129 Journal of Electrical & Electronics Engineering, Australia Vol 22, No.2, 2003 technical paper Studies of in-service and laboratory failures of metal- oxide distribution surge arresters * M Darveniza, TK Saha and S Wright The University of Queensland QLD 4072 SUMMARY: We describe the findings from tests and inspections of arresters withdrawn from service, the results of laboratory studies with multipulse lightning currents and high temporary over-voltages, and comparisons of in-service and laboratory failure modes. We show that gapped metal-oxide arresters are vulnerable to degradation from moisture ingress in the field and that the main causes of gapless arrester failures in the field and in the laboratory include very high temporary over-voltages and lightning strikes with multiple strokes. 1 INTRODUCTION The University of Queensland (UQ) has been involved in surge arrester studies for over thirty years, initially on gapped silicon-carbide (GSiC) arresters 1,2,3 and more recently on metal-oxide arresters. In Australia, as in other countries, the early arresters had porcelain (POR) housings and were mostly gapless metal-oxide (MO); some contained an internal series gap (GMO). Most manufacturers moved to polymer housed MO distribution arresters in the 1990’s. While most modern arresters are of the gapless type, at least one employs a series gap structure. The current Australian arrester standards 4 are based on IEC 99 norms, but American designed arresters sold in Australia are based on ANSI/IEEE C62. It is a common experience that metal-oxide arresters are more reliable than the technologically outdated GSiC arresters, as evidenced by Australian surveys of both transmission and distribution arresters. This is to be expected, as the MO arrester (particularly the gapless type) is much simpler than the GSiC arrester, Also, they have not been in service much in excess of 10 years, so if any degradation due to age is to occur, this should not be evident at this early stage of their life. Even so, during the 1990’s and only on distribution systems, some Australian electricity companies experienced significant failure rates for a few makes, both porcelain and polymer housed and gapped and gapless arresters. At the same time, the UQ group conducted laboratory research into the effects of multipulse (MP) lightning currents and very high temporary over-voltages (VHTOV) on MO arresters (mostly porcelain- housed) and varistor blocks. 5,6,7 Most of the work on arresters was linked to long-term studies to improve the lightning protection of distribution transformers. 8 This paper describes a UQ research project funded by the (then) Australian Electricity Supply Industry Research Board (AESIRB). There are three main parts to the project: i) studies of in-service MO arresters, ii) laboratory studies of arrester failure modes, and iii) comparisons of in-service and laboratory failure modes. 2 IN-SERVICE SURGE ARRESTERS The Participating Companies – 10 Australian companies participated in the research project. They recovered from the field and sent to UQ for test / examination, arresters which - i) had failed in service, or ii) were suspect or had about 5 to 10 years of service. Some companies provided information relevant to the in-service operation of their failed arresters, a few also quoted failure rates. The Arresters - A total of 172 were received; ignoring 48 GSiC arresters ( further study not needed 3 ), there were 124 MO arresters available to the project. Of these, 60 were porcelain housed and 64 polymer, and had been recovered from 11 and 22kV 3-phase 3-wire systems (no neutral) and from 12.7 and 19kV single- wire earth-return (SWER) systems. These were from 8 manufacturers, 4 were Australian (3 had overseas * Presented at V SIPA - V International Symposium on Lightning Protection, Brazil 1999.
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technical paper
Studies of in-service and laboratory failures of metal-oxide distribution surge arresters *
M Darveniza, TK Saha and S WrightThe University of Queensland
QLD 4072
SUMMARY: We describe the findings from tests and inspections of arresters withdrawnfrom service, the results of laboratory studies with multipulse lightning currents and hightemporary over-voltages, and comparisons of in-service and laboratory failure modes. We showthat gapped metal-oxide arresters are vulnerable to degradation from moisture ingress in the fieldand that the main causes of gapless arrester failures in the field and in the laboratory include veryhigh temporary over-voltages and lightning strikes with multiple strokes.
1 INTRODUCTION
The University of Queensland (UQ) has beeninvolved in surge arrester studies for over thirtyyears, initially on gapped silicon-carbide (GSiC)arresters1,2,3 and more recently on metal-oxidearresters. In Australia, as in other countries, the earlyarresters had porcelain (POR) housings and weremostly gapless metal-oxide (MO); some containedan internal series gap (GMO). Most manufacturersmoved to polymer housed MO distribution arrestersin the 1990’s. While most modern arresters are of thegapless type, at least one employs a series gapstructure. The current Australian arrester standards4
are based on IEC 99 norms, but American designedarresters sold in Australia are based on ANSI/IEEEC62.
