Acustic Insulation Analyser for Periodic Condition Assessment of Gas Insualted Substations

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Acoustic Insulation Analyzer for PeriodicCondition Assessment of Gas InsulatedSubstationsAsle Schei, Fellow, IEEE Stig Kyrkjeeide and Vegard Larsen

Allvoltages I

Abstrad A new instrument based on acoustic methods hasbeen designed which is offering non-invasive condition assessmentof Gas Insulated Substations GIs) during normal service. Theinstrument will be able to reveal many of the defects resulting inbreakdown of G Is and will in general have a significant costbenefit. Based on results from several years of field experience ithas been confirmed that the recordings generated by thisinstrument make it possible to detect, locate and recognise themost common defects such as moving particles, particles onspaeers, partial discharges from protrusions and loose shields.The detected signatures from the defects may also be used fo r riskassessment of the CIS.The paper describes the technical background of theinstrument and illustrates how acoustic signals from differenttypes of flaws are presented by and can be recognized from theinstrument. Thereby, the instrument can be used eficiently bymaintenance people without being experts in acoustics.Index Terms-Acoustic, condition assessment, diagnosis,discharge, failure, GIS, insulation, maintenance, monitoring,SF6.

I. INTRODUCTIONIS are often located in important nodes in the grid and a tG ower stations. Therefore the deman ds on availability are

high. To som e extent redundancy is built into the GIS. IEC 71specifies a target rate for availability of 0.1 failure per 100 bayyears. Failure rates of GIS are higher than this (Table 1).

0.9

TABLE 1.FAILURE RATES OF CIS.

125-145

300 3.4420 I 2.2 I 1.8550 I 3.9

Th e general trend is that the failure rate become s lower withtime as mor5 mature designs are introduced. According toNorwegian experience the mean time to repair is in the orderof 14 days, but may be m uch longer. The time to repair andthe outage time after a flashover depends of course on theavailability of spare parts. Typically, the cost of a repair in a420 kV GIS after a flashover will be in the order of 100 300thousand US$. In Norway the typical GIS is of 5 bays. On thisvoltage level, where the failure rate is 2 failures per 100 bayyear, the failure probability pe r year for a 5 bay GIS will be 10. On average the yearly cost will therefore be 20000 US$.The value of non-produced or non-delivered energy is notincluded, but may be more important than the costs of therepair alone. Cigre [I] estimates that about 55 of the defectsleading to breakdow n may be detected by suitable diagnostics.This means that the potential savings by succeeding with acondition based maintenance plan is 10000 US$ per year plusthe value of avoided production or supply losses. In Norwayunplanned supply losses to industry have to be valuedaccordingto a social-economic price o f 5.5 US$ per kWh.Defects leading to breakdown may either be introducedduring manufacturing, erection or operation, e.g. particles

from fast earthing switches. In Norway there is goodexperience with acoustic measurement of the dielectricintegrity of GIS, which in most cases can be directly re lated tothe performance. Such measurem ents are sensitive to amajority of the relevant defects. Based on an annu al conditioncontrol the costs of a maintenance plan based on conditionmonitoring may be estimated. Usually one 5 bay substationmay be thoroughly checked within one day, resulting inlabour cost of about 400 US$. A correction of a severe defectwill cost less than repair after a breakdown. An estimate isabout 15000US$ per corrective action on the 420 kV level. If50 of the potential failures, of which there is a failureprobability of IO , may be detected, the average annuallyexpenditures on corrective actions will be 750 US$. Thecondition monitoring and corrective actions will add up to1150 US$ per year as an average. The annual cost of aninstrument written off over 5 years is about 10000 US$. Incase of a single GIS the annual cost for the maintenance willadd up to about I1000 US$ (i.e. break-even).

A. Schei, S Kyrkjeeide and V. h e n re all wi th TransiNor As 037Tmndheh, N o w a y (e-mail: [email protected],stig.kyrkjeeide@bansinarnond [email protected]).

