Selection of composite insulators for AC overhead lines: implications from in-service experience and test-station results

A. J. Maxwell, I. Gutman, C. S. Engelbrecht, W. L. VoslooS. M. Berlijn & R. Hartings ESKOM, SAHVEC Insulator CentreSTRI (South Africa)(Sweden)

D. Loudon R. Lilja A. Eriksson Statnett SF Svenska Kraftnät Vattenfall Elnät Service(Norway) (Sweden) (Sweden)

21, rue d'Artois, F-75008 Parishttp://www.cigre.org

Session 2002© CIGRÉ33-402

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Abstract: The establishment of composite insulators onthe world market has lead to new opportunities for e.g.improved pollution performance and compact lines, andguidelines for the selection of these insulators areneeded. In this paper we present the results of visualinspections on in-service insulators and at three dedi-cated insulator test stations. It has been found that thesame types of damage and deterioration can be observedfor certain designs progressing at different rates in dif-ferent environments. Most of the changes observed overtime can be associated with design and/or manufactur-ing problems rather than material ageing; there is thus apotential for good composite insulator performance inmost areas if such problems can be avoided. A procedureto be followed when selecting and dimensioning compos-ite insulators is suggested.

1 Introduction

The list of potential advantages of compositeinsulators compared to traditional ceramic designs isimpressive. In particular a low weight-to-strength ratio,hydrophobicity leading to improved pollution perform-ance, resistance to vandalism, etc. For ceramic cap-and-pin insulators, a modular dimensioning procedure wherea creepage distance and profile are selected based on pol-lution performance has been developed (see e.g. IEC60815[1]), based on up to 100 years of service experi-ence. However, the strategy which should be adopted forthe selection of composite insulators is far less clear,largely due to uncertainties in their ageing performance.

In general, ageing of the polymer materials is expected todecide the insulator lifetime, and this may well be themost important issue when selecting such insulators.

M an y p a r a m et e r s e f f e c t a g e in g p h e n o m en ae.g. materials used, hydrophobicity, electric-field controland flange design, design of interfaces, profile. Few use-ful guidelines exist for the selection of composite insula-tors. Unfortunately, knowledge about the condition of in-service composite insulators is extremely difficult toobtain, as outages are required in order that inspectionsmay be performed. Test-station data, where regularinspections can be made is therefore important.

To establish dimensioning rules, a number of insulatorshave therefore been installed at three test stations in envi-ronments ranging from extremely clean to very polluted.In addition, in-service experience consisting of inspec-tion data from almost 300 composite insulators is availa-ble through the CIS (Composite Insulator Status)program, described in a previous CIGRÉ paper[3]. Inthat paper, the program was introduced and preliminaryconclusions were made. This paper deals with detailedinspection results from insulators installed in-service andat the test stations. Finally, the implications of this workfor the future selection of composite insulators are dis-cussed.

It is important to distinguish between pollution perform-ance and ageing, as illustrated in fig 1.

fig 1 An illustration of the emphasis of the present paper onageing of composite insulators and the complementingpaper on pollution performance of insulators.

C eram ic

Pollution A geing

C om posites

A ge in g

Em phasis ofSTR I paper

E m phasis ofthis paper

perform ance

C . Engelbrecht et. a l.

*STRI AB, Box 707, SE-771 80, Ludvika, Sweden.

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Both topics are covered separately by two complemen-tary papers in this years CIGRÉ session. This paper dealsonly with the issue of the selection and dimensioning ofinsulators to minimise ageing, and is based on serviceexperience and insulator test station results. The paper byEngelbrecht et. al.[2], deals with statistical dimensioningmethods to obtain an acceptable pollution performancebased on data from laboratory tests. Both issues must beconsidered to obtain an acceptable design for a pollutedenvironment.

2 Data collection

2.1. Method for data collection

The condition of all insulators covered in thisstudy, i.e. both for the in-service and test station insula-tors, has been established from visual inspections. Allinspections were carried out using the method standard-ised in the CIS inspection guide[4]. Briefly: all insulatorsare carefully inspected over their entire length with theline deenergised. Observations made during this inspec-tion are divided in two groups based on definitions in theinspection guide as follows:• Deterioration: Cosmetic or superficial ageing that

has occurred on the composite insulator as a directresult of exposure to the service environment, electri-cal stress, and mechanical loading. This ageing is notexpected to cause a significant reduction in the insu-lator performance and/or longevity.