It is a common experience that metal-oxide arrestersare more reliable than the technologically outdatedGSiC arresters, as evidenced by Australian surveysof both transmission and distribution arresters. Thisis to be expected, as the MO arrester (particularlythe gapless type) is much simpler than the GSiCarrester, Also, they have not been in service much inexcess of 10 years, so if any degradation due to ageis to occur, this should not be evident at this earlystage of their life. Even so, during the 1990’s andonly on distribution systems, some Australianelectricity companies experienced significant failurerates for a few makes, both porcelain and polymerhoused and gapped and gapless arresters. At thesame time, the UQ group conducted laboratory
research into the effects of multipulse (MP) lightningcurrents and very high temporary over-voltages(VHTOV) on MO arresters (mostly porcelain-housed) and varistor blocks.5,6,7 Most of the work onarresters was linked to long-term studies to improvethe lightning protection of distribution transformers.8
This paper describes a UQ research project fundedby the (then) Australian Electricity Supply IndustryResearch Board (AESIRB). There are three main partsto the project:
i) studies of in-service MO arresters,ii) laboratory studies of arrester failure modes, andiii) comparisons of in-service and laboratory failuremodes.
2 IN-SERVICE SURGE ARRESTERS
The Participating Companies – 10 Australiancompanies participated in the research project. Theyrecovered from the field and sent to UQ for test /examination, arresters which - i) had failed in service,or ii) were suspect or had about 5 to 10 years ofservice. Some companies provided informationrelevant to the in-service operation of their failedarresters, a few also quoted failure rates.
The Arresters - A total of 172 were received; ignoring48 GSiC arresters ( further study not needed3 ), therewere 124 MO arresters available to the project. Ofthese, 60 were porcelain housed and 64 polymer, andhad been recovered from 11 and 22kV 3-phase 3-wiresystems (no neutral) and from 12.7 and 19kV single-wire earth-return (SWER) systems. These were from8 manufacturers, 4 were Australian (3 had overseas
* Presented at V SIPA - V InternationalSymposium on Lightning Protection, Brazil1999.
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‘parents’) and used varistor blocks sourced fromoverseas. Table 1 lists the relevant information aboutthe 124 arresters. It can be seen that over half of thearresters are from one Australian manufacturer whouses overseas blocks and that 4 of the manufacturersare represented by small numbers of arresters. Twomakes were gapped metal-oxide (GMO); one (AO2)was an early porcelain type with a simple internalspark-gap. The year of manufacture was marked on74 arresters and was from 1981 to 1995 with a meanof 1991, so that the mean age of the arresters asreceived at UQ was about 6 years.
The Laboratory Diagnostic Tests – Each MO arresterwas tested for 1.4mA (resistive) reference voltage(VREF) and residual voltage (VRES) at rated 8/20µscurrent, and for GMO arresters for 1.2/50µs impulsesparkover voltage. The test results were assessed bycomparisons between like arresters and with themanufacturer’s published data. Experience provedthat in general it was “easy” to identify thosearresters with unacceptable test results – if onediagnostic test result seemed abnormal, the other waslikewise. During the tests, it became evident that asimple 5kV insulation resistance (IR) measurementwas also an effective diagnostic tool (for arresterswith 1 to 3 blocks, this is quite similar to a referencevoltage test).
Arrester Inspection – After an inspection (for housingdamage, operation of the venting or the earth-leaddisconnector), arresters which were damaged in-
Table 1Relevant information about the metal-oxide arresters
Manuf. Total MO GMO POR POL no. of Field —Dam4 Bad —Tests5 no. goodIdent.3 nos. nos nos. nos. nos. types POR POL POR POL inspect.
service and those arresters with unacceptable testresults were dismantled for internal inspection. Theaim was to find the cause of the failure or the poortest results. Such inspections are rather subjective,but if experienced people are used and if inter-comparisons are made between like arresters(including some with good test results), patterns ofinternal degradation become apparent and enablecauses to be identified. It was also of value to makecomparisons with arresters and blocks which hadbeen damaged during the laboratory tests.
2.1 Test results and inspection findings
Results from Diagnostic Tests - Table 1 shows 30 ofthe arresters were damaged in service (9 porcelain-housed and 21 polymer) and a further 19 hadunacceptable test results (10 porcelain and 9polymer). Only 26 of the 49 had their year ofmanufacture recorded and the means were about1991 and 1993 for porcelain and polymer housingsrespectively. Most of the 49 were of 3 makes - AO2,O3, O8; nothing “sinister” should be read into thesenumbers as they are biassed samples. However, anumber of comments on particular types of arresterscan be made quite legitimately. The first concernsgapped MO arresters. In table 1, manufacturer AO2has 8 GMO arresters of which 2 were damaged inservice– despite having satisfactory impulsesparkover voltages, 3 others displayed abnormalresidual voltage waveshapes at rated impulsecurrent, in that a partial breakdown step can be seen
Notes :1 – several similar types;2 – manufacturer absorbed by another;3 – identifiers are Australian and Overseas4 – Field—Dam means obviously damaged in service such as ruptured or vented housing5 – Bad —Tests means unacceptable diagnostic test results; many were damaged in service
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in each oscillogram, indicating partial failure of oneor more blocks. Of the 12 GMO arresters ofmanufacturer O3, 4 were in-service damaged and 7had unsatisfactory diagnostic test results. Of the 7,2 displayed low values in all three diagnostics, 3 hadlow VREF and IR and good VRES, and 2 had lowVREF and good IR and VRES. Since the series gapsof these arresters are shunted by non-linear SiCresistors, low VREF and IR or low VREF aloneindicate degradation of these resistors.
