Electric Power Systems Research 76 (2006) 778785

Performance improvement of 33 kVin harsh environme

l-Snginement

ber 20er 200

Abstract

Compreh torsin comparis ng impollution tes latorsand wraparo r usin 2005 Else

Keywords: L s

1. Introdu

Recentldistribution lines which are near the sea side or pass throughhighly contaminated areas, have high leakage current leadingto wooden pole fires, insulator fracture and current breakingproblems. All of these abnormalities have caused inadvertent,where eachAll 33 kVtors with d40 mm/kV.

Woodenbut a numbination asof wind, foenough loctact betweeespeciallythe end ofof the solu

CorresponE-mail ad

maqrashi@sq(S. Al-Sumry

e andproaion.

subject to instantaneous polluting conductive mist conditions,e.g. marine salt fogs.

This paper introduces two possible solutions to reduce thewooden pole fire in very heavy polluted areas, namely, adding of

0378-7796/$doi:10.1016/jpole fire causes up to 600 m3 of oil deferment.distribution lines have line-post porcelain insula-ifferent designs at a specific creepage distance of

pole fires can not be attributed to any single factorer of concurrent conditions with insulator contam-a prerequisite followed by atmospheric conditionsg or heavy mist. The leakage currents can generatealized heat to ignite the wood at the point of con-n wood and metal surfaces and start a pole top fire,when pole crossarms and insulators are near or attheir service life. Live-line washing represents onetions to reduce the leakage current on the insulator

ding author. Tel.: +968 99777512; fax: +968 24413454.dresses: metwally@squ.edu.om (I.A. Metwally),u.edu.om (A. Al-Maqrashi), Saif.AR.Sumry@pdo.co.om), Saif.SH.Harthy@pdo.co.om (S. Al-Harthy).

creepage extenders to the existing line-post porcelain insulatorsor replacing the latter by silicone rubber ones.

2. Adding of silicone-rubber creepage extenders

Creepage distance is the shortest distance on insulating sur-face that exists between two conductive parts of an insulator withdifferent potentials. An approach to improve insulators perfor-mance is to increase the creepage distance. This can be achievedby increasing the shed diameter, and therefore the shed projec-tion, with the application of extenders. For example, should thecreepage be increased from, say, 16 mm/kV to 25 mm/kV, thenthe insulator has been effectively upgraded from one suited to aLight pollution category to a Heavy pollution classification[13].

Creepage extenders are made of true HTV silicone rubbermaterial with inherent hydrophobic characteristics and tracking,erosion and UV resistance, and are attached to the insulators bymeans of an RTV silicone rubber adhesive. Even uncoated, the

see front matter 2005 Elsevier B.V. All rights reserved..epsr.2005.10.010I.A. Metwally a,, A. Al-Maqrashi a, S. Aa Department of Electrical & Computer Engineering, College of E

b Power Systems Department, Petroleum DevelopReceived 20 June 2005; received in revised form 27 Octo

Available online 15 Decemb

ensive standard tests were conducted on 33 kV line-post porcelain insulaon to the newly introduced polymeric ones. These tests are the lightnits. The latter were conducted in a salt-fog chamber. Silicone rubber insuund creepage extenders to the existing line-post porcelain insulators ovier B.V. All rights reserved.

ine insulators; Leakage current; Flashover; Silicone rubber; Creepage extender

ction

y, some of Petroleum Development Oman (PDO)

surfacing apoperatline-post insulatorsnt

umry b, S. Al-Harthy bering, Sultan Qaboos University, OmanOman (PDO), Oman05; accepted 30 October 20055

in order to improve their performance in harsh environmentpulse withstand and flashover tests, and the AC artificialshowed superior performance than adding heat-shrinkableg new polymeric insulators.

hence the pole fires. The main drawback of a wash-ch is, though, establishing the correct timing of theIn addition, washing is not effective in environments

I.A. Metwally et al. / Electric Power Systems Research 76 (2006) 778785 779

sheds are capable of operating in the most extreme environ-ments. Creepage extenders increase the flashover performanceof insulator

reducing reducing increasin

There ashrinkableage extend100 mm anGenerally,distance bythe voltageextenders iinsulator adistortionspartial disc

