Available online at www.sciencedirect.com

Electric Power Systems Research 78 (2008) 248254

Performance of RTV silicone rubber coatinggorent, U

mber2007

Abstract

RTV SIR ere phousings is papvoltage subs ementhe specific ivityOn the othe ecordperformance pari 2007 Else

Keywords: R

1. Introdu

The usage installorder to suppress the influence of pollution. Many long-termapplications have been reported worldwide, demonstrating theperformance improvement achieved in comparison to theirceramic counterparts [13]. However there are applicationswhere theis not techity. A typicvoltage subceramic insability of cespecially

In such(RTV) silictive. Coatinceramic insimilar to tlators [47achieved [5

CorresponE-mail ad

thishig

. Infield

data acquisition system, has been employed. The results arecompared to measurements on porcelain insulators, performedsimultaneously in the same substation.

0378-7796/$doi:10.1016/jsubstitution of ceramic insulators with compositesnically or financially in profit for the electrical util-al example is the already installed equipment in highstations [4]. In this case the replacement cost of theulators is considerable. In addition the limited avail-omposite housings, for some types of equipment,

old, such as transformer bushings, is also an issue.applications the use of room temperature vulcanizedone rubber (SIR) coatings can provide an alterna-gs are applied on the surface of the already installed

sulators, imparting a hydrophobic surface behavior,he one demonstrated in the case of composite insu-]. As a result an improved pollution performance is10].

ding author. Tel.: +30 810 223893.dresses: sidkir@ieee.org, k.siderakis@gmail.com (K. Siderakis).

2. Application of RTV SIR coatings

2.1. Application site

Crete is the biggest island in the Aegean Sea, Greece. Due toits coastal development the majority of the 150 kV installationsare located in a proximity to the sea coast. As a result intensepollution problems have been experienced, usually resulting tolong duration power outages. In order to suppress the influenceof pollution, high pressure water washing has been employed asthe primary maintenance method, achieving after years of expe-rience a remarkable decrease of the pollution related flashovers.

However for substations an improved maintenance methodwas required. The financial cost for washing is considerable,not only due to the man hours spent, but also considering that inthe case of substations, live washing is not permitted in Greeceby law and therefore power interruptions were necessary. Theproblem is even more intense in the case of step up substations,

see front matter 2007 Elsevier B.V. All rights reserved..epsr.2007.02.013K. Siderakis , D. AHigh Voltage Laboratory, Electrical and Computer Engineering Departm

Received 24 March 2006; received in revised form 20 DeceAvailable online 29 March

coatings can improve the performance of ceramic insulators, in cases whnot technically or financially in profit for the electrical utility. In thistation is investigated by measurements of leakage current. The measurenvironmental conditions. In the case of condensation the surface actr hand, in the case of light rain the same levels of activity have been r

of the coated insulators, for the specific conditions, is superior in comvier B.V. All rights reserved.

TV SIR coatings; Leakage current measurements

ction

e of composite insulators in outdoor high volt-ations is probably the most efficient solution, in

In150 kVperioding ins installed in coastal systemsisniversity of Patras, Rio, Patras 26500, Greece2006; accepted 9 February 2007

ollution problems are experienced and the use of compositeer the performance of coatings installed in a coastal hights indicated the importance of the wetting mechanism, foron the coated insulators has been remarkably suppressed.ed for both coated and non-coated insulators. The overall

son to the non-coated insulators.

paper the performance of RTV SIR coatings in ah voltage substation, is evaluated after a 7-year timethis direction continuous leakage current monitor-conditions, by a specially designed for this purpose

K. Siderakis, D. Agoris / Electric Power Systems Research 78 (2008) 248254 249

where the maintenance procedures usually require some of thesteam power generators to be stopped.

The usemethod. Asalready inscan be comas long asrequired.

