8/11/2019 00414064.pdf

http://slidepdf.com/reader/full/00414064pdf 1/4

Rocwdllya of the 4 th Intanatlor ConCnenEe n

R o p a b e s d Applicatioru of 131eltrtric MatenahJuly 34.19Y4 BnIbsIle Aumaha

ILZXKAGE URRENT MONlTORING OF OMposlTE

LONG ROD NSULAToRs

Brian Poka ner Colin Lee Don Hawker

Queensland Electricity CommissionLine Design Branch

Austrelia

A b s t m c i

The Queensland Electricity Commission (QEC)has hadover 15 years of field experience with various types ofcomposite insulators on 1 OkV to 275kV lines. Mostofthe insulators were first generetion composites installedin a variety of contaminated environments.In he h e a qcontaminated arcas. various forms of damage to thesheath and sheds were observed and a numberof

insulators flashed over. These failures have also beenexperienced in otherpans of he world.

To gain a better knowledge of the deterioration of thecomposite insulators over time and to evaluate thepollution performance of the various types of polymers,

the QEC developed a sensitive high voltage leakagecurrent monitor and set up a test programfor testingthese insulators.

In th test. a double circuit 275kV tower situatedbetween two cooling towers at the Swanbank PowerStation was chosen for trialing of the compositeinsulators The main polym er types, EthlenePropylene Diene Monomer (EPDM), Ethylene SiliconPolymer (ESP) and Silicon Rubber were tested overa I

month period.

This paper discusses the developmentof the leakagecurrent monitor and results from the monitoringprogram.

Intmduetion

The Queensland Electricity Commission has recentlydesigned high voltage275 kV transmission lines for usein urban and sensitive environmental areas of

Queensland. These lines incorporate compact designsusing modem composite suspension and line postinsulators of both Ethylene D iene Monom er EPDM) ndSilicon Rubber (SIR).

Improvements in material, design and manufacturingmethods for composite insulators have gained thema

reputation for high reliability in medium pollutedenvironments, but some c onflicting opinions remain asto the better polymer type for long term performance.

The QEC installed the first composite insulators in 1978and ha s been closely monitoring their performance sincethen. Following sev eral failures of the early generation

Composite insulators in salt and high polluted areas inAustralia and o verseas in the 1980 s. the QEC initiateda project to de velop a leaka ge cu rrent monitor. It s

primary purpose was to evaluate theshort and long termpollution performance of a number of composite andceramic insulator types at a test site in a highly pollutedm a .

The test site chosen was a double circuit 275 kV towersituated between cooling towers a t the Sw anbank PowerStation. This tower had previously experienced twopollution induced flashovers, one of them involving twocircuits. The tower also enabled analysisof pollulionperformance at a higher than usual voltage stressas mostpublished datahas been ca mc d out at lower transmissionvoltages of around 132 kV.

The t w o main types of polymer materials normally used

at transmission voltages. EPDM and SIR. have beenincluded in the evaluation. as wellas an EPDM ypeblended with silicon oils. Le akage curren t measurem entsbegan at the test site in O ctober 1992 with six samplesof insulators being monitored. Four more samples wereadded in February 1994.

Test S i t e

The tower chosen as the test site was o n a d ouble circuitsection of the 275kV transmission system supplyingQueensland s largest tourist centre.

Energised in 1974 and insulated with high creepagelength fog discs the tower experienced pollution inducedflashovers in 1 981 and 1988, the latter involving bothcircuits and causing catastrophic insulator failure andwidespread blackouts.

The tower is surrounded by four forced draft coolingtowers associated with the Swanbank B Power Stationand is continually engulfed in vapours containing highconcentrations of minerals and salts. Regular b umoffs

of coal slag piles also addto the pollution.

The site was considered preferable over coastal 132kValternatives because of it s proxim ity to the Q ECresearch centre. It also enabled the investigation toconcentrate on industrial type pollution at 275kV.

The majority of published research data addresses theeffects of sea salt pollution on 13 2kV insulators.Industrial pollution has higher levels of sulphates whichare more conductive than chloride based salts, and

therefore produces higher leakage currents. The largerfield stresses at 275kV are also expected to contribute togreater polymer degradation.

8/11/2019 00414064.pdf

http://slidepdf.com/reader/full/00414064pdf 2/4

Table I: Test Insulator Characteristics

No. Shed Shed Coupl Dry-arcMaterial Diam Length Length

( ) ( ) ( )

2

45

6789

IO

SiR-IEPDM-1EPDM-2

SiR-2SiR-3CeramicEPDM-3SiR-4SIR-5Ceramic

140 2490140 2490102 2535

134 2540134 2515254 2628I l l 2434

l l 2434125 2655225 2560

224722472128

22002092

2230223022002196

Test onfiguration

The test insulators are installed in bridging positionsonthe tension tower at each of the three crossam levels.A high quality ceramic disc insulator is installed on topof the composite insulatorst shunt the leakage currentto the Data Centr e via primary protection circuitry. Anearthing switch is also provided to shunt the ceramicdisc and enab le maintenance work to be carried out on

the Data Centre.