• Damage: Changes to the composite insulator thathave occurred as a consequence or progression ofdeterioration and/or external influences. Damagesmay be expected to have a negative impact on theinsulator performance and/or longevity.

For the in-service insulators, visual inspections havebeen carried out within the CIS program by power net-work company personnel. Full details of the CIS pro-gram are given in the previous paper published in CIGRÉ2000[3]. At the three test stations included in this study,inspections were carried out by STRI personnel.

2.2. Climate and environment

The in-service insulators are installed in a widevariety of climates and environments. The commonest

climate types are temperate and subtropical1, for 65%and 24% of the obtained inspection data respectively. Allparticipating utilities are asked, in the inspectionguide[4], to estimate the pollution level of each sitewhere insulators are installed. This estimation is basedon the guidelines given in IEC 60815[1]. Many insula-tors are installed in environments with heavy and very-heavy pollution, i.e. pollution levels III and IV in IEC60815 have been quoted for 31% and 17% of the

inspected insulators repetitively. Some 45% of all theinsulators inspected are installed in coastal environ-ments, and 17% are installed in industrial environments.

The three test stations where composite insulators havebeen inspected are the following: Ludvika, at STRI’soffices in central Sweden; Dungeness, on the SE coast ofEngland; and Kelso, on the NE coast of South Africa.Details of the environmental, climatic and electricalstresses at these test stations are given in table 1.

table 1 Details of the test stations where inspections of insu-lators were carried out.

ESDD (Equivalent Salt Deposit Density) measurementshave been carried out on standard glass cap-and-pin insu-lators over several years at each test station; the results ofthese measurements are presented as a cumulative prob-ability distribution function in fig 2.

fig 2 Distribution of ESDD at the test stations in Ludvika,Dungeness and Kelso.1. Based on Koeppen classification[6]

Ludvika Dungeness Kelso

Location Central inland Sweden

SE coast of England

NE coast of South-Africa

Climate Cold Temperate Temperate

Environment Clean Marine Marine

Max. ESDD (mg/cm2)

0,005 0,15 0,28

Max. test (system) volt-age

110 kVa

(190 kV)

a. These insulators are 30% overstressed.

84 kV(145 kV)

58 kV(100 kV)

T(ºC) -27 to +27 -1 to +30 +6 to +30

Max. rainrate in 10 min period

12 mm 4 mm 26 mm

No of test objects (dura-tion)

31 (8 years)37(5 years)b

b. A total of 68 insulators were originally installed in 1993,and 37 were removed in 1998 after 5 years.

8(5 years)

12(4 years)

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0,0001 0,001 0,01 0,1 1Pollution Severity (ESDD; mg/cm2)

Pro

bab

ility

of

exce

din

g a

bsy

ssa

(p.u

) KelsoLudvikaDungeness

2.3. Insulators inspected

The in-service insulators for which damage anddeterioration are discussed in this report were manufac-tured between 1990 and 1996, with the exception of twoinsulators of an old modular design discussed in section3.1.4. which were manufactured earlier. The test stationinsulators were installed between 1993 and 1997, and thespecific results for these insulators are therefore valid forinsulators manufactured between 1992 and 1997. Noneof the insulators installed at the three test stations wereequipped with corona rings.

2.4. Frequency of inspections

For the in-service insulators, inspections havegenerally been carried out during scheduled outages. Thefrequency of the inspections has not been greater thanone per year. At the test stations, inspections were morefrequent. For Dungeness: 10 inspections were performedduring 5 years between 1996 and 2001. For Ludvika: atleast one inspection per year has been made during 8years, and some insulators have been inspected more fre-quently. For Kelso: 6 inspections have been made during3 years between 1997 and 2001.

3 Results of visual inspections

Results of the inspections of in-service compositeline insulators for which photographs were available, andfrom inspections at the three test stations describedabove, are presented here. It should be noted that the rel-atively large number of damaged insulators presenteddoes not reflect a statistical distribution of the rate ofdamage for insulators installed worldwide due to the fol-lowing:• For the in-service insulators, there is a tendency for

damaged and failed insulators to be reported morefrequently, as they obviously attract the attention ofpower network companies whose personnel decidewhich insulators to inspect.

• Two of the three test stations are in heavily pollutedcoastal environments, since one of the aims of theresearch was to develop design rules valid for severeenvironments.