The first150 kV stelocated inexposed toproblems hthe insulatoIEC/TR 60

In a 3-yehad been coprovided bythe applicaup to today

2.2. Mater

The impof pollutionbic surfaceits capabililated containternal struand the intcase of cerastructure coof the agintoring is neapplication

For thecial requireinsulators avoltage) minsulators amonitoringThirdly comin respect oinsulators.conditions

Amongmonitoringthe above rworldwideon ceramicinformationonset until

2.3. Measu

LeakageIn order to

through a current sensor. Usually this is achieved by insertingthe current sensor between the grounded side of the insulator

e sysulatoa c

or. Fto g

the ne wathe

lectim dce gahe hi

acqdatarren

aneoion ouousuchls ared inally,

eas

e trane hus illustall

kag

orce

perce fgro

atedyed.harg, is il

is c T

0

Qj isbsolN is. Intiontherted

Kn=of RTV SIR coatings was employed as an alternativealready has been mentioned, they are applied on the

talled ceramic insulation, at a user selected time, thusbined with other maintenance procedures. In additionthe materials are effective no other maintenance is

large application took place in 1998, in one of the twop up substations in Crete. Linoperamata substation,a distance of less than 500 m from the sea coast, isthe action of the sea. As a result intense pollutionad been observed, although the creepage distance ofrs used had been appropriately selected according to815 [11].ar period the total number of 2700 ceramic insulatorsvered, employing a total material amount of 4100 kg,three manufactures. It must be mentioned that since

tion no other maintenance method has been employed.

ial monitoring

roved performance of composite materials in the case, including RTV SIR, is the result of the hydropho-behavior provided and especially in the case of SIR,ty to impart hydrophobicity to the surface accumu-mination. This advantage originates from the materialcture, where the levels of the chemical bonds energyermolecular forces observed are lower than in themic materials [1215]. However, this lower strengthrresponds to a material more susceptible to the action

g mechanisms present. Consequently material moni-cessary, especially in cases like this, which is the firstin the specific environment.

selection of the appropriate monitoring method, spe-ments had to be considered. At first the monitoredre part of the substation and therefore off line (out ofeasurements have limited application. Secondly there exposed to the field conditions, thus continuousis necessary in order to evaluate their performance.parative measurements must be provided, not only

f the ceramic insulators but also between the coatedFinally the measurements must be performed in theof a high voltage substation.the methods usually employed [16], leakage currentis probably the most suitable in this case, fulfilling

equirements. It has been applied by many researchersboth in field and laboratory conditions [1721], bothand composite materials and in addition can providefor the total electrical activity observed (from the

a possible flashover).

rements setup

current is distributed on the surface of the insulator.be recorded it has to be collected and then driven

and ththe insInsteadinsulatsensor

alongdistanc

ForThe seing froand thfrom t

Thecentralnine cusimultresolut(continvaluesintervarecord

Finrent) mvoltagrelativsetup iring in

3. Lea

3.1. P

Thereferenout of anot coemplolated cperiodsurface

Qj =

whereof the aA andperiodintegra

Furcalcula

Qacc =tem ground electrode. In this case however due tors supporting infrastructure, this was not possible.

ollection ring was installed at the bottom side of theurther the ring was connected through the currentround. In order to enforce the conduction of currentew path formed, a small part of the insulator leakages excluded (left after the collection ring).

measurements Hall current sensors were employed.on of this type relied on the provided bandwidth, start-

up to 20 kHz, the remarkably low input impedancelvanic isolation of the electronic measuring systemgh voltage side.uired from the sensor data are then transmitted to aacquisition system, which is capable of monitoring

t channels. Sampling is performed continuously andusly for all insulators, at a rate of 2 kHz for each and af 12 bit. Then due to the amount of the measured data

s monitoring), further processing is performed andas min/max current, average, etc. over user defined

e stored. In addition the current waveform is alsothe case of intense activity.

the system is capable of synchronized (with the cur-urements of a three phase voltage system throughsformers and meteorological data, i.e. temperature,

midity, precipitation and wind. The measurementsstrated in Fig. 1. Also Fig. 2 is a picture of collectioned at the bottom of an insulator.

e current measurements

lain insulators

formance of porcelain insulators is considered as aor the RTV SIR coatings evaluation. Therefore twoup of nine simultaneously monitored insulators, werebut instead the regular washing maintenance was