The second comment concerns the porcelain-housedMO arresters of AO2 with voltage ratings of 10,12 or27kV; of the 41 such arresters, 6 were damaged inservice and 7 displayed low values in all tests. Thisindicated severe internal degradation, as wasconfirmed by the inspections. The third commentconcerns the AO2 polymer arresters with voltageratings from 9 to 30kV- Table 1 shows 11 were in-service damaged and 2 had unacceptable test results.The fourth comment is for the 10 O3 MO arresters (9polymer and 1 porcelain), the 7 O4 porcelain MOarresters, and for the 7 O7 polymer MO arresters –all their test results were good. The fifth comment isfor the 5 O8 polymer MO arresters – 4 were damagedin service, the 5th had good test results. The finalcomment is a general one – the validity of thediagnostic tests can be established by inspecting theinternal components, and this is considered next.
Inspection Findings - Where appropriate, theinspection findings are related to test results and tofield information if supplied. The gapped MOarresters are considered first. Of the 8 AO2 porcelainGMO arresters, 2 were in-service damaged and 3displayed abnormal residual voltages. According tothe field staff reports, most were fitted on 3-phase11kV line-to-cable potheads, and the arrester failureswere associated with cable faults and not lightning.Seven were inspected and 6 showed signs of moistureingress despite compressed gaskets at both ends.These arresters have an internal insulating paperliner wrapped around the spark-gap and blocks.Four showed arc marks on the outside of the spark-gap spacer (or the inside of the paper wrap) and nopitting on the spark-gap electrodes, indicating thatthe failure path bypassed the spark gap. Five showedflashover or arc marks on the surfaces of the MOblocks, including the 3 with partial failure steps inthe residual voltages. It seems from theseobservations that the cause of failure is moistureingress through inadequate or aged seals, moisturedegradation of internal components, particularly thepaper wrap which provided an unwanted path forflashover and internal failure. Some arresters alsohad external arc damage to the porcelain , caused byone arrester failing and venting, and so “spraying”fault current arcs onto adjacent ones, in a so-calledconsequential 3-phase fault. Of the 12 O3 polymer-housed GMO arresters, 4 were in-service failures and7 had unacceptable test results. According to the field
staff reports, the failed arresters were on 3-phase line-cable potheads or transformers and failed duringsingle-pole switching with disconnect links; specificmention was made that no lightning had beenpresent prior to or at the time of the failures. Ten(10) of the 12 showed evidence of moisture ingressdue to inadequate seals between the polymerhousing and the end fittings, leading to such internaldegradation as rusty or oxidised springs, corrosionon metal components (verdigris on copper) andcrumbly or part-disintegrated non-linear SiCresistors shunting the series gaps, see plates 1 and 2.Some of the arresters which had passed fault current(8 of the 12) had internal components heavily coatedwith arc products (soot) and this masked themoisture-induced degradation. But there was nosuch masking for the arresters which had not passedfault current because the degradation had notprogressed sufficiently to cause failure. The likelymode of failure is moisture ingress leading to internaldegradation, including that of the non-linear SiCcarbide resistors, which then pass increasing leakagecurrent causing them to become crumbly or partlydisintegrated and in extreme cases leading toflashover at either normal operating voltage ortemporary over-voltage. This short-circuits the seriesgap, placing all the stress on the MO blocks leadingto their failure by surface flashover. Because of the“charge” that most of the GMO arresters had failedor were degraded by moisture ingress throughinadequate or aged seals, some comment must bemade on the seal designs, and these relate to similarexperience with GSiC arresters.2 Compressed gasketseals are used on most porcelain gapped arrestersand do not seem to age well and so eventually allowmoisture ingress. The polymer arrester ofmanufacturer O3 uses simple seals formed bycompressing the polymer (which have internal ridgesthat look like elementary ‘O’-rings) against the endfittings which join the external and internal parts –these clearly do not provide effective seals againstmoisture ingress under field conditions . The in-service failure rate of this type of arrester on one 11kVsystem was 14%.
