-
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:
[email protected] (I.A. Metwally),u.edu.om (A. Al-Maqrashi),
[email protected]), [email protected] (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
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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
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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
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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
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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.
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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
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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.
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I.A. Metwally et al. / Electric Power Systems Research 76 (2006)
778785 785
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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