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11. ACOUSTIC ONITORING OF THE INSllLATlON SYSTEMAcoustic monitoring can be performed during service withexternally mounted sensors and a portable instrument. It is anon-invasive self supported technique [2,3]. Experience showsthat the technique offers several benefits:. ood sensitivity to detection of most defect fypes.Immunity to external noise.Defects may be localized.Defects may be recog nized.Risk assessment based on source characterization.The signals from the defects may vary widely fromcontinuous signals from internal corona to pulse shapedsignals from for example moving particles. There are only alimited number of parameters to describe such a signal:Continuous signals:Peak value over one power cycle- rms value over one power cycleDegree of modulation with the power cycle (50/60 Hz)Degree of modulation with twice the power frequency

Pulse shaped signals:- Peak value of impulse signal.Phase angle of pulse occurrence.Time since last pulse.

(100/120Hz)

In addition filters are useful both for measuring and forevaluation of results. Setting of threshold and time gating fordiscrimination of echoes in pulse shaped signals are alsoessential features for achieving goo d results.A portable instrument that supports all these features hasbeen designed (Figure I and used with good experience byboth suppliers and users of GIs in several countries. Theinstrument can either be operated in continuous mode or in

pulse mode. The instrument is also equipped with aloudspeaker that allows a skilled technician to immediatelyobserve and recognize signals. The instrument performs ameaurement automatically. Afterwards, results andinstrument settings can be downloaded to a PC for furtheranalyses and transfer to a database. The instrument may bepowered either from the mains or from an internalrechargeable battery offering four hours of continuousoperation.

Fig. I coustic Insulation Analyzer (AIA) wi th acoustic emission sensor.

111. ACOUSTICROPERTIES OF GIsThe shape of the detected signal will depend on the type ofsource, the propagation path of the signal and the sensor. Thesound sources are gen erally wide banded (partial discharges inthe range of 10-100 W z articles up to several MHz) [4].

When a signal propagating on the enclosure hits adiscontinuity (i.e. flange), it will partly be reflected and partlytransmitted. Because the materials used in most enclosureshave a very low absorption, a signal will ring for long timedue the mu ltiple reflections for instance from flanges.In the SF,-gas the signals propagate with a speed of about140 d s . The gas acts as a low-pass filter [ 5 ] When the signalhits a surfacelen closure, only a fraction of the energy istransmitted into the enclosure. The coupling between gas andenclosure im proves with increased gas pressure.Some defects (e.g. particles, corona at enclosure surface)act as point sources directly on the en closure. Defects close tothe high voltage conductor (e.g. corona from sharp points),however, first excite a pressure wave in the gas, which thenexcites the enclosure before it finally is picked up by theacoustic emission sensor.

w EFECTSN THE INSULATION SYSTEMDefects in the insulation system of GIS may be left after theproduction and erection and may also be produced duringoperation (e.g particles produced by fast earthing switches).The most im portant defects [ 5 ] are shown in Figure 2.

Fig. 2. The most important defects in a GIS nsulation systemA. Protrusions on earth and live partsA protrusion from live or earth parts will create a local fieldenhancem ent. Such defects will have little influence on the acwithstand level, because the voltage varies slowly and coronaat the tip will have time to build up a space charge that shieldsthe tip. For impulses like lightning surges or very fasttransients produced by disconnector operation, there is notenough time to build up such space charges. Consequently,the lightning impulse withstand level will be heavily reduced.

Usually probusions exceeding 1-2 mm are considered harmful[6,71.B. Partic1es:free m o v i n g andfixed to spacers

Free mo ving particles have little impact on the LIWL, whilethe ac withstand level can be significantly reduced from theirpresence. The reduction will depend on their shape andposition; the longer they are and the closer they get to theHV-conductor the more dangerous they become. If they move

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on to a spacer they become even more dangerous. A particleon a spacer may with time lead to deterioration of the spacersurface.C Voids and defects in spacerselectrical trees and eventually lead to breakdown.D Elecirically and mechanically loose shields

If a field grading shield becomes mechanically loose it mayin the end become electrical floating. A floating shieldadjacent to an electrode will give rise to large dischargesbetween shield and electrode.If these defects are activated (e.g. discharging), thelightning impulse withstand level will be heavily reduced.Moreover, a m echanically loose shield may fall off and createa flashover.

