** Convener: W. BOECK - TU München, Hochspannungslehrstuhl - Arcisstr. 21 80290 München - Germany

LONG-TERM PERFORMANCE OF SF6 INSULATED SYSTEMSby

Task Force 15.03.07* of Working Group 15.03**

On behalf of Study Committee 15

1. INTRODUCTION

The application and experience of gas insulated substa-tions (GIS) has increased rapidly over the past few dec-ades. The long-term performance of the insulating systemof GIS is therefore of increasing interest for both manu-facturers and users. The question is whether there are anyageing processes that cause a restriction of the substationlifetime. Generally, ageing processes cannot be avoidedand any material is subject to ageing. However, for thepresent it is of interest to only consider critical ageingcaused by defects which result in an increase of the sta-tistical failure rate. This report deals with the long-termperformance of SF6 insulated systems including minordefects not revealed by quality assurance tests [1,2,3].

Fig.1:Bath tub curve of the failure rate versus timeA: Dominating teething failuresB: Random failuresC: Dominating aging failures

The statistical performance is usually described by thebath tub curve of the failure rate (Fig.1). Rapid local deg-radation processes at rare but severe defects are of lessinterest since these defects are usually eliminated by rou-tine or on-site tests and repairs during the initial operation

period of the substation. Hence the failure rate decreaseswith time. Therefore, such teething failures are of noinfluence on the long-term performance. Subsequently, ifthe failure rate is constant, the failures are considered tobe caused by random phenomena. As long as this failurerate is below an acceptable value, the insulation system isconsidered to be sufficiently reliable. An increase of thefailure rate indicates that ageing becomes critical.

It is important to consider such critical ageing processes.The critical failure mechanisms have to be clarified inorder to guarantee the correct material properties by suffi-cient production quality, and to determine adequate testmethods and maintenance management procedures. Allparts of the insulation system have to be considered. Inaddition to SF6 gas as the main insulation, there are manydifferent components made of epoxy resin and other ma-terials. The long-term performance of the gas, the bulk ofsolid insulating components and their surfaces is of sig-nificant importance. It has to be considered as a result ofthe relevant physical phenomena whether critical ageingcan happen at all or whether it can be excluded by normalroutine tests or diagnostics. Otherwise it is necessary toinvestigate the statistical long-term performance and tosecure it by special quality control.

The dielectric strength and long-term performance ofinsulating materials is mainly affected by physical andchemical defects and contamination of the pure insulationsubstances. Therefore, two regions of the insulation sys-tem of a GIS have to be considered separately.

A The gas space inclusive of the interface to its environs,i.e.the surfaces of solid insulating or conducting mate-rials (chapter 2).

*Members:BOECK (Convener), DIESSNER (DE), ENDO (JP), FESER(DE), GIBOULET (FR), GIRODET (FR), HALLIDAY (GB),KOLTUNOWICZ (IT), LI MING (SE), LUNDGAARD (NO),MEIJER (NL), MUFTI (UAE), NEUMANN (DE), PIETSCH(CH), PRIEUR (FR), RIECHERT (CH), RUTGERS (NL),SLOWIKOWSKA (PL)

A B C

a < 1 a = 1 a > 1

f

t

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

Session 2002© CIGRÉ15-301

2

B The internal bulk of solid insulating materials (chap-ter 3).

Almost all individual defects and contamination of eachof the above regions are restricted to their own locality.Only a few mutual interferences between both regionshave to be considered like the humidity absorption andrelease of solids by diffusion. Otherwise they can be con-sidered as completely separate insulation regions withdifferent types of defects and contamination as well asdifferent and independent long-term performances.

2. INSULATION PROPERTIES OF THE GAS ANDSURFACES OF SOLID MATERIALS

2.1 Moisture and SF6 decomposition products

The only intrinsic feature of the pure gas which has anessential influence on its dielectric strength is SF6 den-sity. It is therefore monitored and kept constant by a lowleakage rate.