For the description in this chapter of observations madeduring inspections, composite insulators have beendivided into three different components as follows:• The shank of the insulator i.e. the housing material

covering the fibreglass core• The sheds• The area around the end fitting connecting the fibre-

glass core and housing material to the flange

Observations made on the insulator shanks are describedin 3.1. No significant damage or deterioration hasoccurred on the insulator sheds, and this issue is there-fore not discussed further. Observations made in the areaaround the end fittings are described in section 3.2.

3.1. Observations made on insulator shank

The housing of the inspected insulators has beenmanufactured and attached to the fibreglass core usingseveral different design concepts, as illustrated in fig 3and described below:

fig 3 Different concepts used for manufacturing.

• In design concept 1, the sheds are vulcanized individ-ually onto a continuous extruded sheath covering thefibreglass core.

• In design concept 2, the polymer housing, completewith shank and sheds, is injection moulded onto thefibreglass core. An axial moulding line can be foundon such insulators. On some such insulators, a spacerring which may be visible from the outside is used toposition the core in the centre of the mould.

• In design concept 3, separately moulded shank/shedsections have been joined together without vulcanisa-tion using a sealant. This design can be identified bylifting a shed and looking underneath.

• In design concept 4, a non-vulcanised joining ring isused to connect longer shed/shank sections.

3.1.1. Erosion on the shank: design concept 1

At both of the coastal test stations, i.e. Dungenessand Kelso, erosion has occurred on the shanks of insula-tors manufactured using design concept 1, with a contin-uous sheath without moulding lines. This damage isshown in fig 4 from the heavily polluted marine test sta-tion at Kelso. For these insulators, identical flanges, pro-

Concept 1 Concept 2Separately

Extruded

core

Concept 3

core

moulded

sheath

Concept 4

core

Joiningring

Separatelymouldedsections

core

Moulded housing

(Spacer ringused on someinsulators)

sheds

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files, materials, and manufacturing methods were used,but the creepage distances and shed spacings vary. Sim-ilar results were obtained from the other coastal test sta-tion with heavy marine pollution in Dungeness with thedeepest erosion occurring for the design with the shortestshed separation and longest creepage distance (see fig 5).

These results seem to indicate that: the use of alonger creepage distance does not automatically protectthe insulators from erosion.

fig 4 Erosion damage at Kelso on insulators made withdesign concept 1. The insulators are identical apart fromshed spacings and creepage distances as indicated.The photographs were taken after 4 years of testing.

fig 5 Erosion on insulators made with design concept 1 atDungeness. The photographs were taken after 5 yearsof testing.

At the test site in Ludvika (clean, inland area), no sucherosion damage has occurred on insulators of this design.Neither has any such damage been reported within the in-service experience on identical insulator of differentvoltage classes installed in a variety of environments.

3.1.2. Damage and deterioration around the moulding line: design concept 2

Light erosion has been observed for insulatorsmanufactured using design concept 2, at both Kelso andDungeness (polluted coastal test stations), as seen in thephotographs shown in fig 6. These insulators have beenmanufactured using an injection moulding techniquewith an axial moulding line.

fig 6 Light erosion occurring between the sheds of insulatormade with design concept 2 installed at: (a) Kelso (pho-tographs after 4 years of testing), and (b) Dungeness(photograph taken after 4.5 years of testing).

On other identical insulators (of different voltageclasses), superficial splitting has been observed in themoulding line on an in-service insulator installed in apolluted industrial environment, and on all such insula-tors at the test station in Ludvika (clean, inland area).Such splitting is only approximately 1 mm deep and hasnot increased significantly in size with time when moni-tored in Ludvika, it is thus regarded as deteriorationrather than damage. It is situated at the same position (6large sheds from the top of the insulator) on 7 identicalinsulators installed in the Ludvika test site. This deterio-ration can probably thus be linked to the manufacturingprocess for this design.

fig 7 Splitting in the moulding line on insulators made withdesign concept 2 in (a) a polluted industrial environmentafter 5 years in service, and (b) in Ludvika (clean inlandarea) after 5 years of testing.

Other different insulators made using design con-cept 2 have exhibited splitting and light erosion in themoulding line. Such insulators are manufactured fromtwo different housing materials (i.e. one material perindividual insulator), with identical geometric designand manufacturing methodology; the materials arelabelled here as material 1 and material 2. In-serviceinsulators manufactured with housing material 1 haveexhibited splitting and light erosion along the mouldingline as shown in fig 8(a). The insulators were installed ina flat area some 10-20 km from the sea with pollutionlevel II according to IEC 60815. These insulators alsoexhibited splits perpendicular to the insulator axis notshown here.