3. Replaceinsulators

Siliconeused for moSiR is the mvoltage outin various p220 kV tranproperty [5insulator ishydrophobof electric fia very impolators in serof SiR insuend-fittings

In genecaused byand electridegradationmer degradical attackattack incluband arcinstructure omerizationpolymer mical strengtpolymer mlead to high

4. Test res

This secried out: (1tests accord

Test sporcew polEs.

ollutestsdesigt, th

e to d% imto dehe ser (2

temd co

vo

es un

est sa

. 1s andof all samples.

tandard lightning impulse withstand and ashover

these tests, all ten stages of the impulse voltage gen-were used to generate the lightning impulse voltage/50s). This voltage is applied to the insulator top, whileulator base is grounded. A capacitive divider is usedsure the voltage. The voltage waveform was recordedigital storage oscilloscope, where the high voltage wasted by a ratio 6987:1. This test was done in dry/clean

luted) conditions under recorded values of the room tem-re, pressure and humidity. First, the impulse withstandas done by applying ten shots to the insulator [710].a) shows the generated standard 1.2s/50s lightninge waveform with 170 kVpeak value for the impulse with-est, i.e. the basic impulse level (BIL) = 6.31 per unit (pu)s by:

the surface electric stress,the leakage current andg the electric strength of insulators.

re two types of creepage extenders, namely, heat-creepage extenders (HSCEs) and wraparound creep-ers (WACEs). HSCE and WACE add a nominal ofd 150 mm to the creepage length, respectively [2].it is recommended to increase the existing creepageat least 20%. Creepage extender does not upgradeclass of the insulator. The disadvantage of creepage

s that the different surface properties of the ceramicnd the polymer extender can create severe voltageat the transitions between the materials, resulting inharges and flashover [13].

ment of existing porcelain by silicone rubber

rubber (SiR) as outdoor insulation material has beenre than 25 years with good service experience. Today,ost used polymeric material for medium and high-

door insulation [4]. After about 11 years of operationolluted areas, SiR insulators removed from 110 kV tosmission lines still presents a good hydrophobicity]. Generally, the hydrophobicity distribution of SiRnon-uniform. It is found that the local deterioration oficity has close correlation with the local strengtheningeld. Water droplets induced low-energy discharge isrtant reason for the hydrophobicity loss of SiR insu-vice. In order to improve the long-term performancelator, it is better to limit the electric field level near5 kV/cm.

ral, surface degradation of polymeric insulators isenvironmental attack related to weather conditionscal activity. Environmental attack includes thermal

from ambient temperature, ultraviolet induced poly-ation, acidic attack from acid rain or acid fog, chem-from salt contamination, sandblast, etc. Electricaldes partial discharges caused by leakage current, dry

g and corona near the metal fittings. The chemicalf the polymer changes by chain scission (depoly-), oxidation, etc. As the result of these changes, theay lose elasticity, become brittle and lose the mechan-h. As well, increased surface roughness or loss of theatrix could cause loss of surface hydrophobicity and

leakage current [6].

ults and discussion

tion presents the lab tests. Two types of tests were car-) standard lightning impulse withstand and flashovering to IEC standards [710] and (2) standard AC arti-

Fig. 1.B: used(f) F: netwo HSC

ficial pTheseferent

Firsvoltagthe 50latorstests, tchambity andpolluteappliedvoltag

4.1. T

Figdesigndetails

4.2. Stests

Inerator(1.2sthe insto meaby a dattenua(unpolperatutest wFig. 2(impulsstand tamples of 33 kV line-post insulators(a) A: used porcelain, (b)lain, (c) C: used porcelain, (d) D: used porcelain, (e) E: new SiR,ymeric, (g) insulator A with two WACEs and (h) insulator C with

tion tests under different pollution severities [11,12].were conducted on 33 kV line-post insulators of dif-ns and materials as can be seen in Fig. 1.

ese insulators were tested under standard withstandetermine if the insulators pass the test or not. Then,pulse flashover voltage tests were done for the insu-termine the U50% for each insulator. In AC pollutioname set of insulators were tested inside a salt-fogm 2 m 2.5 m) with a controlled relative humid-perature of 100% and 25 C, and under clean andnditions to characterize the leakage current versus

ltage (IV curves) and to get the AC wet flashoverder different pollution severities.