The inspection findings for the gapless MO arrestersare considered next, starting with the 41 AO2porcelain ones of 10.5, 12 or 27kV rating. The internalcomponents of 20 were inspected; of these, 13 werefaulty either because of damage in service or becauseof unacceptable diagnostic test results, and all but 1showed clear signs of internal damage due to thepassage of fault currents ( the 1 was an arrester whoseporcelain housing was broken in two, probably dueto damage in transit). Five (5) of the faulted arrestershad been fitted on line-to cable potheads and theirfailure was associated either with a cable fault or withsingle-pole switching using disconnect links. Similarsingle-pole switching of a 500kVA transformer wasassociated with one other arrester failure. As therewas no lightning, the likely cause of failure for these
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6 arresters is very high temporary over-voltage(VHTOV). Nearly all showed evidence of surfaceflashovers on the MO blocks; but unlike the VHTOVdamage caused by laboratory tests, chips, fusing andrupture effects were also evident on some of theblocks, as would be expected because of the flow offault current, see plate 3. Of the remaining 6 faultedarresters, 1 was said to have failed during a severethunderstorm and no cause was given (or is evident)for the other 5. Ten (10) of the inspected arrestershad not passed any fault current - 8 were perfectinside and only 2 showed any signs of degradationdue to the ingress of moisture. However, 2 of the 12faulty arresters did show signs of moisturedegradation which contributed to their failure (theprime cause of 1 was VHTOV, while no cause wasgiven for the other). A further 4 arresters had someinternal degradation possibly due to moistureingress. This relative freedom from degradationattributable to moisture ingress is in marked contrastto what was reported above for AO2 porcelain-housed GMO arresters and whose gasket seals weresimilar. A final observation is that 5 of the arresters,which were free of damage internally due to faultcurrent, did show some external porcelain damagecaused by progressive 3-phase faults. The nextinspection findings to consider are for the 19 AO2polymer-housed arresters of 9 to 30kV ratings, ofwhich 11 had in-service damage, 2 had ‘bad’ and 6had ‘good’ test results. The voltage ratings of 3arresters were 9 and 12kV and their MO blocks andmetal spacers were surrounded by full length fibre-glass casings. Of the 3, 2 were faulty in that theirpolymer housings were ruptured and their fibre-glass casings were split due to the passage of faultcurrents. The blocks of one (the 9kV arrester) werepunctured and shattered, while the blocks of theother failed by surface flashovers with some of theMO material in contact with the metallisation brokenoff at one end. No cause was given (or is evident)for these failures and so could be due to VHTOV orextreme lightning such as multiple strokes orcontinuing current between strokes. There were 16AO2 polymer arresters of rating 21 to 30kV, probablyall from SWER systems; 10 had in-service damageand had passed fault currents internally so that thepolymer housing was ruptured on 9 (see plate 4) butwas only expanded on the 10th. Most (7) of the fibre-glass casings were split while 3 were arced on theoutside. The MO blocks of 8 showed surfaceflashovers with some seeming to penetrate and causesome damage to the MO below the surface; 2 (of the8) showed clear white deposits on their MO blocksindicative of degradation due to moisture ingress andthese caused surface flashovers. The blocks of 2arresters were punctured and split, suggesting failureby thermal instability. The field staff reports attributethe cause of 6 arrester failures to lightning, 1 wasdefinitely not lightning, and no cause was given for3 arresters although 2 of these had the white deposits
on the blocks suggesting that moisture ingress wasthe underlying cause of failure. These arresters wereon SWER lines, which have very high impulseinsulation levels (because they are on wood poles)and which have long distances (typically at least15km) between transformers, thus placing a veryonerous discharge duty on the arrester nearest to adirect lightning strike; also, VHTOV is extremelyunlikely on a SWER line. The failure rate of the SWERarresters was as high as 18% during the 1997/98storm season. Only 6 of the higher voltage AO2polymer arresters had no power fault damage; 5showed no signs of internal degradation or damage,1 had the abnormal residual voltage waveshapealready referred to and its blocks showed surfaceflashover marks caused by the applied impulsecurrents. There were 5 polymer MO arresters ofmanufacturer O8 – the 21kV unit was fine in allrespects whereas the 4 other arresters failed in servicewith no cause suggested by field staff. All thepolymer housings and the fibre-glass casings aroundthe blocks were ruptured by fault currents, see plate5; plate 6 is a failed AO1 MO arrester. The blocksfailed by surface flashovers, often the arc damagepenetrated into the MO material below the surface.The most likely cause of each failure is severelightning, probably multiple strokes.
3 LABORATORY STUDIES
3.1 Experiments with SP and MP currents
The UQ lightning impulse current generator canproduce SP currents up to 100kA or MP currents to15kA with 5 or 6 successive pulses and time intervalsin the range 15 to 150µs. Sequences of SP and MPcurrents can be applied alone or with powerfrequency voltage applied as part of operating duty(OD) tests. The procedure was to first demonstratethat the arresters or varistor blocks could withstandthe standard SP tests and then to investigate theeffects of MP. Degradation or damage of the blockswas identified by examining current and voltageoscillograms for abnormalities, by comparing the‘before’ and ‘after ’ characteristics using 1.4mAreference voltage and residual voltage as diagnostics,and for OD tests checking thermal stability bymonitoring current flow with the 50Hz voltagemaintained for up to 30 minutes after the impulseswere applied. After each set of tests, the blocks wereinspected for any signs of visible effects (the arresterswere dismantled for inspection).
The SP/MP experiments were in three groups – i)on commercial distribution arresters, ii) on one makeof varistor block with different dielectric surfacecoatings, iii) on several makes of blocks in varioussurroundings.
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Distribution arrestersSP and MP tests using 8/20µs currents were madeon 21 porcelain arresters of 6 makes; most were of5kA rating, with a few of 10kA rating. All passedthe standard (SP) tests satisfactorily. However MPtests with currents up to 11kA caused 12 of the 18arresters of 5kA rating to fail at currents from 5 to9.5kA. Most (10 of 12) failed by flashover of theblocks, some across the surface of the dielectriccoating leaving a faintly visible track, some byflashover just below the surface and leaving anobvious track, and some by flashover deep in thedielectric coating and leaving very obvious damagewhich involved a little of the MO material as well.The other 2 arresters failed by thermal instability. No10kA arresters failed during the MP tests, (maximumavailable current 11kA). Full details of the MP testresults on distribution arresters are in6.