New SF6 according to IEC 60376 has a permitted mois-ture content of not more than 15 mg/kg (122 ppmv). Inmany cases, moderately higher values can be permittedfor the filling gas (IEC 60480, Ed.2). Usually, after com-missioning the humidity of the gas will increase with time(some weeks) due to the release of moisture from thesurfaces of all types of solid material and the moisturefrom the bulk of the insulating material. Besides that thedew point may change faster with the temperature of thegas and the enclosure. Nevertheless for permitted mois-ture contents in accordance with IEC 60694 (dew pointtemperature not higher than - 5°C) there is no influenceon the insulation properties of the gas itself. Even thesurface of insulating components do not get dewy in theinstance of such gas humidity. This can only happen dur-ing a temperature drop below the dew point temperature.Since the dew point temperature is below 0°C the surfacecan only be frosted. Usually the temperature drop at themetal enclosure occurs first. Its large internal surface isfrosted, the gas dried and the insulator surface is not af-fected at all. Furthermore the use of absorbers is commonpractise to keep the moisture content at a low level and itis also normal for the gas humidity to be checked duringmaintenance procedures

SF6 has an extraordinarily high chemical stability. Fordissociation a quantum energy of 6 eV is required. Partialdischarges (PD) do not make a significant contribution todissociation, but arcing during normal switching opera-tions will contribute to a greater degree. Since the decom-position products may acidify insulation surfaces theyhave to be limited by absorbers. Circuit breakers andswitching compartments are provided with special ab-sorbers whereas insulation compartments are usuallyequipped with common absorbers. Absorbers in circuitbreakers are exchanged during routine maintenance work,whereas the other absorbers remain active for decades.

Therefore, under normal service conditions, moisture anddecomposition products of SF6 have no bearing on thelong-term performance of SF6 or the surfaces of solidinsulating components.

2.2 Mobile particles

Mobile particles belong to another category of defects.The movement of particles is stimulated by AC fields andthey can cause breakdown when they approach live partsduring their motion. Consequently mobile particles ofcritical size can reduce the AC Withstand Level (ACWL).They are regard as being the most frequent and deleteri-ous type of defects within the gas [1]. Under the influenceof the electric field they usually move into low field re-gions which act as natural particle traps and are storedthere. Stimulation of this motion is often promoted byconditioning procedures during testing without effectinglive parts. However, some particles may migrate fromthese natural traps during temporary AC overvoltages ordue to vibrations caused for example by circuit breakeroperations. Moreover, all of these are considered as ran-dom phenomena which may result in random failures butnot in ageing. Mobile particles may also be caused byabrasion within switching devices, but particles of a criti-cal size are only caused by malfunctioning switchingdevices. Large particles may also be caused during heavycircuit breaker (CB) operation, but they should not be of acritical nature due to the CB design. These particles willbe removed during repair or the obligatory maintenanceprocedures for such switching devices. In addition freeparticles can easily be detected by electrical or acousticdiagnostic methods [1] during testing or in service opera-tion due to their high signal emission. Therefore freeparticles do not contribute to critical ageing.

2.3 Fixed particles on live parts or insulator surfaces

A third category of defects are fixed particles or protru-sions at live parts. Such defects which are of critical sizemainly reduce the required Lightning Impulse WithstandLevel (LIWL). During the commissioning tests of GIS,80% of the rated LIWL has to be verified. According to aCIGRE recommendation [2] this can be achieved forexample by an AC voltage test with 36% of the ratedLIWL. Additionally the 5 pC PD inception voltage shallnot exceed 29% of the rated LIWL. For GIS ranging from123 kV up to 550 kV the phase to ground service voltageis between 14% and 20% of the rated LIWL [3]. There-fore the service stress will not exceed 48% to 68% of the5 pC PD inception voltage. For permissible uncriticalprotrusions which have not been excluded due to the testrequirements or service experience [3] either very low orno PD activity at all will exist in service. The size andshape of these protrusion and their insignificant influenceon the dielectric strength will not change with time. Alsoaccording to experience, if there are undetected criticalprotrusions with stronger PD activity they will usually bereduced in size with time and the dielectric strength willbe increased.

3

For these type of defects, breakdowns are typically causedby overvoltages. The associated failure rate depends onthe statistical occurrence of these overvoltages. Howeverthere is no critical ageing since the dielectric strength isroughly constant with time.

This category of defects also includes particles whichadhere to the surfaces of solid insulating materials. All ofthe related phenomena and consequences are the same asfor protrusions at live parts, but in addition, chemicaldegradation of the insulation material may occur due tolong-term PD activity. Therefore, it is of additional inter-est to consider whether such defects may cause progres-sive tracking and flashover. This and all further trackingproblems are considered in section 2.5.