(a)

SC

D =

18

mm

/kV

(b)

SCD

= 2

3 m

m/k

V

(c)

SCD

= 2

8 m

m/k

V

(a)

SCD

= 1

8 m

m/k

V

(b)

SCD

= 2

3 m

m/k

V

(b)(a)

(a) (b)

- 4 -

Other identical insulators (of a different voltage class)installed in Ludvika have also exhibited splitting in themoulding line as shown in fig 8(b).

fig 8 Splitting and light erosion in the moulding line: insulatorsmade using design concept 2 and material 1. (a) After 3years in service. (b) After 5 years of testing at Ludvika.

Insulators with identical design made from material 2exhibited light erosion in the moulding line at the coastaltest site in Dungeness as shown in fig 9.

fig 9 Insulator manufactured using design concept 2 withhousing material 2 installed at Dungeness (pollutedcoastal site). Photograph taken after its removal, show-ing light erosion traces in the moulding line. The insulatorwas tested for 5 years.

Based on these results, the moulding line appears to be apreferential area for erosion and splitting on such insula-tors. Similar deterioration occurs in different environ-ments.

On one insulator manufactured using design concept 2,installed in Ludvika, a split is present across the mould-ing line. This split is directly above a vent used to allowair to escape during the injection moulding process.According to the manufacturer, a knife has been used inorder to remove excess rubber after the mould wasremoved, and the cut has gone too deep on the surface.This split grew somewhat between 1998 and 2001 from 7mm to 9 mm.

fig 10 Split across the moulding line of insulator made withdesign concept 2, which has increased in length inlength and depth between 5 and 7 years of testing.

Similar splits across the insulator axis have also beenreported in the in-service data from two different pollu-tion environments.

3.1.3. Erosion around the spacer ring: design concept 2

For the insulators described above and shown infig 8 and fig 9, a spacer ring is used in the injectionmoulding manufacturing technique in order to hold thefibreglass core in the centre of the mould. Erosion hasoccurred on material 2 of this design at the pollutedcoastal test stations at Dungeness and Kelso, but not inthe clean, inland area at Ludvika. Erosion is concentratedaround the spacer ring, with some traces of erosion in themoulding line, as seen in fig 11.

fig 11 Insulator manufactured using design concept 2 withhousing material 2 installed at Dungeness. Exhibitingerosion around the spacer ring. Photograph taken sub-sequent to the insulators removal after 5 years of testing.

Insulators with an identical design made from material 1are installed in Ludvika, Kelso and Dungeness. Similardamage has occurred on these insulators in particulararound the spacer rings in the coastal sites (i.e. Kelso andDungeness).

fig 12 Erosion and burning of insulators around the spacer ringwith housing material 1, design concept 2 at (a) Dunge-ness after 5 years in service and (b) at Kelso, after 4years of testing.

(a) (b)

7mm9mm

(a) 5 years (b) 7 years

(a) (b)

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For identical insulators with housing material 1 installedat Ludvika, the cleanest of the test sites, no erosion orburning has occurred in the area of the spacer rings. Alocal colour change has occurred so that the ring hasbecome more visible as shown in fig 13. Insulators madewith material 2 of this design at Ludvika show no tracesof colour changes at the spacer ring.

fig 13 The spacer ring on insulators with design concept 2 andmaterial 1 has become more clearly visible as the testprogressed in Ludvika, although no burning or erosionhas occurred. Photograph taken after 5 years of testing.

In-service insulators manufactured using the samedesign, and made with material 1 have exhibited splitsaround the spacer ring as shown in fig 14. The insulatorswere installed in a flat area some 10-20 km from the seawith pollution level II according to IEC 60815.

fig 14 Splitting next to spacer ring on an insulator made withdesign concept 2 and material 2. The photographs weretaken after 3 years in service.

The deterioration and damage observed here is thusfound in several different environments and for differenthousing materials are used.

3.1.4. Splitting and failure of insulators made with de-sign concepts 3 and 4.

For insulators made with design concept 3, theinsulator is constructed by moulding a series of housingsections individually onto a continuous fibreglass core.Several of these insulators have, however, from in-serv-ice experience exhibited splitting between adjacent sec-tions, such a case is illustrated in fig 15(a) from aninsulator installed in 1982. In one case, such an insulatorinstalled in 1978 has failed after 21 years in service. Thisis presumably due to water ingression to the corebetween adjacent sections causing tracking and erosionof the rod leading eventually to mechanical failure. Insu-lators of this design are no longer manufactured.

fig 15 Insulators made with design concept 3, exhibiting (a)splitting between adjacent modules and (b) mechanicalfailure. Insulators installed in 1978 and 1982 respec-tively.The photographs were taken after 17 and 18 yearsin service respectively. Photograph (b) was taken shortlyafter the insulators removal.