mples

shows the 33 kV line-post insulators of differentmaterials, and with and without CEs. Table 1 gives

780 I.A. Metwally et al. / Electric Power Systems Research 76 (2006) 778785

Table 1Details of the test samples

Status Material Number andtype of sheds

Distance betweenlarge sheds (mm)

Top/bottom sheddiameter (mm)

Top/bottom trunkdiameter (mm)

Axial length(mm)

Arcing distance(mm)

Creepage distance(mm)

A Used Porcelain 12 Regular 38 197/197 105/108 590 503 1510B Used Porcelain 11 Regular 27 191/191 92/108 520 493 1488C Used Porcelain 13 Irregular 50 202/202 103/115 525 490 1480D Used Porcelain 11 Irregular 70 197/230 72/108 540 475 1495E New Silicone Rubber 17 Irregular 41 135/105 53 580 460 1460F New Polymeric 16 Irregular 52 134/113 60 560 500 1520

of the peak phase operating voltage. Thereafter, the voltage wasincreased in steps to reach to the flashover level across the insu-lator surface. Ten shots were applied at each voltage level witha time interval of 5 min. All impulse tests were conducted atthe room temperature of 25 C and relative humidity of 55%.Fig. 2(b) illustrates an example of the flashover on the tail underpositive polarity.

Fig. 3 illustrates a comparison between U50% for all insula-tors under positive and negative lightning impulses, where allflashovers occurred on the tail. It can be seen that the nega-tive flashover occurs at higher voltage level. The high-voltageconductor (connected to the insulator top) and the earth planeforms a non-uniform field arrangement. Therefore, this trendis attributed to buildup and accumulation of the space charges.These charges distort the electric field in a way that under posi-tive the electric field across the airgap becomes highly distorted.The high-field region (very close to the high-voltage conduc-tor fittings) is in time moving further into the airgap because

Fig. 2. Lightnwithstand vol

Fig. 3. U

electrons aing the heaionization.On contrarcharges (el

izater v

(E anoughor Ee among all insulators tested (see Table 1). It is worthning that insulators AD are used ones, therefore theirunder negative applications are lower in comparison withf insulators E and F. In addition, the arcing distance of

or F is higher than that of insulators B and C (see Table 1).tors with HSCEs are indicated as (CE), e.g. insulator ASCEs is indicated as A (CE).ulse tests with CEs were conducted for insulators A ando of each of HSCEs and WACEs were added for twoors of type A and three HSCEs for type D. The aim ofst is to find out the effect of CEs on the U50% of theor. Table 2 introduces a comparison of the applicationboth types of CEs. From Fig. 3 and Table 2, it can bethe ionflashovlatorshave rinsulatdistancmentioU50%,those oinsulatInsulawith H

ImpD. Twinsulatthis teinsulatof theing impulse waveforms for insulator A: (a) waveform of lightningtage and (b) waveform of lightning flashover voltage.

Table 2U50% in kVpe

Polarity

Insulator A wInsulator D wInsulator A wInsulator D wInsulator A w50% flashover voltage under positive and negative impulses.

re quickly attracted in the positive terminal and leav-vy positive ions behind, i.e. extending the region forThis leads to a decrease in the flashover voltage.

y for the application of negative impulses, the spaceectrons) are repelled into the low-field region andion region is drastically reduced leading to a higheroltage. As can be seen the SiR and the polymeric insu-d F) have the highest negative U50%. All insulatorsly the same U50% under positive application, wherehas the lowest U50% because it has the lowest arcingak under positive and negative polarities with and without CEs

Positive Negative

ithout CEs 307 406ithout CEs 284 426ith 2 HSCEs 314 447ith 3 HSCEs 289 424ith 2 WACEs 356 446

I.A. Metwally et al. / Electric Power Systems Research 76 (2006) 778785 781

seen that there is a little effect of the HSCEs on the U50% ofthe insulator (10%) because of increasing the arcing distancefrom 503 mm to 535 mm. When using two WACEs for insulatorA, the positive U50% of the insulator becomes higher by 15%than that without CEs as a result of increasing the arcing distancefrom 503 mm to 596 mm.