2.4 Floating metal parts

Deformed or incorrectly installed stress shields andfloating components with poor electrical connectionsbelong to a further category of defects. For this type ofdefect there is either no PD at all or very severe PD activ-ity in case of floating components when they are periodi-cally discharged. If these discharges do not occur inhighly stressed areas the dielectric strength is not affected,as long as the amount of the resultant dissociation prod-ucts can be absorbed. In very rare instances incorrectlyinstalled parts may move due to vibrations or mechanicalimpacts for example circuit breaker operations. Thesesevere defects have to be eliminated by repair. Other suchlike or similar mechanical defects - for example looseningbolts etc - which may lead to failures are beyond thescope of the report.

2.5 Surface tracking on insulating materials

Tracking is the formation of a visible path across thesurface of solid insulating components like spacers. It istypically caused by test flashover. In severe cases it is apermanent conducting path usually of carbon, resultingfrom the interaction between surface discharges andspacer material. The tracking performance of the spacersdepends on the spacer material, number of flashovers andtheir arc energy.

The arc energy is mainly emitted as thermal radiation andcauses carbonization. The track is a partially conductingcarbonaceous path on the surface of the insulator. In anSF6 atmosphere however, the formation of free graphiticcarbon is restricted. In the presence of free fluorine, theformation of a C-F compound is more likely than theformation of free carbon on the surface [4]. In most casesthe carbon consists of isolated carbon particles. If thecarbon layer is thick enough to form a conductive path,the dielectric strength of the spacer decreases. Experiencehas shown that such carbonised tracks either have highconductivity or they have an insignificant effect on theinsulation properties.

A high-energy test flashover (500-2000 J/cm) can causeepoxy spacer damage with the result that both the surfaceresistance and the electrical strength are reduced. Inde-pendent of the materials, the specific erosion depth (ero-

sion depth/flashover) is on average 10 µm with a ten-dency to a lower erosion pattern in the case of Al2O3filled materials [5,6,7].

However, for modern insulating materials no reduction ofthe electrical strength occurs in the case of low-energyflashovers (<200 J/cm). The decomposition products inthe vicinity of the discharge track, surface charges orother small contamination particles on the spacer can beremoved, so that a new flashover does not follow theexisting surface tracks. Another possible reason for thatphenomenon is that electrode-protrusions, perhaps in theform of foreign particles, may melt or be expelled fromthe region of high field. This means that the low-energyflashover can be accepted in a conditioning procedure [5].

According to laboratory experiences where impulse testconditions and low energy flashovers prevail it has beenshown that the dielectric withstand is not affected by aspacer surface flashover [8]. Although, even in real GISlow energy test flashovers predominantly occur in thelower voltage ranges.

In Sweden [9] slight black tracks were observed on GISspacer surfaces due to flashover during the AC tests. Thespacers usually withstood the subsequent AC test andlightning impulse test voltages.These tracks did not causefailure of the GIS during the many years of operation ofthe substation.

According to [10], tracks have been found on spacersfollowing inspection of the GIS during maintenance pro-cedures after 10 to 20 years of service. These tracks ap-parently originated from self-restoring flashover duringon-site testing, however they have not impaired the serv-ice reliability of the GIS. In long duration tests on 420 kVepoxy spacers, two spacers with tracks from flashover dueto particle contamination (successful spark conditioning)survived under 550 kV without flashover for one year.

Fig.2: Typical example of surface "track" marks [11]

In another paper [11] it is also reported that low energytrack marks were found on in service GIS spacers duringnormal inspection. During an extensive internal examina-

4

tion of an eight circuit-breaker mesh connected 420 kVGIS installation, approximately 300 spacer insulator sur-faces were examined. Three insulators had carboniseddisruptive discharge marks across their surface. A typicalexample of surface "track" marks is shown Fig.2 [11].The marks are clearly the result of low energy dischargesand are characterised generally by a very thin white linebetween two carbonised paths. These tracks are not due to50 Hz AC on-site tests. A full-scale research program,which included many hundreds of disconnector switchingoperations, failed to reproduce the phenomena.