Insulators manufactured using design concept 4,installed in Ludvika, have a rubber ring used to jointogether adjacent sections of the polymer housing.Rather than vulcanising such a seal, a sealant is usedbetween the polymer surfaces. These rings have startedto loosen. This can be clearly seen in fig 16. On one ofthese insulators, sealing grease has started to leak outfrom the seal. Such grease leakage has also been reportedfrom in-service experience from several environments.

fig 16 Splitting occurring at the sealing ring used on insulatorsmade with design concept 4. The insulator shown isinstalled in Ludvika in a clean inland area. The photo-graph was taken after 5 years of testing.

3.2. Observations made on area around insulator end fitting

There are several different types of end fittingdesigns amongst the insulators inspected. Four suchdesigns which will be discussed in this section are illus-trated in fig 18:1. A “cup-shaped” fitting is filled with housing polymer

to seal the housing to the flange2. A similar cup-shaped fitting is used, and a sealant is

placed between the polymer housing and the flange3. The polymer housing is moulded onto the flange, and

the first shed is immediately above this.4. A direct transition is used, whereby the bottom shed

is moulded over the flange.

(b)(a) (b)

Ringloosening

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fig 17 Various end fitting designs used for the composite insu-lators tested.

3.2.1. Peeling on insulators with end fitting design 1

Peeling has occurred on insulators with end fit-ting design 1, in two different environments, i.e. theclean, inland environment in Ludvika and a pollutedindustrial area. The seal has loosened from the high-volt-age flange as shown in fig 18. The seal is positioned inthe region with the highest electric field. In addition, theflange is designed in such a way that water can collect inthe “cup-shaped” fitting after rain, probably increasingthe duration of discharge activity. Once the seal hasfailed, the distance for water to penetrate to the fibreglasscore is relatively short as seen from the dissected insula-tor in fig 18(d). Dissection of the insulator did not revealany extra sealant between the bottom of the core andmetal flange.

fig 18 Peeling of rubber from flange on insulator with end fittingdesign 1: (a) from the HV and (b) from the ground flangeat the test site in Ludvika, and (c) on an in-service insu-lator in a temperate industrial area. In (d) an insulatorhas been dissected. The no. of years of testing for (a)and (b), and in service for (c) is indicated in the figure.

According to the observations in Ludvika, peel-ing on the HV (bottom) flange fig 18(a) has proceededconsiderably more rapidly than on the grounded (top)flange fig 18(b). This is probably due to the higher elec-tric field on this insulator which has not been equippedwith corona rings at 190 kV system voltage.

This insulator has been made with an extremely thin(approximately 1mm thick) polymer sheath, crazing anderosion is observed on the shank immediately above theend fitting such that core exposure might be expected tooccur after further testing. Core exposure has beenreported from in-service experience with other identicalinsulators, which have elsewhere failed due to brittlefracture in the area immediately above the end fitting[7];a thicker polymer sheath is thus essential.

End fitting design 2 in fig 17 is similar to end fittingdesign 1, although an additional sealant has been addedbetween the sheath and flange. No problems with thisseal have been found. However, the ability of the end fit-ting to collect water adding to discharge intensityremains, as for design 1. This may be the reason whydeep erosion is observed in fig 4(c) at Kelso, as thisdesign has the shortest flange-shed spacing, the electricfield here is higher than for the designs with shortercreepage distances.

3.2.2. Corona marks: insulators with end fitting design 3

On insulators with end fitting design 3 illustratedin fig 17, the distance between the bottom shed and thehigh-voltage flange is very short. This has resulted incorona marks on the bottom shed of these insulatorsinstalled in the clean, inland test site at Ludvika as illus-trated in fig 19.

.

fig 19 Corona burning marks observed on the bottom shed ofinsulators with end fitting design 3, installed in Ludvika.The photograph was taken after 5 years of testing.