The Data Centre houses current transducers, an Analogto Digital converter, signal conditioning equipment anda Multiplexer.

The Multiplexer sends signals via a fibre-optic cable toa Command Centre mountedon the ground. Th is Centrehas a microcomputer w hich processes and stores the dataonto memory cards. The fibre-optic cable was selectedto shield electrical noise and for electrical isolation.Solar cells are installed at both locations to charge thepower supplies in each of the centres.

In addition to the leakage current monitor. a weatherstation has been installed adjacent to the CommandCentre for recording the following environmental data:

temperaturehumiditysolar radiationultra violet radiationbarometric pressurerainfall

Test Insirlotors

Table gives the characteristics of the comp osite andceramic insulators being tested. Insulator types1 to 6were installed in August 1992 and types 7 to IO inFebruary 1994. Insulator number 6 is a string of 18porcelain discs and insulator number O is a two pieceporcelain long rod. This paper presents the results fromthe first 6 samples . The results from the latter4

sam ples will be presented in a future pape r.

Leakag e No of DistLength Sheds Between

Sheds(mm) (mm)

61 1261125403

5255480177766100610064807800

4 94 956

3628I832324748

454540

6 071

1466 66 65045

Identical insulators were installed, but not energised.alongside the test insulators for the purpose of allowingperiodic analysis of the pollution.

As recommended by the manufacturers eachof thecomposite insulatorswere installed with grading rings toimprove the electric stress distribution at the live end.

eakage urrents

The amount of leakage current flowing along the surfaceof an insulator is dependent on many factors, the majorones being:

type of weathershed materialamount of pollution depositedthe hydrophobicity of the materialthe applied voltagethe environmental conditions

The hydrophobic performanceo the insulator will varybetween the different polymer family types (EPDMorSiR) and between differenttypes in each family group.Published research datafor both indoor and outdoor testsindicates that silicon rubber exhibits the better

hydrophobic properties in all pollution conditions.The major environmental con ditions affecting leakagecurrents are humidity and rain. On a new clean insulatorwhich has a high surface resistivity, the leakage currentis usually low and primarily capacitive. When pollutionbuilds up on the surface and the insulator becomeswetted by high humidity or rain, the resistivity reduc es,and the leakage currentsr i s e and become resistive.

The effects of the pollution on leakage current willdepend on the am ount of solub le and insoluble depositson the surface layer.

When soluble deposits become wet during rain, thesolution will lower the surface resistivity and causeleakage currentsto rise. Insoluble deposits do not per secause leakage c urrents to increase, but w ill assist inholding soluble deposits on the surface.

53

8/11/2019 00414064.pdf

http://slidepdf.com/reader/full/00414064pdf 3/4

Future QEC research will analyse the small cyclemagnitude and phase angle variatio ns for each insulatortype at varying degrees of pollution and age. It isexpected that each insulator will exhibit it s ow nsignature for various degrees of pollution and polymerdegradation.

Test Resul ts

eakage C u r r e n t s

Leakage currents are continuously monitored andrecorded as an integrated average over a20 minuteperiod. The 20 minute rainfall measuremen ts werenot made because the weather station only recordsrainfall on a daily basis. Rainy days were notedfrom Bureau of Meteorological data and from theweather station. It was found that rainfall has thelargest effect on leakage currents followed byhumidity.

A comparative performance over a 24 hour periodfor a SiR, an EPDM and ceramic disc insulator isshown in Figure 1 The SIR insulators have lowerleakage current than the EPD M, and both compositeinsulator types have cu rrents less than a third that ofthe porcelain string.

Figure 2 shows the long term trend of leakagecurrents over a 15 month period. It compares theporcelain disc with an EPDM and SiR insulator.The SiR insulators exhibit negligible variations inleakage current over time, whereas the EPDM andceramic materials show an upward trend, with thelatter showing the largest increase over time.

The first two months of recording coincided with adry period and leakage currents on the ceramicinsulator have increased w ith accum ulation ofpollutio n. After heavy rain in December andJanuary 1992 , the discs have been washed andleakage currents have reduced to their initial value.

llII

Figure 2

They began increasing again to double the initialvalue until a year later when the downward trendresumed during the wet season months of Decemb erand January. The washing after a year only causesa marginal reduction of leakage current as thepollution harden s on the sheds and remain s partiallyinsoluble. Also virtually no washing occurs on theundersides of the sheds which constitutesapproximately two thirds of the creepage length.

The pollution performance of the SiR compositeinsulators has been good with no signs ofdegrad ation of the polym er. Even with a variationin leakage lengths on the three SiR insulators from4801 to 61 12 mm, there were only sm all differencesbetween the leakage currents.