Such low energy track marks found on in-service GISspacers were initially attributed to fast transients gener-ated by switching operations. However it was shown laterthat the low energy flashovers occurred after openingoperations. The time delays range from a few minutes toseveral hours. Obviously they were caused by surfacecharge accumulation during the residual DC stress afteropening operations [12]. An investigation indicated thattrapped charges on the spacer surface play an importantpart in providing the initiating mechanism for the flash-over [13]. They are caused by the ion flow due to theresidual DC voltage of floating conductors after openingoperations.

Fig.3:Particle radially glued on epoxy insulator. Note discoloration around the particle tips [14].Right tip: 1mm above the insulator surfaceLeft tip: on the insulator surface

Particles adhering to spacers have not been found to causetracks on spacers under long-term AC stress, unless theparticles initiated flashover. Even an extremely long par-ticle of 16mm length radially fixed on an epoxy spacershowed no signs of tracking besides a slight discolorationof some mm length around both tips after a 10 000 h test(Fig.3) [14].

In general it can be concluded that tracking phenomenaremaining on surfaces of solid insulating componentsfollowing successful high voltage testing are of no influ-ence on the long-term performance of GIS.

3. INSULATION PROPERTIES OF THE BULKOF INSULATING SOLID MATERIALS

3.1 Physical Phenomena at defects

The main solid insulating materials are epoxy resin withinorganic fillers, fibre-reinforced epoxy and bushing in-sulation impregnated with epoxy, phenol resin or SF6.The two main types of defects for these materials arevoids and metallic inclusions [15].

Voids originate during casting processes as result of ab-normal shrinkage, evaporation of impurities or delamina-tion at electrode surfaces. In fibre-reinforced epoxy usedfor example for drive rods, mechanical stress may causedetachment at fibres [16]. Such voids contain remnantgases from the curing process and sometimes SF6. Theycan be detected by PD measurement if a sufficient testingtime and sensitivity can be realised and pulse shapeddischarges occur. Such discharges may lead to an in-creased surface conductivity, space charges and a tempo-ral PD extinction over a period of time. Even glow dis-charges occur which are pulseless and cannot be detectedby usual PD pulse detectors [17]. These phenomena donot usually occur in new material, consequently they haverarely been observed with new components during eitherquality or routine tests in factory, or commissioning testson-site. In factory test laboratories it is not a problem toachieve sufficient sensitivity to detect voids which areconsidered as critical. The sensitivity which can normallybe achieved during routine or on site tests is not usuallysufficient to detect critical voids in solid materials. Addi-tionally, metallic inclusions cannot be detected at all byPD measurement. Nevertheless, 30 years experience withcasting techniques has confirmed that quality control inthe factory can reliably eliminate such critical defects. Forthat purpose the long-term performance has to be consid-ered and investigated. Of significant importance is theevaluation of the statistical lifetime and to exclude criticalageing.

3.2 Approximation of the long-term performance bythe Weibull distribution

Modern statistical models to simulate the long-term per-formance of insulating systems are mostly based on theWeibull distribution [18]. In the case of solid insulatingmaterials the two parameter Weibull distribution is pre-dominantly applied [19]. It defines the cumulative failureprobability F in dependence on the field stress E and theoperation time T by

FE

ETT

m a

= − −

163 63

exp (1)

The shape factors m and a depend on the material and itsquality and for the reference values E63 and T63 follows acumulative failure probability of 63%. In practise F isprimarily presented in form of the Weibull plot.

5

W FF

( ) lg ln=−1

1 (2)

by the linear Weibull-Log function

W F mE

Ea

TT

( ) lg lg=

+

63 63(3)

For other chosen reference values E0 and T0 there will bea parallel shift of the straight Weibull-Log line.

W F mEE

aTT

const( ) lg lg=

+

+

0 0(4)

During the normal short time breakdown test the fieldstrength is increased within such a short time, that thesecond term of the breakdown probability function can beconsidered as constant and the linear Weibull function

W F mEE

const( ) lg=

+

0(5)

has to be applied as presented in Fig.4 for epoxy resin[20].