Similar burning marks and loss of hydrophobicity werealso observed on other insulators installed in Ludvikawith a separation between the HV flange and lower shedof less than ca. 45 mm. None of the insulators installed inLudvika were equipped with corona rings, and it is clear

(1) (2)

(3) (4)

Sealant

(a) (b)

(c) (d)

8mm

3years 4 years

5 years

- 7 -

that these should be used at such voltage levels (equiva-lent to a system voltage of 190 kV) to avoid such corona-induced problems.

3.2.3. Deterioration around the moulding line: end fitting design 4

The need for corona rings is reinforced from dete-rioration observed on insulators with end fitting design 4in the clean, inland test site in Ludvika, where the mould-ing line has started to split, immediately above the HVflange on two identical insulators, one of which is shownin fig 20(a), but not further up the shank. In addition,some light erosion and deepening of the moulding linehas been observed between the first few sheds above theHV flange as shown in fig 20(b).

fig 20 On insulators with end fitting design 4, (a): splitting in themoulding line under the bottom shed and (b): light ero-sion and deepening in the moulding directly above theend fitting. Photographs taken after 7years of testing.

The mould line splitting in fig 20(a) can also beassociated with the lack of corona rings at this site, and isprobably due to corona from the flange. The deteriora-tion seen in fig 20(b) is likely due to corona from waterdrops and patches in this high field region, as no suchdeterioration is observed higher up the insulator.

4 Discussion of observed damage and dete-rioration

Several modes of damage and deterioration haveoccurred at the test site in Ludvika, despite the fact thatthe environment is very clean, and the leakage currentsare very low (less than 1mA). Such changes can clearlybe related to design and manufacturing problems ratherthan material ageing due to leakage currents, and indi-cates the importance of good design, quality control andhandling for composite insulators for all installationenvironments.

A summary of the time passing between observations ofdifferent changes which have occurred since the begin-ning of the test is shown in fig 21. It is clear that thesekind of damage and deterioration occurred fairly quicklyin this clean environment, and that in most cases all iden-tical insulators of a given design were effected after a

few years. This further strengthens the idea of thechanges being due to design or manufacturing issuesrather than material ageing.

fig 21 Deterioration and damage modes in the clean environ-ment at the test site in Ludvika showing how many insu-lators of each design are effected

As indicated in chapter 2, none of the insulators installedin Ludvika were equipped with corona rings. Much ofthe observed deterioration can be associated with coronafrom the flange or from water in the area close to the HVflange where the electric field is high. Grading rings areclearly essential at voltage levels over 190 kV. A con-servative approach would be to use corona rings for volt-ages over 130 kV for typical applications.

It is also clear that certain geometrical designs and man-ufacturing faults lead to the same type of damage at dif-ferent sites progressing at different rates, as can clearlybe seen for erosion of the insulator shank shown in fig 4and fig 5, and for the spacer rings on the insulators shownin fig 11 to fig 14. Furthermore, for identical designsusing different polymer materials, insulators deteriorateat different rates in several different environments. How-ever, the types of damage and deterioration are the same.

In heavily polluted coastal environments such as the teststations at Kelso and Dungeness, very high demands areput on the design of composite insulators. In clean areas,design weaknesses may only lead to deterioration such aslight erosion and minor splitting. However, in coastal

(a) (b)

1 2 3 4 5

Polymer joining ring on

Corona marks under bottom shed

Y ears in serv ice0

> 8 0 % o f in su la to rs

50% to 80%

< 50%

Polymer seal lifting fro m top f lang es

L EG E ND Splitting in moulding line

6 7

Splitting in moulding line

a ffecte d

affecte d

a ffec ted end fitt ing design 4 fig 20

end fit ting design 4 f ig 2 0

d esign c onc ep t 2 , f ig 7

S p lit acro ss m ou ld in g lineon des ign con cep t 2 fig 10

8

ab ov e H V flange

abo ve H V flan ge:L ig h t e ros ion in m ou ld in g line

en d f ittin g design 1, fig . 18 (b )

Polymer seal lifting from H V flan geend fit ting design 1, fig . 18 (a)

des ign con cep t 4 , fig 1 6 .

en d f ittin g d esig n 3 , fig 19 .

- 8 -

areas they may lead to serious damage when heavy pol-lution and leakage currents are present. Some clearexamples of such design weaknesses are:• the extremely short shed spacing with large sheds

used on some of the insulator shown in fig 4(c) andmade with design concept 1 (see fig 3) which causeda high level of erosion.

• the flange design on end fitting concept 1 and 2 (seefig 17) which can collect water in this sensitive area.

