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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: [email protected],
[email protected] (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
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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)
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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.
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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
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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.
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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
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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