The tests are done in the usual manner with small materialsamples of some mm thickness. For filled epoxy resinwithout any prestress, m-values between 6 and 8 can becalculated from test results in [20,21,22]. In the case oflong-term prestresses of less than 12 kV/mm there was noinfluence on the shape factor m [20] (Fig.4). For higherprestresses between 15 and 20 kV/mm the shape factor mis reduced to values between 4 and 6 [20,21] (Fig.4).Evidently the material properties have changed with timedue to PD discharges or other phenomena at such highprestresses. Obviously, of considerable interest is thelong-term performance for the maximum working stressin a GIS or GIL, which is usually in the range of (2,5-5) kV/mm. According to these results no change of theshape factor m has to be expected under these serviceconditions.

3.3 The lifetime curve

For consideration of the long-term performance, the life-time curve is very efficient. It presents the breakdownfield strength E over the operation time T for a certainbreakdown probability F and can be derived fromEqu.(4). Usually it is presented as Log-Log plot for aconstant cumulative failure probability F by the linearfunction

lg lg ( )EE n

TT m

W F const0 0

1 1

= −

+ + (6)

The voltage endurance coefficient

nma

= (7)

can be determined by the gradient -1/n of the linearlydeclining lifetime curve.

If the components E0 and T0 of a point on the lifetimecurve are chosen as reference values, equation (7) be-comes

lg lgEE n

TT0 0

1

= −

(8a)

or

T TEE

n

=

−

00

(8b)

as it is usually applied. But these equations are only validfor a single lifetime curve of a certain breakdown prob-ability F. When life time curves with different failureprobabilities are considered equation (6) has to be used.

Lifetime curves are usually experimentally gained byseveral constant field long-term tests with differentstresses. For each stress it is performed with many test

Fig.4: Distribution of breakdown stress on unstressed (broken lines) and prestressed (unbroken lines) epoxyresin specimens [20]

6

components such as real spacers, post insulators or mate-rial samples to get the scatter of the time to breakdown.Results of a few publications are compiled in a commonlifetime curve in Fig.5 [23].

Fig.5: Lifetime curves of epoxy resin gained bycompiled published results

The resulting voltage endurance coefficient n ranges from10 to 20 with the median mainly being applied.

n = 15 (9)

The main advantage of such lifetime curves is, that theypermit us to estimate the statistical life expectancy. Dueto the obligatory high reliability of a complete GIS or GILan extremely low failure probability is demanded for theirsolid insulating components. In [22] for instance a cumu-lative breakdown probability of less than 0,01% after 50years is considered to be necessary for a post insulator inorder to guarantee sufficient reliability for a GIS. Fromthe experimentally determined lifetime curve 1 (Fig.6) fora failure probability according to the test procedure ofF1 = 10% the lifetime curve 2 for the demanded failureprobability F2 = 0,01% can be gained according toEqu.(6) by a vertical parallel shift of

EE

FF

m2

1

2

1

11

=−−

ln( )ln( )

(10)

By short time breakdown tests with thin material samplesthe shape factor m was found to be 8. Thus the lifetimecurve 2 in Fig.6 for the demanded failure probability F2 isachieved by a parallel shift of E2/E1 = 0,42. During thecomplete expected lifetime of 50 years it stays beyond themaximum service stress.

3.4 Failure probability of a total substation and an ex-tended GIL

The resulting cumulative failure probability FN of a wholeGIS with N installed post insulators can be calculated bythe enlargement law

F FNN= − −1 1 2( ) (11)

For instance a GIS with total of 100 insulators wouldhave a cumulative failure probability during the expectedlifetime of 50 years of 1% corresponding to a mean timebetween failures of 5000 years, which is considered as

adequate. Life time curve 2 (Fig.6) is valid for the wholeGIS with a failure probability of 1%. Besides the insula-tors already considered there are of course other solidinsulating components installed in GIS. However onlythose components which are stressed by the maximumelectrical field strength have to be considered. In thisexample these are the post insulators.

In the case of a very large GIS or a very long GIL with ahigh number N of insulators, the failure probability wouldbe greatly increased by Equ.(11). In such a case the threeparameter Weibull distribution has to be applied even forthe failure probability of solid insulating components as itis normal for gas insulation [30]. It has to be taken intoaccount that below a certain inception field stress EI nobreakdown can occur. This is approximated by the choiceof a sufficiently high shape factor m in the case of the twoparameter Weibull distribution. The failure probabilitygets very small for low field stresses but never zero. Bythe exchange of E by E - EI in the basic equation (1) andall further equations, it is considered that in the threeparameter Weibull distribution the failure probability isexactly zero below EI.