The leakage current on the EPDM compositeinsulators are up to two times higher than the SIRinsulators. The currents did however, drop to acomparable level after a prolonged period of heavyrain. The pollution performance of these insulatorsis still acceptable in these conditions.

The EPDM-2 insulator has shown some evidence ofchanges occurring to the polymer housing. Whitediscolouration is appearing on a number of sheds(towards the live end) with the apparent leechingou t of filler or silicon additive . This effect is notevident on the unenergised insulator hanging at thesame location.

The string of porcelain discs has performed poorlywhen compared with the composite insulators.

Figure 1

8/11/2019 00414064.pdf

http://slidepdf.com/reader/full/00414064pdf 4/4

Table 11: Analysis of Pollution

Polymer Chemical Compo und (pglcm’) Equiv.Ty Pe Salt

Sodium Potassium Magnesium Calcium Chloride Sulphate Density

EPDM-T 2.32 . I 5 0.49 0.81 2.93 2.7 14.13EPDM-U 4.81 .34 1.45 2.49 8.55 11.1 33.71

SiR-T 2.90 2.5 0.80 1.70 3.83 6 15 20.58SiR-U 5.28 .29 1.28 4.48 6.96 19.8 31.99

Note: EPDM-T denotes the top surface of the weathershed and EPDM-U, the undersides of the sheds.

It has the highest leakage length of the testedsamples, 7776”. but produces leakage curre nts 3times higher than SIR insulators during normalweather conditions and upto 7 times higher duringrainfall. The long term trends show an increase inleakage current with time. This is indicative ofpollution buildup on the discs as the leakagecurrents do not retum to their initial values afterheavy rain.

Pol lu t ion A nalysis

Two unenergised insulators (one EPDM and theother S in ) were recovered in February 1994 after aperiod of heavy rain. There were considerableamounts of wind-bome dust (of aluminium andsilica) which had to be removed before an analysisof the soluble pollutants coul d be made. Thisanalysis is given inTable 11 The results show theEPDM and SiR insulators attracting similar types ofpollutants with the main ones being, Sulphates andChlorides of Sodium, Potassium, Calcium andMagnesium.

The w ind-bome dust w as more difficultto removefrom the SIR insulator. The silicon material attractsand holds the non soluble dust (by encapsulatingwith it‘s oils), making it difficult to remove.

The equivalent salt deposit density was measured inthe range 14.13 pg/cm’ to 33.71 pg/cm’, whichisgenerally regarded as light pollution. This contrastswith what is known for porcelain dis cs on this towerwhich require washin g every 18 months. Clea rly,the composite insulators have a lower pollutionretention than the porcelain discs, and consequently,lower leakage currents.

The equivalent salt deposit density measurementsshow much higher levels of pollution on theundersides of the sheds. This is because theinsulators are hung in suspension and the topsurfaces are washed by rain. There is a similar levelof pollution on the undersides of both insulatortypes, but on the top surfaces, the silicon materialhas higher values. Despite this, the siliconmaterial has produced lower leakage currents, thus

supporting the manufacturers claim that the siliconoil continually migrates to the surface to maintainthe material’s hydrophobicity.

Environmental Effects

There has been no visible evidence of polymerdegradation from solar or U V adiation in thisperiod of analysis. There have been only smallvariations of temperature and barometric pressurewhich has not significantly affected the leakagecurrents.

Conclus ions

At this stage, the test results show the SiR family ofinsulators are performing better than the EPDM andceramic insulators. The initial valuesof leakage currentfor the SiR insulators were approximately half ofthoseor the EPDM and a quarter of those for ceramic

insulators and there isless variation in leakage currentsove r time. The low variations show that the SiRinsulators have consistently maintained hydrophobicity.

The EPDM insulators have provided mixed results.EPDM-I has performed reasonably well in theconditions, but there is some concern over theperformance of the EPDM-2 insulator under heavypollution and storm conditions. The insulator is showingsome evidence of degradation on the polymer housingwith white discolouration on a number of sheds towardsthe line end. This appears tobe due to the leeching outof the tri-alumina hydrate or the silicon oil.

The analysis of pollution from the shedsof unenergisedinsulators indicates that the composite insulators areretaining lower levels of surface pollution than theceramic discs. The ceramic disc has consequently notperformed as well asthe composite insulators. Leakagecurrents were at much higher lev els and have increasedwith time.

The difficulty with removing the dust from the SIRmaterial suggests the material is attracting and holdingthe pollution tothe surface. Thi s effect has not affectedthe material maintaining it’s hydroph obicity in the shortterm.

A cknowledgment

The authors wish to thank the QECfor permission to

publish this paper. The assistanceof the QEC NorthgateTest Section for developing the leakage current monitoris also acknowledged.

5