For these two the monthly distribution of the accumu-e, given as the average value in the 7-year monitoringlustrated in Fig. 3. The value of charge on the insulatoralculated according to the following equation:

|i(t)| dt Qj =N

n=0|i[n]|t (1)

the charge in coulombs calculated by the integrationute current values, i[n] the value of leakage current inthe amount of values sampled during the integrationthis case the sampling frequency is 2 kHz and theperiod T = 60 s, thus N = 120,000.the value of the accumulated charge per month isaccording to the following equation:

1|Qj[n]| (2)

250 K. Siderakis, D. Agoris / Electric Power Systems Research 78 (2008) 248254

Fig. 1. A schmaterial mon

where Qacclated) in coperiod andthis case 1.

Fig. 2. Installpost-insulator

Fig. 3. Monthly distribution of the accumulated charge, given as the averagevalue in the 7-year monitoring period, for the porcelain non-coated insulators.

The measurements indicate that the problem is confined in a3-month period, starting from August until October, while dur-ing the rest of the year the activity intensity is remarkably lower.The measuthe observeAs illustrat

inceserv

uen, thetheematic diagram of the data acquisition system employed for the

ages, sput inthe inflperiodtion ofitoring.

is the total amount of charge per month (accumu-ulomb,Qj the amount of charge for the nth integrationK is the number of integration periods per month, in73 108 per day.

ation of the current collection copper ring at the bottom of a 150 kV.

total numbtion. Otherand mechanphenomenafrom Janua

3.2. Corre

The inteto the envir

Fig. 4. Monthrelated, in therements of leakage current come in agreement withd surface activity and the power outages reported.

ed in Fig. 4, the majority of the pollution related out-1969, when the first 66 kV transmission line was

ice, have been reported during this period. Althoughce of pollution is found to be confined in a 3-monthimpact of the pollution related outages to the opera-transmission system is remarkable, since 43% of theer of the outages observed, are correlated to pollu-causes for outages in Crete are lightning incidentsical failures due to the stress applied by strong winds,responsible for the outages observed in the period

ry to April.

lation to the environmental parameters

nsity of the pollution problem is strongly correlatedonmental conditions observed, especially on the windly frequency distribution of power outages, total and pollutionTransmission System of Crete, from 1996 up to today.

K. Siderakis, D. Agoris / Electric Power Systems Research 78 (2008) 248254 251

Fig. 5. Average wind speed per month, calculated for the 7-year monitoringperiod.

activity, which is the primary contamination transfer mechanismand precipimechanism

In Fig. 5calculateditoring perthere are tcal year, onanother ining these twtransferredthe range otaminants athe summesary for a cduring this

Furthermulated onself-cleaninmechanismduring thetion are remperiods of

Fig. 6. Avera75% per mon

increase of the accumulated contamination is observed, sincethe degree of insulator cleaning is limited.

At the time of deposition the transferred contamination isalready humid since the sea is the primary source. Howeverthe amount of water transferred by the wind is not enough tosupport the formation of a conductive surface film. In addition,due to the increased ambient temperatures observed during thisperiod, the humid contamination is dried upon deposition on theinsulation surface. Consequently surface wetting is required forthe flashover development.

In the absence of precipitation, condensation, due to the radi-ation of heat, is the wetting mechanism present. Observed duringclam and cloudless nights is capable of supporting the develop-ment of a surface conductivity. As illustrated in Fig. 6, duringAugust and September, although precipitation is low, the proba-bility for RH to exceed 75% is increased. The threshold of 75%is set, considering the phase transition of sodium chloride parti-cles [22]. It is worth mentioning that the majority of the poweroutages observed during the period of intense activity have taken

uring the night and early in the morning, as also illustrated7.sequult odevensais a

ue tolity oulatimec

p simightthe r

TV S

ig. 8as thtation which on the other hand is the primary cleaning.the values of the average wind speed for each month,