3.5 Prediction of critical ageing

Assessments with the lifetime curve are very instructiveand they give hints but no clear information whether criti-cal ageing happens within the considered lifetime. Anessential question has to be considered whether the failurestatistics indicates a critical ageing of the failure rate f

fa

TE

ETT

m a

=

−

63 63 63

1

(12)

It characterises the failure types (Fig.1). For a < 1 thefailure rate decreases with time and is considered to becaused by teething faults. For a = 1 the failure rate isconstant and is considered to be caused by random phe-nomena. For a > 1 the failure rate increases with time andis considered to be caused by critical ageing.Therefore the shapefactor a is very important for theevaluation of the long-term performance of the insulatingsystem [19]. It can be determined directly by a constantfield stress test. In this case only the second term of

Fig.6: Lifetime curves of post insulators [22]1: 10% band with 10% puncture probability2: 0,01% band with 0,01% puncture probability

7

Equ.(4) has to be considered since the first term remainsconstant.

W F aTT

const( ) lg=

+

0

(13)

Such tests have to be made with high stresses in order torealise acceptable testing periods. For field stresses be-tween 15 and 30 kV/mm a common value of aH = 0,9 wasfound for epoxy resin with filler (Fig.7)[20]. This indi-cates that all breakdowns are caused by teething or ran-dom faults but not by ageing.

Fig.7: Probability F (%) of time to breakdown ofepoxy resin [20]

Under real in service conditions the shape factor aL be-comes 0,4...0,53. It can be calculated by Equ.(7) and (9)with the shape factor m = 6...8 for unstressed epoxy resinor after prestresses of less than 12 kV/mm. Critical ageingis consequently excluded. Therefore, modern qualityepoxy resins with fillers guarantee a high reliability andan excellent long-term performance of solid insulatingcomponents without any critical ageing.

Such considerations are of course of interest for all ap-plied insulating materials. However the relevant data areonly available for epoxy impregnated paper as it has beenused for bushings in the past [31]. For stresses between 13and 16 kV/mm the shape factor aH has been found torange from 1,3 to 1,9. This proves that at such high fieldstresses the failure rate increases with time due to criticalageing. The voltage endurance coefficient has been foundto be nH = 8 for stresses beyond 9 kV/mm and nL = 19for stresses below 9 kV/mm. Evidently, at 9 kV/mm thereis a change of the long-term performance of this material.The normal working stress in service of 3 kV/mm is wellbelow this threshold value of 9 kV/mm. No shape factorsaL for such service stresses are published. However theshape factor m is assumed to be constant within the entireconsidered field stress range. Therefore the shape factoraL for service stresses can be calculated according toEqu.(7) by

ann

aLH

LH= (14)

and becomes aL = 0,55...0,80. These values are higherthan for filled epoxy resin but there is no increase of thefailure rate and critical ageing.Evidently the material has a lower threshold value forcritical ageing than filled epoxy. The bushing insulationconsists of a more complex composite material than ep-

oxy with the usual filler. Additional care is necessary toensure the obligatory quality. This has to be consideredfor all such complex materials as for example fibre-reinforced epoxy resin. Its long-term performance has notbeen reported as yet. Critical defects due to detachmentprocesses at the fibre surfaces have to be avoided by ade-quate measures, which are well developed [16]. Forbushings meanwhile improved insulating materials arepredominantly in use today.

4 SUMMARY AND CONCLUSION

The various parts of the insulation system that have beenconsidered which are manufactured in sound conditionaccording to the quality requirements do not reveal anyageing mechanisms which cause critical ageing. There-fore, overall the entire sound insulation system should notexhibit an increase of the failure rate with time.

This is supported for 123 kV GIS which have been in-stalled in Germany since 1967 [32]. Fig.8 presents thefailures by dielectric faults per 100 bays and year over theindividual operation time until failure occurrence. Thetemporary increase and higher values of the failure rate

after approximately 11 and 17 years were caused by somecomponents which have been exchanged. These compo-nents were overstressed and showed clear ageing phe-nomena after that time. Nevertheless besides that there isa general reduction of the failure rate with time, whichproves that all further failures were caused by teethingfaults during the first 6 years of operation. After 30 yearsof operation there is no general ageing which recognisa-bly affects the long-term performance. The lifetime canbe estimated to be much higher than 30 years. This is inline with the findings based on considerations about thecritical physical phenomena and investigations of thestatistical long-term performance.