A defect within a spacer will give rise to discharges,

v. ACOUSTIC SIGNATURES A N D RISK ASSESSMENTThe parameters detected in continuous and pulse modetogether with the loudspeaker signal give clear indications o fthe type o f flaw. Furth er details concerning the various defectsare explained in the following.

A. Free moving pariiclesParticles will jump around when the electric field is strongenough so that coulomb forces exceed gravity. Each time theparticle hits the enclosure it emits a wide banded transientacoustic pulse which travels back and forth on the section it iscontained within. The sensitivity is sufficient to detectsub-millimetre particles with a good signal to noise ratio. Thesignals are easily recognized by large amplitudes, a high crestfactor (amplitude-to-rms ratio) and a large scatter in the peakvalue in continuous mode m easuremen t. Th e signals are pulse

type signals, and if the instrument is switched to pulse modeand the source really is a particle, the patterns like thoseshown in Figure 3will be observed [8][9]. The 50/100 Hzcorrelation is weak (F igure 4).LEVEL I INTERVALf V

I

Caln : 3 200 m r

1500 mv LEVEL SYNC.

90 inn no 380Fig. 4. Acoustic amplitudes versus phase angle tiom 1000 impactsenclosure.

Gal : 3 DW,the CIS

From these patterns a lot about the particle characteristicsand behaviour can be inferred. The dangerous particles arethose which are elongated and give high jumps. Of course, thedesign of the GIS (i.e. insulation distances and possibility forthe particle to arrive at a spacer) should also be considered.Not all moving particles in a GIS are dangerous. This is alsoshown by our return o f experience, where substations withmultiple particles has operated safely for years. However, alarge percentage o f major failures is cau sed by particles. It istherefore very essential to have good methods for riskassessment in case a particle is revealed, i.e. decide aboutparticle size, movem ent and location.The recorded patterns are tightly connected lo the particlecharacteristics and its behaviour. From the particle patterns (asthe one in Figure 3 it can be possible to estimate the particlecharacteristics such as the length of the particle, its mass andits elevation height [IS]. In the phase plot (Figure 4)symmetrical patterns in the two half cycles will appear if theparticle keeps its charge during flight, while an asymmetrywill appear if the particle looses charge (i.e. discharges)during flight.When measuring on a GIS, the raw signal contains both thedirect incident wave at the senso r and the m ultiple reflections.Only the directly incident signal should if possible bemeasured as this signal will not be polluted by the geometryof the enclosure. This can be done by gating, i.e. choosingappropriate live- and dead-times. Particle movement may beinitiated either by a high voltage or a mechanical shock (e.g.circuit breaker operation). During measurement a hammer tapmay b e applied to activate the particle in order to detect it.

Compared to conventional partial discharge (PD) detectionthe acoustic method is very sensitive as can be seen fromFigure 5. For the conventional method ( E C 270 [14]) asensitivity of 5 pC is considered good, and will only beachievable in a screened set-up. From th e figure it is seen thatwith such a sensitivity it will be difficult to detect 5 nunoarticles (and smaller) with the conventional method. For theFig. 3. Acoustic amplihldes versus elevation time fmm 1000 impacts to theCIS enclosure..acoustic system, however, the signal-to-noise ratio is verylarge even for 2 mm particles.

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a. 1 0 100A wmm a w r m t h mWlFig. 5. Acoustic sensitivity for detection of free, bouncing particlescompared to conventional partial discharge measwments.

E ProtrusionsA prohusion will create a field enhancement. If the ac fieldexceeds a certain level, a discharge will first occur at thenegative peak. When further raising the voltage, the num berof discharges will increase and large discharges will occuralso at the positive peak. These discharges create pressurewaves that generate a continuou s acoustic signal with a 50 Hzmodulated envelope.Figure 6 a) shows how the acoustic signal level varies withthe peak level of discharges in a 300 kV CIS. The fact thatmuch o f the energy in the waves stays within the section withthe defect, results in a drop in the signal level once a flange iscrossed asshown in Figure 6b).