according to measurements, during the 7-year mon-iod are illustrated. The measurements indicate thatwo periods of intense wind activity during a typi-e during the winter, from November to March andthe period from July to August. As a result, dur-o periods an increase of the contamination amountis observed. However it is important to notice thatf wind speeds observed result to a progressive con-ccumulation up to the critical value. Practically forr, an average period of 18 days is considered neces-ritical amount to be formed, considering that washingperiod is necessary every 23 weeks.the amount of the contamination that is finally accu-the insulator surface also depends on the insulatorg capability. Precipitation is the primary cleaningin these specific conditions. As illustrated in Fig. 6,

summer period the levels of the observed precipita-arkably low. Consequently although there are two

intense wind activity, during the summer period an

place din Fig.

Conthe resand theis condwhichever dcapabiaccum

sationdevelocalm ngap in

3.3. R

In Fgivenge precipitation and probability for relative humidity to exceedth (data available after a 20-year monitoring period).

Fig. 7. DailyTransmissionto today.ently the problem of pollution in the case of Crete isf the limited insulators cleaning during a dry periodelopment of an additional wetting mechanism, whichtion. The problem is enhanced by the wind activity,lso increased during the considered period. How-the wind forces observed and the limited cleaningf the wetting mechanism a progressive contaminantson is observed. Also due to the features of the conden-hanism, wetting and contaminants deposition cannot

ultaneously, since condensation is observed durings. Therefore in the most severe case there is a timeange of 610 h from deposition to wetting.

IR coated insulators

the monthly distribution of the accumulated charge,e average value for the porcelain insulators (Fig. 3)

frequency distribution of pollution related power outages, in theSystem of Crete, for the period of intense activity, from 1696 up

252 K. Siderakis, D. Agoris / Electric Power Systems Research 78 (2008) 248254

Fig. 8. Monthly distribution of the accumulated charge, given as the maximumvalue in the 7-year monitoring period, for the RTV SIR coated porcelain coatedinsulators.

and the maximum value for the coated insulators, in the 7-yearmonitoring period, is illustrated. The improvement achieved bythe application of the coatings is evident from the values pre-sented especially during the period of intense activity. Howeverit can be seen that the surface activity on the surface of the RTVSIR coatings becomes comparable to the activity observed onthe surface of uncoated insulators, in the period from Decem-ber to April and especially in March. During this period, inaddition to condensation, light rain is also a wetting mecha-nism that cboth coatedthis case issurface is

nism, but it is not enough to completely clean the insulatorsurface.

The correlation between light rain and surface activity isevident from the recordings of Fig. 9, where the simultaneoussurface activity on a porcelain insulator and a coated porcelaininsulator in respect to precipitation are illustrated. The activityin the case of the porcelain insulator is higher; however it iscomparable to the activity recorded on the coated insulator.

3.4. Investigation of the surface activity

Fig. 10a is the leakage current waveform on the surface ofthe non-coated insulator, in the case of a dry band discharge,during the incident of Figs. 9 and 11a is the correspondingcurrent waveform on the surface of the coated insulator. AlsoFigs. 10b and 11b are the FFT analysis spectrum for each wave-form, respectively.

The peak current values observed are higher in the case ofthe porcelain insulator. A delay of the current onset is evidentfrom the waveform, indicating the presence of a dry band whichblocks the flow of current. Further a discharge follows, whenthe voltage stress applied along the dry band exceeds the corre-sponding dielectric strength, which however may occur beforethe peak voltage value is reached. In the case of the coating,a delay is also observed for the discharge onset. The currentpeaks that appear however in this case occur at the voltage peak,denoting a possible correlation to the surface electrical field.

lthouponde vo

serve

Fig. 9. Simulcharge on a nan support the development of surface activity, onand non-coated insulators. The important feature in

that the amount of water deposited on the insulatorshigher in comparison to the condensation mecha-

Also acorres

negativare obtaneous measurements of the surface charge observed on a RTV SIR coated insulatoron-coated 150 kV insulator. (b) Positive/negative charge on a coated 150 kV insulatogh in the case of porcelain the dry band discharges to a continuous activity both under the positive andltage stress, in the case of RTV SIR more dischargesd under the positive polarity. This is reflected to the

and on a similar non-coated, under light rain. (a) Positive/negativer. (c) Precipitation at the time of measurements.