Varistor blocks with different surface coatingsSP 4/10µs currents from 40 to 90kA and MP 8/20µscurrents from 6.5 to 11kA were applied to one makeof 5kA rated varistor block coated with differentsurface coatings. These included a simple glasscoating, glass coating plus silicone varnish, glasscoating bonded to a silicone moulding, and acomplete silicone moulded arrester. Some blocks ofeach type were first tested with SP 8/20µs currentsand all withstood up to 40kA. A very differentpattern of results was obtained with MP 8/20µscurrents. Most of the varistors with simple coatingsfailed by surface flashover at currents between 6 and10kA, while those with a glass coating bonded to asilicone moulding mostly withstood MP currents to11kA (only one failed at 11kA). The flashoversoccurred on or just below the surface of the coatings.A similar failure pattern was observed with the highmagnitude 4/10µs currents. The blocks with simplecoatings failed by surface flashover at currents from65 to 90kA, whereas the silicone moulded varistorsshowed no gross effects when subjected to currentsup to 90kA (in one or two cases, some small spots ofdamage were observed on the coating). Full detailsof these results are given in [7].
Varistor blocks in different surroundings MP 8/20µs currents of increasing magnitudes were appliedto 4 makes of varistor blocks, each with a differentproprietary surface coating. The blocks were testedin various surroundings - normal air, air at reducedpressure (1/2 atmospheric), SF6 at atmosphericpressure, and air at relative humidities in the range40 to 98% (the blocks were first held in the highhumidity environment for up to a week).Subsequently, the blocks were also tested afterimmersion in rain water The tests on each block ineach condition simply involved the application ofincreasing MP currents till failure occurred (or theupper limit of 13kA was reached) and determinationof the failure path (either on the surface, below the
surface and in the coating, or by puncture throughthe metal-oxide material, or some combination ofthese). The blocks were normally allowed to cooldown to room temperature between trains of MPcurrents. The 4 makes of blocks (A to D in Table 2)were supplied by 3 Australian arrester manufacturerswho source their varistor blocks from overseas.
Somewhat surprisingly, apart from reduced pressureand water-immersed blocks, the MP currents neededto cause block failures were nearly independent ofthe surrounding gas environment. All the B, C andD blocks withstood MP currents up to 13kA in air, invery humid air (98% RH) and in SF6. Make A blocksalso did not fail in humid air and in SF6, but failed innormal air (RH about 60%) at MP currents in therange 11.4 to 13kA. So it can be stated that increasedhumidity did not cause failures at lower MP currents,and that SF6 did not appear to increase the MPcurrents needed for block failure. In contrast, the1.2/50µs impulse voltage needed to flashover apolymer block of similar dimensions was muchgreater for SF6 than for air (by a factor of 2.6); also, itis well known that high humidity (>85%) nearlyalways reduces impulse flashover voltages acrossinsulating surfaces. As expected, MP tests with theblocks in a reduced pressure (1/2 atmospheric)environment did reduce the failure currents for someblocks ( one of A at 11.9kA and two of C at 11.8kA )but not for the others (of makes A and C or any of Band D).
As it is “conventional wisdom” that moisture ingressis detrimental to the insulation of most high voltagecomponents including arresters and their varistorblocks, a further series of MP tests was made on the4 makes of blocks after they had been immersed inrain water for periods up to 60 minutes. After theimmersion, the free water was shaken off and theblocks were tested immediately in their obviouslywet state – some had surfaces which werehydroscopic and they looked to be evenly wet, whileothers were hydrophobic and were unevenly wet.Even so, none of the wet blocks of makes C and Dfailed at MP currents up to 13kA, only one of wet Bblocks failed (at 12.3kA) and all the wet A blocksfailed at 12 to 13kA. Mass measurements were madeto determine how much water was absorbed by the
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blocks in the 60 minute immersion. The results : A-0.12%,B-0.11%,C-0.85%,D-0.45%, seem to have littlecorrelation with the MP performance. In comparison,the water absorbed by make A blocks was less than0.04% after exposure to air at 98% RH for a week.There was one very noteworthy observation duringthe MP tests on the moist and wet blocks – theapplication of withstand MP currents invariablycaused a cloud of steam to be released from the blockwhen the MP was applied. In fact, the application ofthe MP current “drove off” all the absorbed water.This is a remarkable result, as are probably all theresults with high humidity and water immersion.
Whenever failures occurred, observations were madeto determine the failure paths. Visually, all appearedto be external flashovers. Subsequent inspectionshowed that many of the discharge paths were onthe surface of the block coating, typically leaving atrail of spots where the discharge arc “touched” thesurface as can be seen in plates 7, 9 or leaving surfaceflashover marks as in plate 8. The second type offailure path can be seen in plate 9, surface flashoverspots with damage at both ends of the block. Thelatter appears to be due to a concentration of currentflowing from the metallisation to the MO blockmaterial before transferring to the surface. Theresulting discrete arc damages the metallisation andcracks or chips the nearby MO in a localised way.Plate 10 shows the third type of failure – a dischargepath initially below the surface of the coating, causinga crack which sometimes also penetrated into the MOmaterial. Again, end damage occurs with this typeof failure. So far, no reference has been made tofailure by internal puncture of the MO block – in fact,this did not seem to occur even though some of thepost-test diagnostics suggested possible internalblock damage.
