows from zeroat the stator's stem and decays astern. It follows that, ifcompared with an induction motor having the same pole sizeand number, and working at the same crest-gap density,tractive capability is less because of the transient nature ofworking fields. A linear stator with four fully excited polepitches has the equivalent of one tractively effective field polewhen slip is 10%. To assess the separate influence of sterndrag, changes in field-pole energy are examined as stern fieldsemerge. The paper derives 11 expressions for operating power,some in terms of the stator, others of the track, and these arecollectively consistent with power received from the stator atthe gap. Results are applied to predict power-slip characteristicsfor three linear motors.

IEE PROC, Vol. 129, Pt. C, No. 2, MARCH 1982 69