• the use of a spacer ring, if not adequately designedwith a reliable bond between the ring and the housing

• the presence of seals in high-field regions, as this putsextremely high demands on the quality of polymer-flange seals in particular

• inadequate bonding between adjacent rubber sectionsconnected together to make a longer insulator shank

• the use of polymer sheath on the fibreglass core lessthan 3 mm thick

Caution should be exercised when selecting and dimen-sioning insulators for such heavily-polluted areas whichshould be assessed individually. The use of longer insu-lators may e.g. be advisable in such areas to avoid rapid,excessive ageing.

If properly designed and manufactured, non-vulcanisedjoints may perform well. However, such joints are apotential risk, and one piece solutions are to be preferred.Spacer rings may also perform well if properly manufac-tured, however the results presented here illustrate a riskpresent with such a design. In principle, the use of com-ponents of different materials introduces more potentialrisks as additional electrical and/or residual mechanicalstresses may occur.

The presence of the moulding line appears to have con-centrated the erosion in this area. However it is not thecase that the moulding line in itself constitutes a weak-ness as erosion damage has occurred anyway on insula-tors with continuous sheaths. Good performance shouldbe possible for composite insulators in such extremecoastal areas if the design weaknesses described aboveare avoided, and material formulations with the bestresistance to erosion are used.

In addition, careful handling of composite insulatorsprior to mounting and during maintenance is important toavoid handling-induced damage. For this reason, a han-dling guide has recently been published by CIGRÉ[8].This guide also includes a number of photographs ofdamage caused by inadequate handling. Two cases ofdamage which can probably be traced to handling (notshown here as good quality photographs were not avail-able) have been reported within the in-service experiencecollected within the CIS program.

As indicated above, the majority of the changes observedon composite insulators can be attributed to manufactur-

ing or design problems, including the lack of coronarings at higher voltages, rather than material ageing. Fur-thermore, some of the damage reported in the in-serviceexperience can probably be related to careless handlingof the insulators prior to insulation or during mainte-nance. This indicates that most of the problems with com-posi te insu la tors are so lvable , and tha t goodperformance can be expected with improved quality con-trol and screening tests as well as careful handling.

5 Implications of results for selection and dimensioning

The fact that the majority of damage found is dueto manufacturing or design problems instead of materialageing, and that the same type of damage is observedoccurring at different rates depending on the severity ofthe site, has important implications for the selection ofinsulators. The focus on design and manufacturing issuesincreases the importance of reliable screening test andgood quality control. Our suggestion is to divide theprocess of composite insulator selection as follows.1. Preselection process:

• Give preference to insulators which do not haveany known design problems (see. 5.1.1.)

• Check performance in screening tests (5.1.2.);insulators that have not passed these tests shouldbe disregarded

• Ensure that the manufacturer’s quality control isadequate (5.1.3.).

2. Selection and dimensioning:• Follow selection and dimensioning rules to avoid

ageing in the relevant environment (5.2.).• Check the performance of the insulators in pollu-

tion tests (see Engelbrecht et. al.[2])

This chapter of the present paper will deal with the prese-lection process and selection and dimensioning of com-posite line insulators based on ageing results.

5.1. Preselection process

5.1.1. Give preference to insulators without known de-sign problems

Based on the evidence presented in this paper, thefollowing general design rules have been formulated sothat known in-service problems can be avoided:• The thickness of the housing should be at least 3 mm.• Grading rings should be used for voltages at and

above 130 kV.• Insulators with one continuous polymer housing

bonded to the core are to be preferred.• Insulators without extra components of different

materials added within the housing are to be pre-ferred.

• Avoid designs with very short distance betweensheds, i.e. the distance between adjacent sheds over

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the sheath should be greater than 25 mm for non alter-nating shed designs.

• Flange designs which do not catch and retain waterare to be preferred.

5.1.2. Check the performance in screening tests

Screening tracking and erosion tests are an impor-tant tool to be used to verify different insulator designsfor possible design and/or manufacturing process weak-nesses. The manufacturer normally has reports of all per-formed tests, and they can be easily obtained to check theperformance.

The IEC is presently looking into common screeningtests for polymer insulators. The aim of such tests is notto estimate the lifetime of the insulator, but to reveal anydesign or manufacturing weaknesses. This work coversonly AC applications. However, the use of the IEC61109 5000 hours test, but performed at a DC voltage,seems promising. Based on our experience[9], this testwas able to detect different design weaknesses, andamong them even some which were similar to thoseobserved in service and which have not been detected inthe AC 5000 hours tests, see example in fig 22.

fig 22 Spacer problems observed in long-term testing inDungeness and in the 5000 hours DC screening trackingand erosion test.