3.2 High temporary over-voltage experiments
Occasionally, surge arresters in service are subjectedto TOV magnitudes well in excess of the 1.4punormally encountered on effectively eartheddistribution systems. Two known causes are – ferro-resonance and contact with a higher voltage line (eg,contact between a 33kV line conductor and a lower11kV line). The resultant fault current usuallyobscures the initial failure path, but failed arrestersrecovered from the field suggest initial flashover ofthe block surface. Laboratory tests were made on 3makes of either arresters or varistor blocks using50Hz voltages. TOV magnitudes in the range 1.4 to2pu (of rated voltage) were applied and the resistivecomponent of block current was monitored todetermine the onset of thermal instability (identifiedby a fast increase in current). Of interest here are thefailure paths if the 50Hz voltage was applied longenough for the arrester to fail completely. Sincefailure was by thermal instability, gross damage andrupture of the block was expected. However, all the
block failure paths observed were either flashoveracross the surface of the coating or below the surface,and were similar to those caused by MP currents.Two examples are shown in plates 11 and 12.
50Hz voltages were also used to examine the capacityof the blocks listed in Table 2 to withstand voltagesequal to their continuous operating voltages (COV)and 10% above COV after they had been immersedin rain water for 10 minutes. All the blocks testedwithstood COV and (after further immersion )COV+10% for at least two minutes, without showingany signs of “distress” other than some occasionalspots of minor arcing at various locations. Duringthese tests, the total leakage currents were alsomeasured. Even though the blocks were wet, therewas no large increase in the currents between thoseobserved initially (usually from 0.2 to 0.6mA for B,C, D blocks and 3 to 7mA for A ) and those afterseveral minutes when the currents were at finalvalues (from 0.15 to 0.4mA for B, C, D and 0.7 to 3mAfor A blocks).
4 COMPARISONS OF IN-SERVICE ANDLABORATORY FAILURE MODES
Section 2 described studies made on 124 MOdistribution arresters withdrawn from service. Somehad obviously been damaged in the field; some wereshown by test to have failed in the field; yet otherswere found to have experienced in-service internaldegradation but not to the extent to have causedfailure. These studies have identified a number offailure modes and some potential failure modes. Forgapped MO arresters, it is clear that the principalcause of failure is moisture ingress through faulty orinadequate seals on both porcelain and polymerhousings. The moisture ingress degrades the seriesgap structure by either destroying the properties ofthe insulation associated with the gaps or bydamaging the non-linear SiC grading resistors.Eventually all the system voltage is applied to thevaristor blocks, leading to their failure and so failureof the arrester. It is clear that the presence of lightningis not a part of this process, but there is evidence thatthe likelihood of this mode of failure is increased bythe occurrence of high temporary over-voltages. Forgapless MO arresters, moisture ingress does notappear to be a significant factor. While the cause ofblock failure in such arresters is often masked by thedamaging effects of fault currents, most of the blocksfailed in service by flashover of the surfaces orperhaps near-surface flashovers. In some cases, thisoccurred when it was known that there had been nolightning associated with the failure and so the blamemust be attributed to high temporary over-voltages.In other cases, the failure was attributed by field staffto lightning. So the blame must be attributed tolightning of great severity, which can be the case fora lightning strike with multiple stroke currents,sometimes accompanied by long-duration
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continuing current, or by a very large-magnitudestroke current. The last is unlikely becausedistribution arresters are tested with current to 65kAfor 5kA rated arresters (and 100kA for 10kA ratings),and the largest arrester discharge currents recordedon normal distribution lines never exceed 50kA - itmay be different on single-wire earth-return (SWER)lines because they are carried on wood poles withhigh impulse strength and because the spacingbetween arresters is usually greater than 15km. Soapart from the case of SWER lines, the likely causeof failure by surface or near-surface flashover mustbe multiple stroke lightning.
Consideration is now given to laboratory test failuremodes. No specific tests were made on gapped MOarresters as their block failure modes would be thesame as for gapless MO arresters. In section 3.1, itwas shown that the MO varistor blocks can fail bysurface flashover when subjected to MP currents inthe range 5 to 13kA and that block rupture by MPcurrents was rare. Further, the likelihood of MPsurface flashover was dependent on the quality ofthe surface coating applied to the block andindependent of the presence of humid air or evenfree moisture on the block surface. These laboratoryresults seem to be very consistent with the findingsfrom the field and so confirms the assertion thatmany of the arrester failures attributed to lightningin the field are caused be multiple stroke lightning.Also, the laboratory tests described in section 3.2 withhigh temporary over-voltages always caused failureby surface flashover or near-surface flashover, as isthe case for failures in the field when no lightningwas present. Further, it was shown that free wateron varistor blocks did not cause failure at COV or atCOV+10% and so normal operating voltages are notthe cause of non-lightning arrester failures. Byassociation with laboratory tests, the cause of suchfailures must be high temporary over-voltages, asmight be experienced by arresters on line-to-cablejunctions or on cable-connected transformersswitched with single-pole disconnect links.