4.3. Standard AC articial pollution tests

The AC test circuit consists of a 380 V/400 kV high-voltagetransformer, the test object (insulator), series shunt resistance of10 k with the insulator to measure the leakage current passingthrough it, a varistor across the shunt to protect the oscilloscope,and the capacitive divider for measuring purpose. The clean fog(100% relative humidity) was generated by atomization of tapwater ( 130S/cm). All tests were conducted at a constanttemperature of 25 C.

For dry/clean (unpolluted) conditions (Figs. 4(a)6(a)), theleakage currents are purely capacitive (i.e. the phase angle = 90) with very low amplitudes. In the case of clean fog (seeFigs. 4(b) and 6(b)), the leakage currents become purely resis-tive ( = 0) with much higher amplitudes, contrary to insulatorE (Fig. 5(b)), where its hydrophobicity nature prohibits the for-mation of thin conductive layer of water. Although insulator Fis a new polymeric one but its leakage current increases signifi-cantly during the clean-fog condition (see Fig. 6).

Generally for dry/clean insulators, the leakage current pathis capacitive, the current magnitude is small and current shapeis more orand generathe leakage

Fig. 4. Oscillfog condition

Fig. 5. Oscillograms of I and V for insulator E: (a) dry condition and (b) clean-fog condition.

and simultaneously, current amplitude increases, especially forinsulators with small vertical distance between sheds, e.g. insu-lators A and B (see Figs. 4(b) and 7(b) and Table 1).

com

re shcur

scillograms of I and V for insulator F: (a) dry condition and (b) clean-ition.less sinusoidal. Clean fog wets the insulator surfacetes a conductive layer [13]. As wetting progresses,

current path changes from capacitive to resistive,

ograms of I and V for insulator B: (a) dry condition and (b) clean-.

Thetions athe IV

Fig. 6. Ofog condparisons of all insulators for dry and clean-fog condi-own Fig. 7(a and b), respectively. It can be seen fromves that the rms current is proportional to the voltage

782 I.A. Metwally et al. / Electric Power Systems Research 76 (2006) 778785

Fig. 7. IV cuclean-fog con

accordingmainly capcondition fdesired appto 27 kVrminsulator (Etions becauthat isolatehydrophobelongationtinuous wawhich led tperiphery oleakage cushortest creas can be sa small leawhere its oand the verest of all poAt clean-folayers on awill drop ding anotherof the lowea reductioncondition, oleakage cudistance be

Oscillograms of I and V for insulator A with and without two HSCEs,dry and clean-fog conditions.

V characteristics with and without CEs underfog condition

his teg chf 25sedthatreethoue eff75%tivel

ACot onrves for all 33 kV line-post insulators: (a) dry condition and (b)dition.

to these facts. At dry condition the current becomesacitive, but it becomes mainly resistive at clean-fogor all insulators except for the SiR one (E). The

Fig. 8.and for

4.4. Iclean-

In tsalt-foature owere u

clearlytion, thand withat thaboutrespecusing WCEs nlied voltage is up to 19 kVrms, but it was increaseds to include the temporary over voltages. The SiR) gives the lowest leakage current for both condi-se of its hydrophobicity property. It was observedd water droplets are formed over the surface of theic insulator E and there was no significant water dropat the rim of the sheds. On the other hand, a con-ter film was formed on porcelain insulators (AD),o much bigger pendant water drops distributed on thef the underskirts. In Fig. 7(a) under dry condition, therrent for insulator C is the highest because it has theepage distance among the porcelain insulators testedeen in Table 1. The porcelain insulator D also giveskage current at clean-fog condition due to its design,verall design has a conical shape with irregular shedstical distance between sheds is 70 mm, i.e. the high-rcelain insulators tested (see Fig. 1(d) and Table 1).g condition, the condensation of the fog will makell sheds. The droplets falling from the small shedirectly on the surface of the lower/larger shed mak-thin layer which will drop faster. Then, the surface

r/larger sheds will be at higher resistance leading toin the leakage current. In Fig. 7(b) under clean-fogn the other hand, insulators A and B give the highest

rrent because both of them have the smallest verticaltween sheds, see Table 1.

