INSULATORS J

Coating GlassInsulators forService in SevereEnvironments

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RTV silicone material has beenused to prevent pollutionflashovers at substationsfor more than 30 years. Morerecently, this same remedialmeasure has also been adoptedby power utilities worldwide toresolve service problems due toextreme contamination affectingline insulators. These days, thereis even a trend to specify siliconecoatings at the design stage ratherthan dealing with the requirementsand costs of subsequently coatinginsulators in the field. While the

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demand for such a solution isclearly expanding, questionsremain in regard to optimalcoating material, performance andexpected service life.

This article, contributed by Jean-Marie George, Sandrine Prat andFabien Virlogeux of Sediver inFrance, reviews some of the workbeing done at their Saint YorreR&D facility as well as in externallaboratories.

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Among the initial motivations behindthe development of RTV siliconecoatings on glass were requests byutility maintenance staff seekingto maintain the benefits of glasson their overhead lines but alsolooking to avoid any requirement forperiodic washing under challengingcontaminated service conditions. Forexample, one of the benefits of glassinsulators is the ease with whichinexpensive yet reliable diagnosticscan be carried out using unaidedvisual observation alone. This isin contrast to the more complexdemands linked to inspecting othertypes of insulators and is especiallyvaluable when it comes to ensuringsafe live line working.

Selection of Coating MaterialRTV silicone coatings are availablein a range of different chemicalcompositions and therefore theimpact of environmental andelectrical stresses must be evaluatedon each alternative material. Ingeneral, the addition of thesetypes of coatings will result in asubstantial difference in pollutionwithstand capability of an insulator,due to the hydrophobic nature ofsilicone. Still, coating longevity,performance and ageing ultimatelydepend on the ideal chemistry aswell as on method and quality ofappl icat ion.

Screening of available siliconecoatings was based not only onextensive company experienceover years of testing differentpolymeric housing materials but alsoconsidered important performancefactors such as hydrophobicproperties and resistance to erosionand tracking. While the applicabilityof tests such as the inclined planeis still being debated for siliconerubber, there are a number ofalternative methodologies to evaluateand rank silicone coatings. Amongthese, the long-term multi-stressprogram implemented by Terna,the grid operator in Italy, has alsobeen performed at the laboratoryin Saint Yorre. Tests such as this

have conclusively demonstrated thaterosion resistance can vary greatlyamong different coatings and favorthose protected by means of addedalumina tn-hydrate (ATH) filler.

Ageing & Longevity of CoatingWhen dealing with polymericinsulating materials, ageing isalways a central issue and compositeinsulators have been used longenough to give some basic indicationsbased on field experience. Accordingto engineers at Sediver, utilitymaintenance crews sometimes haveto replace composite insulatorsafter about 15 to 20 years ofservice (and in certain situationseven less depending on product orenvironment). The principal areaof weakness in the case of certaindesigns comes from possible erosionof the rubber housing and seals whichare essential to prevent exposure ofthe core to moisture ingress.

Coating applied to a toughened glassinsulator is fundamentally differentfrom the basic design of compositeinsulators. While clearly looking for amaterial that offers the best erosionresistance, even should a coatingbecome eroded or damaged, theintegrity of the insulator is never atrisk, i.e. whatever may happen tothe coating does not compromisethe inherent properties of toughenedglass, which still performs nodifferent from a non-coatedinsulator. R&D work in this field bySediver has therefore been focusedon three aspects of coatings:

. Erosion resistance

. Hydrophobicity evolution undervarious stress conditions

. Performance under pollutionSeveral ageing tests have beenestablished or are being developedto better understand the behavior ofcoatings under diverse environmentalconditions. For example, longterm AC and DC tests in chamberscontaining clean or salt fog atvarious stress levels and differentregimes have been runningcontinuously at the Saint YorreR&D ageing laboratory. Particular

Different erosion levels related toselection of ATH filler.

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Coating longevity, performance and ageingultimately depend on optimum chemistry as well

as method and quality of application.61

Tests being performed on coatingsSediver ageing laboratory.

attention has been given to tryingto ensure the best correlationbetween test results and actualfield experience. In fact, one of themain findings from the differentmethodologies tested is how difficultit is to duplicate real conditionsusing short-term laboratoryprocedures. For this reason, severalstri ngs of coated glass insulatorshave been installed at outdoor

test stations selected because oftheir challenging environmentalconditions. The target here has beennot only to develop more knowledgebut also to verify the consistency ofresu Its obta i ned versus those fromlaboratory testing.

Establishing the proper balancebetween erosion resistance andhydrophobicity has been approachedthrough a novel philosophy thataims to combine HydrophobicityClassification from IEC TS62073with an internally developederosion class chart, designatedSediver’s CE classification. Whileinitially only used internally, thisclassification system is now startingto be adopted elsewhere sinceit provides a relatively accuratemeasure of the dynamics betweenthese key performance parameters.

Hydrophobic properties of alternativecoatings are evaluated both throughtest results and observations made inthe field.

It should be noted that findingshave to be viewed with some cautiongiven the wide possible diversityamong severe service environments,e.g. from the deserts of Peru tothe coastlines of Sicily. Similarly,behavior under AC or DC has to beanalyzed with regard to the specificimplied stresses. Such informationhas been accumulated for almost20 years and over the past decade amonitoring program covering over amillion coated glass discs has beenin force. This has included yearlyeval uations of sam pies removed fromlines.