K. Siderakis, D. Agoris / Electric Power Systems Research 78 (2008) 248254 253

Fig. 10. (a) Lcoated 150 kVin (a).

frequency stively, wheat even freq

4. Evaluat

The meof the envimechanisminsulators.increased fllate July unably low probserved cocomparisonsignificantthat condenface, althoconductive

For thisrisk due toinsulators wmance of tnor any otheakage current waveform during a dry band discharge on a non-insulator during light rain. (b) Frequency spectrum of the current

pectrums as illustrated in Figs. 10b and 11b, respec-re in the case of RTV SIR the presence of harmonicsuencies is evident.

ion of the coated insulators performance

asurements of leakage current reveal the influenceronmental conditions and especially of the wettingto the performance of both coated and non-coated

In the case of porcelain intense surface activity andashover risk have been recorded for the period fromtil October. This period is characterized by remark-

ecipitation, which in combination to the wind activityrrespond to increased contaminants transfer rates, into the rest of the year. However the difference is not

and for the increased amount accumulated, the factsation is not capable of cleaning the insulation sur-

ugh it can support the development of the surfacefilm, must also be considered.period, which is characterized by increased flashoverpollution, the performance of the RTV SIR coatedas remarkably superior to the corresponding perfor-

he non-coated insulators, although neither washinger maintenance procedure has been performed to the

Fig. 11. (a) LSIR coated 1current in (a).

coated insuthis improvand especibehavior thmulation thamount to bsition and wcoating surmolecules mity is suppr

Howevesupplied onditions formrevealed that levels cohydrophobinfluence ain Figs. 10a

At the pperformancfrom the leactivity inapplied streakage current waveform during a dry band discharge on a RTV50 kV insulator during light rain. (b) Frequency spectrum of the

lators since the coatings application. The reason fored performance is the hydrophobic surface behaviorally the SIR capability to transfer the hydrophobice accumulated contamination. The progressive accu-at occurs (an average period of 18 days for a criticale formed) and the time gap that exists between depo-etting permits the recovery of hydrophobicity on the

face, as a result of the low molecular weight (LMW)igration [1315,2326]. Thus leakage current activ-

essed.r in the case of light rain larger amounts of water arethe insulator surface supporting changes of the con-ed. The measurements of leakage current performed

at in this case surface electrical activity can developmparable to the porcelain insulators. However the

ic surface behavior of RTV SIR appears also to haves illustrated from the comparison of the waveforms

and 11a.resent stage, after 7 years of service the coatingse is satisfactory throughout the year, as illustratedakage current measurements. However the observedthe case of light rain is an issue that rises, since theess to the coating material in this case can possibly

254 K. Siderakis, D. Agoris / Electric Power Systems Research 78 (2008) 248254

support aging mechanisms. Therefore the application until todayis considered successful, especially for the period of intenseactivity. However the influence of the activity during light rainto material aging needs further monitoring and investigation.

5. Conclusions

RTV SIR coatings can be used in order to improve the pol-lution performance of ceramic insulators. In this case, after a7-year service period, such an improvement has been achieved,as it has been verified by continuous leakage current measure-ments. The measurements also revealed the importance of thewetting mechanism present in each application. Condensationwetting appears to have limited effect on the performance ofthe coatedof water fotaminantsdepositionmance.

On thewater whicwhere a losurface eleare more inobserved innon-coated

In conclvice can bHowever thrial aging ithe case of

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Performance of RTV silicone rubber coatings installed in coastal systemsIntroductionApplication of RTV SIR coatingsApplication siteMaterial monitoringMeasurements setup

Leakage current measurementsPorcelain insulatorsCorrelation to the environmental parametersRTV SIR coated insulatorsInvestigation of the surface activity

Evaluation of the coated insulators performanceConclusionsReferences