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

age at failure occurrence

failu

res

/ 100

bay

s

failures / 100 bays

mean value

mean value of 3 years

Fig.8: Failure rate of 123 kV GIS in Germany due todielectric faults [32]

8

References

[1] CIGRE WG 15.03Effect of particles on GIS insulation and theevaluation of relevant diagnostic toolsCIGRE Report 15-103, 1994, Paris

[2] CIGRE JWG 33/23.12Insulation co-ordination of GIS: Return of experi-ence on-site tests and diagnostic TechniquesElectra No 176, 1998, pp. 67-97

[3] CIGRE WG 33/23.12Insulation co-ordination of GIS: Questions on theinfluence of on-site tests and dielectric diagnosticsCIGRE Report 23/33-03, 1992, Paris

[4] F.Y. Chu, J.M. BraunGas-insulated substation spacer surface degrada-tion analysisFinal Report of Project 1360-10, Ontario HydroResearch Division, 1986

[5] D. Darwanto, H.C.Kärner, R. SchwarzThe influence of test flashover on the insulationstrength of spacer surface in SF6 gas insulatedsubstations6th International Symposium on High Voltage En-gineering, No. 23.05, 1989, New Orleans,No 23.05

[6] A.J.L. Verhage, J. Tom, H.F.A. Verhaart,C.S. VosThe electrical conductivity of insulators afterflashover in SF65th International Symposium on High Voltage En-gineering, 1987, Braunschweig, No. 15.11

[7] H.F.A. Verhaart and A.J.L. VerhageInsulator flashover in SF6 gasKema Scientific & Technical Reports 6(9):pp.179-228,1988

[8] B. Gänger, J. VigreuxMay the GIS SF6 insulation be considered as self-restoring?Electra No. 75, March 1981, pp. 31-39

[9] Mats LeijonCommunication on GIS operation in Sweden,1997, 03-25

[10] A. Diessner, G.F. Luxa, W. Mosca, A. PiginiHigh voltage testing of SF6 insulated substationson siteCIGRE Report 33-06, 1986, Paris

[11] B.M. PryorTracking of in-service GIS barrier insulatorsIERE Workshop on Gas-insulated Substations,1990, Toronto, pp. 4.15-4.16

[12] C.X. Wang, C.J. Jones, B.F. Hampton, B.M. PryorThe Use of Novel Monitoring Techniques to In-vestigate the Distribution, Behaviour and Effect ofTrapped Charges in GasCIGRE Report 15-105, 1994, Paris

[13] C.X. WangPhysical model for surface charge supported flash-overGaseous Dielectrics VII, pp. 519-526, PlenumPress

[14] L.E. Lundgaard, B. Skyberg, A. Schei, A. DiessnerMethod and instrumentation for acoustic diagnosisof GISCIGRE Report 15-309, 2000, Paris

[15] J.M. Braun, N. Fujimoto, G.C. Stone, S.A. Boggs,G. Addis, K.-J. Diederich, A. Diessner, G. Luxa,A. GirodetLong-term reliability of cast epoxy insulators ingas insulated equipmentCIGRE Report 15/33-07, 1990, Paris

[16] A. KutilQualification of fibre-reinforced insulating materi-als based on PD analysisIEEE Trans. on Dielectrics and Isulation Vol. 5,No.4, 1998, pp. 603

[17] T. Tanaka, K. GotoAgeing of polymeric and composite materials– Aspects of interfacial performances in ageing –CIGRE SC15 Symposium: Service - Ageing ofmaterials in HV equipment, 1999, Sydney, No.5

[18] Y. Kako, E. Takahashi, H. Okubo, A. Kishi,T. OkamotoInfluence of minute particles on breakdown char-acteristics of insulating oilCIGRE Report 15-03, 1986, Paris

[19] W. BoeckIsolationsverhalten technischer IsolationssystemeETG-Fachberichte 49, 1993, p. 111

9

[20] M. Honda, A. Aoyagi, M. Koya, N. Kobayashi, M.TamuraV-t characteristic of epoxy resin insulation forsustained AC voltageIEEE Trans. on Power App. and Syst. vol. PAS-103, No.5, 1984, pp. 1017-1023

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