the currenttrend was aD but with

4.5. IV ch

In this tsalinity soland NaCl c

Fig. 9. The IWACEs, andst, the insulator types A and D were tested inside theamber (100% relative humidity and at room temper-C). Two CEs were used for insulator A, while threefor insulator D. From Figs. 8 and 9, it can be seenthe effect of the CEs on insulator A. At dry condi-

is no remarkable change in the leakage current witht the CEs. At clean-fog condition, it is obviously seenect of the CEs on decreasing the leakage currents byand 44% for adding two HSCEs and two WACEs,

y. The latter effect is due to the gap left in the case ofEs (see Fig. 1(g)). It is worth mentioning that addingly reduces the leakage current peaks but also shiftswaveforms to be more capacitive (see Fig. 8). Similarlso observed when using three HSCEs for insulatorlower efficiency.

aracteristics under salt-fog condition

est, the insulators were dipped for 20 min in a highution with electrical conductivity of = 100 mS/cmoncentration of 70 g/l [12]. Thereafter, the insulatorsV curves for insulator A with and without two HSCEs and twofor dry and clean-fog conditions.

I.A. Metwally et al. / Electric Power Systems Research 76 (2006) 778785 783

were tested inside the salt-fog chamber at the rated phase voltageof 19.05 kVrms.

For a continuous application of sinusoidal voltage, the leak-age current causes drying of the wet pollution layer and, there-fore, both the current amplitude and pattern can vary with time[14]. Consequently, a continuous fog generation at a constantrate was used throughout the whole experimental investigations.As the electric stress increases, additional short current peaksmay appear at the signal crest due to discharges across dry bandareas, as well as between water droplets; especially the dynam-ically moved water droplets [14]. Water droplets on insulatorsurfaces elongate under electric stress. Corona and dry-banddischarges may appear from and between the droplets causingcurrent pulses with high amplitudes.

Continufor 30 mintive. The lbands arebands to cdischarges,magnitude.the arc gentain short dand the leaFigs. 10 an

Fig. 10range. It cabetter perfocurrent isThese resudition; seewhere insuthe hydropinvestigateFig. 11(athe peak oFrom Figsgive higherworth mencan be attrinsulator, wtive salt-foble dry bapulses.

Oscillograms of I and V for insulators D and A: (a) insulator D without) insulator D with three HSCEs; (c) insulator A with two HSCEs; (d)r A with two WACEs.

C wet ashover voltage for medium, heavy and verypollution levels

thermore, most of insulators were tested to get the ACshover voltage under medium, heavy and very heavy pol-levels [8,12]. The insulators were dipped for 20 min inns of NaCl with electrical conductivities of 100 mS/m), 167 mS/m (140 g/l) and 214 mS/m (200 g/l) to simulatedium, heavy and very heavy pollution levels, respec-

[8,12]. Thereafter, each insulator was tested inside theous fog was generated and the voltage was applied, where the leakage current pulses become repeti-eakage current dries the conductive layer and dryformed [13]. The high-voltage flashes over the dryreate partial arcs. Dry-band arcing causes surfacewhich change both the leakage current shape andDepending on contamination levels, extension of

erates a non-sinusoidal arc current, which may con-uration impulses superimposed on the 50 Hz currentkage current becomes intermittent as can be seen ind 11.illustrates the current pulses for insulator E in mAn be seen that the silicone rubber insulator (E) hasrmance, where other results reveal that its leakage

about 40% of that for the polymeric insulator (F).lts are consistent with those under clean-fog con-Figs. 5 and 6 for insulators E and F, respectively,lator E gives much lower leakage current due tohobicity property. In addition, the effect of CEs wasd for insulators D and A as shown in Fig. 11. Fromand b), it is noticed that the use of CEs reducesf the leakage current pulses by more than 90%.

. 11(c) and 12(d), it can be seen that the WACEsleakage current than that for the HSCEs type. It is

tioning that high leakage currents for insulators ADibuted to the surface roughening of the field-agedhich causes of trapping a large volume of conduc-

g pollution. These insulators rapidly establish unsta-nd conditions, i.e. unstable high-amplitude current

Fig. 10. Oscillograms of I and V for insulator E.