The absence of any current zeros in a DC voltage seemsto give an additional acceleration factor in the tests.Under AC, every discharge is extinguished at the currentzero, and it may not start again if the conditions havechanged slightly. Under DC voltage, the discharges aremuch more stable, increasing the stress. Although thisway of doing the tests is not included in the standards, itcould be used as an alternative.

5.1.3. Manufacturers quality control

Many of the observed deterioration/damageswere related to some manufacturing problems related tothe specific manufacturer, even though type-test reportsare available for such insulators. To overcome this prob-lem, it is suggested that power network companies check

that manufacturers have an appropriate quality system,including appropriate quality-control tests.

5.2. Selection and dimensioning process

5.2.1. For clean inland environments

For clean inland environments similar to Lud-vika, which are dominating worldwide, no pollution-related ageing problems are expected to occur. It wasfound that insulators with creepage distances as short as12 mm/kV performed well, and no lower limit for thecreepage distance was detected. Since the damage anddeterioration occurring was due to manufacturing and/ordesign problems, all of the materials commonly in use attransmission voltage levels are judged to be suitable forsuch areas, as long as the design problems discussed ear-lier are avoided, and the quality control is good. It isimportant that grading rings are used for insulators atvoltages above 130 kV for typical applications, to avoidcorona from the hardware and water induced corona onthe insulator surface. The length of the insulators canthen be selected from the usual lightning- or switching-impulse requirements.

5.2.2. Heavily polluted coastal environments

Good performance should also be possible in pol-luted environments which represent a small minority ofapplications, as long as the design problems discussedearlier are avoided, and quality control is good. How-ever, caution is advised for such areas which should beassessed from case to case; longer insulators may some-times be required to minimise ageing. For materials withthe ability to recover their hydrophobicity, a shortercreepage distance may be possible than for other poly-mer materials. The choice of the insulator creepage dis-tance and profile should be made based on appropriatepollution tests and analysis, as discussed in the comple-menting CIGRE paper[2].

6 Conclusions

Based on the results of visual inspections of com-posite insulators at long-term test stations and in-service,the following conclusions can be made:• The same types of damage can be found for the same

designs progressing at different rates in differentenvironments.

• Most of the damage and deterioration observed canbe associated to design and/or manufacturing prob-lems rather than material ageing.

• There is a potential for very good composite insulatorperformance in all areas if such problems can beavoided.

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The following procedure is suggested to power networkcompanies when composite insulators are to be selectedand dimensioned:

Preselection process:• Give preference to insulators without known

design problems • Check performance in screening tests• Ensure that manufacturers quality control is ade-

quate

Selection and dimensioning:• Follow selection and dimensioning rules to avoid

ageing in the relevant environment.• Check the insulators’ performance in pollution

tests, see the complimentary CIGRÉ paper[2].

7 References

[1] IEC 60815 “Guide for the selection of insulators inrespect of pollution environments”, 1986

[2] Engelbrecht C. S. et al. “Dimensioning of Insula-tors for salt pollution: Novel Procedures and a Lab-oratory Test Method”, CIGRÉ 2002, SC 33.

[3] A. J. Maxwell & R. Hartings, “Evaluation of opti-mum composite insulator design using serviceexperience and test station data from various pollu-tion environments.” CIGRÉ 2000, 33-203

[4] STRI Guide: “Composite Insulator Status Program:Field inspection of composite insulators”, Guide 3,98/1, 1998

[5] STRI Guide: “STRI Hydrophobicity ClassificationGuide 1”, Guide 1, 92/1, 1992

[6] “Brief Guide to Koeppen Climate ClassificationSystem”, www.fao.org/sd/eidirect/climate

[7] Munteanu et. al. “Experience and Applications ofNewest Generation of Insulators in the Network ofIsrael Electric.” INMR 2000 Barcelona

[8] CIGRÉ composite insulator handling guide: WG22.03, ELECTRA no. 195 - April 2001, p.51.

[9] Gutman, I.; Hartings, R.: “Single Stress and Multi-stress AC/DC Tracking and Erosion Tests for Com-posite Insulators”, Proceedings of the NordicInsulation Symposium, Copenhagen, Denmark,June 10-12, 1999, p.p. 271-278.

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