5 CONCLUSIONS
Like the paper, the conclusions are in three parts –
In-Service Arresters -10 companies recovered fromthe field 124 MO arresters of ratings from 9 to 30kVforthe project. They were from 8 manufacturers andincluded gapped metal-oxide (GMO) and gaplessmetal-oxide (MO) arresters in porcelain and polymerhousings in roughly equal numbers. Their years ofmanufacture were from 1981 to 1995, mean about1991, so their mean age was about 6 years. About 30arresters arrived at UQ in an obviously damagedstate and a further 19 were found to be damaged orinternally degraded by diagnostic tests. They wereopened and their internal components wereinspected. A number of good arresters were also
opened and inspected for comparison. The mainfindings are –
1. The condition of the 22 GMO arresters from 2manufacturers was uniformly unacceptablewith 6 obviously in-service damaged and afurther 10 with unacceptable diagnostic testresults. The internal inspections revealed why– moisture ingress through ineffective seals hadcaused degradation of the internal series gapstructures and other components. Even GMOarresters with satisfactory test results showedsigns of internal degradation due to moistureingress. Clearly the seals were not adequate. Thein-service failure rate of such arresters on one11kV system was as high as 14%
2. The pattern was different for the 33 gapless MOarresters which were obviously in-servicedamaged or had unacceptable diagnostic testresults. Their internal components generallyshowed the effects of power frequency faultcurrents which damaged the MO blocks andtheir protective casings to varying degrees andwhich often ruptured the arrester housings.Inspection of their internal components onlyoccasionally indicated degradation attributableto moisture ingress. In most cases, it was notobvious why the arresters had failed. Themajority showed surface or near-surface damageto the MO blocks. This type of damage is quitecommon in laboratory tests with very hightemporary over-voltages (VHTOV) and withmultipulse (MP) lightning impulse currents.Field reports from a SWER system indicated thatone make of polymer MO arrester experienceda failure rate of 18% during one thunderstormseason.
3. A surprising proportion of the faulty arrestershad been installed on line-to-cable potheads orwere associated with single-pole switchingusing disconnect links. These are the likely causeof VHTOV.
4. Apart from inadequate seals on gapped MOarresters, the factor that seems to have mostinfluence on the performance of metal oxidearresters in the field is the quality of the surfacecoatings or casings which surround orencapsulate the MO blocks. This statement canbe made in part because of the findings fromlaboratory tests with VHTOV and MP lightningcurrents.
5. The effectiveness of the diagnostic tests used inthis study of MO arresters recovered from thefield were confirmed by the findings from thesubsequent inspection of the arresters. Thediagnostics included - a 1.4mA (resistive)reference voltage, the residual voltage at rated
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8/20µs current and a 5kV insulation resistance.The last is simple to measure and is surprisinglyeffective.
Laboratory Studies - Experiments with single pulse(SP) and multipulse (MP) lightning currents andwith high temporary over-voltages (TOV) were madeon various MO arresters and on various varistorblocks including blocks with different surfacecoatings and in different surrounding environments.The main findings are –
1. Most failures caused by MP 8/20µs currents areby surface or near-surface failure of the varistorblocks at current magnitudes of 6 to 13kA for5kA rated blocks. The likelihood of MPflashover is very dependent on the quality ofthe dielectric coating on the side of the blocks.The same statement can be made for high-magnitude 4/10µs SP currents. The MP currentsneeded to cause failure of 4 makes of blocks werenearly independent of the surrounding gasenvironments of normal air, very humid air andSF6 (the maximum available MP current was13kA). Even immersion in rain water forperiods up to 60 minutes only lowered the MPfailure currents for some of the blocks and thenonly to 12 to 13kA.
2. The application of very high 50Hz temporaryover-voltages (TOV) to arresters and blocks alsoresulted in failure by surface or near-surfaceflashovers. The TOV magnitudes for suchfailures were in the range 1.4 to 2 per-unit ofrated voltage. It was found that the applicationof continuous operating voltage (COV) and evenCOV+10% after water immersion for 10 minutesdid not cause 4 makes of (wet) varistor blocksto fail.
Comparisons - The failure of gapped MO arrestersin the field was clearly caused by moisture ingressand the resulting degradation of internalcomponents. So the following relate to gapless MOarresters:
1. Moisture ingress does not appear to be asignificant factor in the failure of such arresters.
2. Field staff say that some arrester failures occurwhen no lightning is present. Such failures seemnearly always to be on line-cable junctions and/ or during single-pole switching withdisconnect links. These are circumstances thatcan give rise to high TOV probably attributableto ferro-resonance. The failures mostly involvesurface or near-surface flashover of the arresterblocks. This is also what happens with veryhigh TOV in the laboratory.