Fig. 11.CEs; (binsulato

4.6. Aheavy

Furwet flalutionsolutio(70 g/lthe metively

784 I.A. Metwally et al. / Electric Power Systems Research 76 (2006) 778785

Fig. 12. Effewithout HSCE

salt-fog chFig. 12 shothe NaCl cwithout HSvoltage isthree sampity of meastrend. Siliceven under(unused) oHSCEs leamore thanheavy pollu

The stattant role inbridging. Tciated withfor shed brupper surfadistance being effect oflat angle cthe top parthe larger tthe lower ifrom Table

Therefolators by sCEs to thethe HSCEscan be seewhen addinincrease (the case offact that ea

length, while each HSCE adds 100 mm. At high concentrationof NaCl, the order of best insulator performance, i.e. the lowest

flass, A

nclu

ndar/50-pos170t neorstle eor (dryery ciableconean-fty, aer ccree

t in talt-fge eby

ers lmor

eavyers gng ime cuhe re ruers temect of NaCl concentration on AC wet flashover voltage with ands.

amber by applying a continuous fog for 20 min [12].ws the dependence of AC wet flashover voltage ononcentration for insulators (A, C and D) with andCEs, and for insulator E, too. The reported flashover

the average value of three readings, for each of theles of the same type, taken to check the repeatabil-urements. Insulators A, C and D showed a commonone rubber insulator (E) gave the best performancevery heavy pollution condition because it is a new

ne and has the hydrophobicity nature. Adding theds to an increase in the AC wet flashover voltage by9%, 16% and 23% for the medium, heavy and verytion levels, respectively.us of the insulator surface material plays an impor-handling the excess water to inhibit or promote shed

drop intwo CE

5. Co

Sta1.2sof linelevel (highesinsulatis a litinsulat

Athave vapprec

Siliand cleproper

Unduse ofcurren

level (screepapulsesextendage byvery hextendlightnileakagfore, tsiliconextendimprovhere are two other important factors which are asso-the insulator profile, which proved to be relevant

idging [15]. They are the water accumulation on theces of the insulator sheds upstream and the verticaltween sheds. The former can occur due to a cascad-riginating from the upper wet weathersheds, whereauses shed bridging due to water accumulation ont of the shed [15]. The latter is self-explanatory, i.e.he vertical distance between two consecutive sheds,s the possibility of shed bridging [16] as can be seen1 and Fig. 12 for insulators D and E.

re, the replacement of the existing porcelain insu-ilicone rubber ones is much better than adding theformer. Further improvement in the insulators withcan be achieved by adding more than two units as

n for insulator D. Moreover, insulator A was testedg two WACEs. The results reveal that there is a slight5%) in the AC wet flashover voltage in comparison toadding two HSCEs. This trend can be attributed to thech WACE adds 150 mm to the insulator creepage

extenders cinsulator w

Acknowled

The firsport and coproject CTtheir gratitoratory atUniversity.

Reference

[1] Using Cfor FlasSTN/stn

[2] Creepag[3] Huakun

productshover voltage, is E, D with three CEs, D or C withwith two CEs, C and A.

sions

d lightning impulse tests (IEC-60060 withs) on all insulators (different designs and materialst type) have shown that they can withstand the BILkVpeak = 6.3 pu) and polymeric insulators have thegative flashover voltage. Used porcelain line-postgive the lowest impulse flashover voltages. Thereffect of the creepage extenders on the U50% of the15%).

condition and under AC application, all insulatorslose low values of the leakage currents and with noeffect of adding the creepage extenders.rubber insulator has the lowest leakage current at dry

og conditions because of its excellent hydrophobicitynd the highest AC wet flashover voltage.lean-fog condition with AC voltage application, thepage extenders leads to a decrease in the leakage

he range of 3375%. While under medium pollutionog condition: IEC-60507), the use of heat-shrinkablextenders reduces the peak of the leakage currentmore than 90%. Adding heat-shrinkable creepageeads to an increase in the AC wet flashover volt-e than 9%, 16% and 23% for the medium, heavy andpollution levels, respectively. Wraparound creepageive slightly higher (5%) flashover voltages underpulse dry and AC wet conditions, contrary to the

rrent under clean-fog and salt-fog conditions. There-eplacement of the existing porcelain insulators bybber ones is much better than adding the creepageo the former or using polymeric insulators. Furthernt in the insulators performance with the creepagean be achieved by adding more than three units perhich is impractical.