This work has confirmed good overallpreservation of hydrophobicityincluding in service areas wherewashing cycles once had to beperformed each quarter. Thestress level encountered in theseapplications translates into somereduction of hydrophobicity aroundthe pin area, which is expectedgiven the electric field distributionprevailing in that region. For example,the hydrophobicity status of insulatorsafter 7 years of service in a pollutedcoastal environment equivalent tolevel E7 (as described in the IEC60815-1) can be explained bymodeling electric field that showsthat the pin section typically suffersmore than the rest of the insulator.

The overall hydrophobicity of thesecoated insulators, even at the liveend, seems to be preserved and canbe classified between WC1 and WC3.What is also clearly evident based onlaboratory tests on short strings aswell as on field observations is thatthe complete string never loses all itsoverall hydrophobicity performance.This in spite of some hydrophilicareas appearing on localized areasof individual units, mostly near the

Sediver CE coating erosionclassification chart

Hydrophobicity (WC 3) near pinElectric field modeling.

(pollution level E7 according to EC 6081 5-1).

energized side. Similarly, when smallsections of coating are removed, thesame approach applies arid salt fogtests have confirmed no differencein performance versus fully coveredu n its.

In parallel to such visual andhydrophobicity comparisons, thelevel of desired low molecularweight species (silicone [MW) leftin the RTV silicone coated surfaceafter several years in the field orafter thousands of hours in anaccelerated ageing chamber has alsobeen i nvestigated . I n th is regard,a Soxhlet test has been utilized inorder to allow a quantitative measureby extracting the [MW left in thesilicone. While further such researchwill help to better understand thedynamics of hydrophobicity recoveryprocesses, one major trend hasalready been confirmed throughresearch. Although top and bottomsurfaces are coated with the samematerial and surface thickness, thereis a difference in the amount of[MW species remaining after a fewyears such that there is a noticeabledecrease at the beginning of the lifeof the insulator (corresponding tothe initial years in service). However,the trend indicates that a possiblestability in residual [MW content ofthe silicone coating is achieved overtime. One possible explanation — stillbeing investigated — is that after aninitial phase of settling, the levelremains relatively stable, ensuring thenecessary hydrophobicity as well ashydrophobicity recovery.

Field MonitoringVarious performance attributes havebeen monitored in the field includingoverall condition of the insulators,coating adherence, thickness,hydrophobicity and hydrophobicityrecovery. There have also beenmeasurements of pollution and itsconductivity. Apart from the benefitof allowing laboratory test proceduresto be calibrated against field findings,

this has also allowed measuringthe stress gradient on the coatingalong the string’s length. Some unitscan be partially WC5, as discussedabove , bu I overa I I 1lìe stri ng remainedhydrophobic in all the differentservice onviroi nieots i rivestigated.

To help monitor the evolution ofcoating performance along the string,

a geometric approach was establishedwhereby the string is divided into 3sections of length: bottom 25%; top25%; and middle 50%. Up to now,only light erosion (i.e. type CE2)has been observed in the bottomportion of strings. Similarly, in regardto hydrophobicity, only some areasin the same portion of string havebeen affected. This demonstrates ahigh hydrophobic buffer effect andresilience of the silicone coatingapplied to glass insulators.

Pollution PerformanceService performance has confirmedthat the risk of pollution flashoverhas been largely eliminated bythe use of coatings in place ofany periodic washing. Given this,special focus has been placed on

Top: Soxhiet testingChart shows evolution of 1MW species

content over years in service.

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String sectioning for coatingevaluation.

—Top LMW content

——Bottom 1MW content

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DC salt fog tests at Sediver laboratory.Average U% performance with base 100

for non-coated glass.

artificial pollution tests to verify this.For example, clean fog pollutiontests with solid deposit layers wereperformed at STRI in Sweden andthe findings were later confirmed atthe HV laboratory in Bazet, France.One of the challenges in this regardrelates to the preparation anddeposition of the contamination on ahydrophobic surface prior to testing,with or without recovery. Resultsconfirm a substantial i ncrease inwithstand voltage compared to anon-coated string with a performancesimilar or even superior to anequivalent composite insulator.

Additionally salt fog pollution tests(at 8Og/l) in DC were performed onsilicone coated glass insulators atthe R&D laboratory in Saint Yorre.Once again, the coated insulatorsoffered substantial improvementversus normal non-coated glass.This result is in line with findingsfrom monitoring an actual DC lineequipped with silicone coated glassrci iId-rrc

Future DevelopmentsWhile R&D on RTV coatings isnow concentrated on chemistry,ageing, performance and testingmethodologies, there is also greatvalue in continuing to monitor thecondition of coated insulators in thefield. Up to now there have beenno reports of flashover, washingor replacement of any of theseinsulators. At the same time, theSediver CE classification table andmethodology discussed earlier isbeing ‘fine-tuned’ to better describethe real condition of insulators andthis approach could one day formthe basis for a standard method ofevaluation.

Better understanding theperformance of silicone-coatedinsulators is also providing newdirections for development. Forexample, testing and monitoringof insulators where the coatingwas applied only to the bottomof the insulator began severalyears ago and there are alreadysuccessful applications in LatinAmerica with such ‘undercoated’insulators. Similar trials are inprogress in North America andAfrica with equally good results.This development may providefurther opportunities for technicalpublications on the topic. E1

Silcone coated toughened glass insulator

Toughened G ass nsu ator

C 5C 100 150 200

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