-
INSULATORS J
Coating GlassInsulators forService in SevereEnvironments
:: . :
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
I. •::•- •..
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.
II
-
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.
rj
•1.-
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.
120 -----—.-— -
String sectioning for coatingevaluation.
—Top LMW content
——Bottom 1MW content
C0
60
Cw
40
20
0
Time
: I
-
/
r.Tf1 ¶.:‘ . .. ,
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
L