gements

t two authors wish to acknowledge the financial sup-operation with PDO Company under the researchR 2004-69. All authors would like to express

ude for the technicians in the High-Voltage Lab-ECED, College of Engineering, Sultan Qaboos

s

reepage Extenders on Pin Type Insulators, The Best Solutionhover Problem in Wet and Polluted Areas, www.nri.ac.ir/En/20E.pdf.e Extenders, http://catalog.tycoelectronics.com/TE/bin/TE.Hkce Heat-Shrinkable Creepage Extenders, www.huakun.net/5.htm.

I.A. Metwally et al. / Electric Power Systems Research 76 (2006) 778785 785

[4] ABB Switchgear, Silicone Rubber in Outdoor Insulators, first ed., ABBCatalogue Publ. SESWG/I 0000en, 1998.

[5] W. Shaowu, L. Xidong, C. Zixia, W. Xun, L. Zhi, Z. Yuanxiang,Y. Yu, W. Liming, G. Zhicheng, Hydrophobicity changing of sili-cone rubber insulators in service, Paper #15-305, CIGR E, Session2002.

[6] S.H. Kim, E.A. Cherney, R. Hackam, The Loss and Recovery ofHydrophobicity of RTV Silicone Rubber Insulator Coating, IEEE PES90 WM 024-0 PWRD, 1990.

[7] Tests on Indoor and Outdoor Post Insulators of Ceramic Material orGlass for Systems with Nominal Voltages Greater than 1000 V, IECStandards, IEC 60168, 2001.

[8] Insulation Co-ordinationPart 2: Application Guide, IEC Standards60071-2, 1996.

[9] Insulators for Overhead Lines with a Nominal Voltage Above1000 VPart 1: Ceramic or Glass Insulator Units for a.c. SystemsDefinitions, Test Methods and Acceptance Criteria, IEC Standards60383-1, 1993.

[10] High-Voltage Test Techniques. Part 1: General Definitions and TestRequirements, IEC Standards 60060-1, 1989.

[11] Composite Insulators for a.c. Overhead Lines with a Nominal VoltageGreater than 1000 VDefinitions, Test Methods and Acceptance Crite-ria, IEC Standards 61109, 1992.

[12] Artificial Pollution Tests On High-Voltage Insulators to be Used on a.c.Systems, IEC Standards 60507, 1991.

[13] F. Amarh, G.G. Karady, R. Sundararajan, Level crossing analysis ofleakage current envelope of polluted insulators, IEEE Power Eng. Rev.21 (August (8)) (2001) 4649.

[14] M.A.R.M. Fernando, J. Lambrecht, S.M. Gubanski, Modeling non-linearleakage currents on artificially polluted polymeric surfaces, in: Confer-ence on Electrical Insulation and Dielectric Phenomena, Annual Report,vol. 1, October 2528, 1998, pp. 5255.

[15] A. De La O, R.S. Gorur, J.T. Burnham, Laboratory wet tests on artifi-cially contaminated nonceramic and ceramic insulators, in: Tenth Inter-national Symposium on High Voltage Engineering, Montreal, Canada,August 2529, 1997.

[16] R.S. Gorur, A. De La O, H. El-Kishky, M. Chowdhary, H. Mukherjee,R. Sundaram, J.T. Burnham, Sudden flashover of nonceramic insula-tors in artificial contamination tests, IEEE Trans. Dielectrics ElectricalInsulation 4 (February (1)) (1997) 7987.

Performance improvement of 33kV line-post insulators in harsh environmentIntroductionAdding of silicone-rubber creepage extendersReplacement of existing porcelain by silicone rubber insulatorsTest results and discussionTest samplesStandard lightning impulse withstand and flashover testsStandard AC artificial pollution testsI-V characteristics with and without CEs under clean-fog conditionI-V characteristics under salt-fog conditionAC wet flashover voltage for medium, heavy and very heavy pollution levels

ConclusionsAcknowledgementsReferences