3. Some arrester failures in the field are attributedto “severe” lightning. Such failures are mostlyby surface or near-surface flashover of thearrester blocks. This is precisely what happensin the laboratory when MP currents of sufficientmagnitude are applied to arresters and tovaristor blocks. So arrester failures attributedby field staff to “severe” lightning are probablycaused by multiple stroke lightning groundflashes. Information from the field suggests thatMO arresters on single-wire earth-return(SWER) lines are relatively vulnerable todamage by severe lightning. Presumably, thisis because SWER lines mounted on wood poleshave high impulse insulation levels and becauseof the large spacing between SWER transformersprotected by arresters.
ACKNOWLEDGEMENTS
The work reported was carried out with AESIRBfunding in the UQ High Voltage Laboratory. Fourstudents contributed to the experimental work:L Reddy Tumma (PhD), Putri Khalid, Azrina Azizand Michael Haskins (BE). Ten Australian electricitycompanies provided arresters recovered from thefield and their cooperation is gratefullyacknowledged. So is the assistance from threeAustralian arrester manufacturers.
REFERENCES
1. M Darveniza, DR Mercer. Service performanceof distribution lightning arresters andtransformers. Elect. Eng. Trans. IEAust., Sept.1966;EE2:97-112.
2. M Darveniza, DR Mercer, RM Watson. Anassessment of the reliability of in-service gappedsilicon-carbide distribution surge arresters. IEEETrans. Pwr. Del, Oct 1996;11:1789-97.
3. M Darveniza, DR Mercer. The effects of multiple-stroke lightning on distribution surge arresters.IEEE Trans. Pwr Del, July 1993;8:1035-44.
4. AS 1307-1996. Surge arresters Part 2: metal –oxide surge arresters with no gaps for ACsystems. Standards Australia, NSW 2140.
5. M Darveniza, D Roby, LR Tumma. Laboratoryand analytical studies of the effects of multipulselightning currents on metal-oxide arresters. IEEETrans. Pwr Del, July 1994;9:764-71.
6. M Darveniza, LR Tumma, B Richter, D A Roby.Multipulse lightning currents and metal-oxidearresters. ibid,July 1997;12:1168-75.
7. M Darveniza, TK Saha. Surface flashovers on
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metal-oxide varistor blocks. IEEE 6th Int. Conf.On Conduction and Breakdown in SolidDielectrics (ICSD98), Vasteras, Sweden, paper09-3, 4p.
Plate 1: Failed 10kV gapped metal oxide arrester,bad test results, degradation due tomoisture ingress
Plate 2: Close up of moisture degradedcomponents, GMO in Plate 1
Plate 3: Failed 10.5kV MO arrester,vented by faultcurrents, bad test results
Plate 4: Failed 30kV MO arrester, damaged by faultcurrent
Plate 5: Failed 11.2kV MO arrester,damaged by faultcurrent
Plate 6: Failed 10kV MO arrester, damaged by faultcurrent
8. M Darveniza. Lightning arrester protection ofdistribution transformers – revisited.JEEEA;22(2):
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Plate 7: Flashover spots and marks on two C blockscaused by MP 11.8kA, in 1/2 atmosphericpressure air
Plate 8: Flashover marks on make B block causedby MP 12.3kA, block in rain water for 60minutes
Plate 9: Flashover spots and block damage at bothends caused by MP 13kA, block A in air
Plate 10: Full length crack to A block caused byMP 12kA, block in rain water for 60minutes
Plate 11: High temporary overvoltage damagein the laboratory, TOV 1.5pu
Plate 12: High temporary overvoltage damagein the laboratory, TOV 1.6pu
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PROFESSOR MAT DARVENIZAMat Darveniza was born in Innisfail, Australia in 1932. He graduated B E (Elect)in 1953 and D Eng in 1980 from the University of Queensland and Ph D in 1959from London University. He joined the staff of the University of Queensland in1959 and retired as Professor in Electrical Engineering (Personal Chair) at theend of 1997. He is currently Professor Emeritus and is a Professorial ResearchFellow at the University. He is also Principal Executive Officer with Lightningand Transient Protection Pty Ltd. Prof Darveniza is an Hon Fellow of IEAust,Life Fellow of IEEE, and Fellow of ATSE. He is also a Foreign Member of IVA(Sweden) and in 1990 was awarded an Hon D Sc (Eng) by Chalmers University(Sweden).
TAPAN SAHATapan Kumar Saha was born in Bangladesh and came to Australia in 1989. DrSaha is a Senior Lecturer in the School of Information Technology and ElectricalEngineering, University of Queensland, Australia. Before joining the Universityof Queensland in 1996 he taught at the Bangladesh University of Engineeringand Technology, Dhaka, Bangladesh for three and a half years and at JamesCook University, Townsville, Australia for two and a half years. He is a seniormember of the IEEE and Fellow of the Institution of Engineers, Australia. Hisresearch interests include power systems, power quality and conditionmonitoring of electrical plants. He has published widely in these fields.
STEVEN WRIGHTSteven Wright was born in Australia in 1964. He joined the University ofQueensland as high voltage laboratory supervisor in 1995. Prior to that, heworked as an electrical fitter mechanic for six years with Queensland ElectricityCommission and as a scientific artificer for six years with the Queensland ScienceMuseum.
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