~ ~~~. tb zd JO o w m z mwu 1w q wnaton Ern,

a) bFig.6. a) Acoustic pulse amplitudes as a function of maximum electric(apparent) discbarge levels in a 300 kV CIS 121, b dependence of sensorlocation on acoustic pulse ampliNdes 21.Even if no good method presently is available forcalibrating the acoustic method for corona type signals,several independent investigations using different types ofenclosures demo nstrate sensitivities in the 2 5 pC range [IO]-

[12]. The standards (IEC 517) set a limit for acceptance of 10pC at 110 of system voltage, while Cigre SC 33 advises alevel of 5 pC at 80% of test voltage [ I ] . As explained, theacoustic method will in most cases have a sensitivity betterthan this. However, for an acoustically detected protrusionwith corona, it can be hard to m ake a ml judgement of therisk based on limits for the electrically measured PD level asgiven in the standards, due to uncertainties in the connection

between acoustic signal level and PD level. This far, it is onlypossible to give an estimate of the discharge level from themeasured a coustic signal. How ever, the position may be takenthat there should be no corona within the CIS at normaloperating voltage, as a protrusion will reduce the flashovervoltage for steep transients and over time decompose the gasand create products which may be harmful to insulatorsurfaces.If the discharge source is located at the ground side, theabsorption of the high frequency signals will be small as thepropagation path through the gas is short. C onsequently, thefrequency content of the detected signals will be shiftedupwards compared to the case where the source is furtheraway from the enclosure like from corona at the centreconductor. This makes it possible to decide whether the sourceis on the high voltage or ground side. If the signal disappearswhen a high-pass filter is applied, it will be on the highvoltage side, and v ice versa.To get a corona discharge from the ground side of a CIS along and sharp particle/prohusion has to exist. A metallicobject resting on a painted interior surface will act as afloating metallic object and create discharges towards thepainted surface at the rising flanks of the voltag e.C Floating shields

Sometimes field grading shields will become loose and startto vibrate. They may also loose the electrical contact andbecome electrically floating. A large floating metallic objectwill be capacitively charged, and when the w ithstand voltagebetween the object and its base is exceeded a largedischarge/arc will occur. Such discharges occur at the risingflanks of the voltage, and will produce a large continuoussignal with mainly a 100 Hz envelope. The signal level willusually be stable and have a low crest-factor.D. Voidr and defects in spacers

V o i d s a n d defects ns ide spacers w i l l create d i s c h a t g e s oncethe initiation voltage is exceeded. Usually such voids arefound during the quality control in the factory. Because thesound absorption in filled epoxy is very high, the chance todetect them with acoustic measurement is small.Measurements indicate a sensitivity of some hundreds of pCfor a discharge occurring inside the spacer close to the centreconductor.E. Particles on spacers

A particle that moves on to a spacer may behave in manyways. It may move around on the spacer where it maydischarge and dep lete charges. This is particularly relevant forhorizontal spacers (e.g. located at the bottom of a riser). Itmay also become fixed to the spacer and create dischargestowards the spacer surface. The spacer surface is not a selfhealing insulation material and may during time bedeteriorated (e.g. carbonized ) and eventually break down.The knowledge of acoustic signals generated from particleson spacer surfaces is relatively limited. Signals frommechanical impacts may occur and because they are veryenergetic they may be detected at the outside of the en cl os m

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even if the absorp tion in the spacer is high. The particles canalso generate sound waves in the g as that propagate like thosefrom a protrusion. If the particle is located at the inside of aconical spacer, the spacer may act as a barrier for the pressurewave and reduce sensitivity.Some early studies showed that signals from large particleson spacers could he detected. At low voltage levels smalldischarges (in the pC range) occurred regularly on the risingflank of the voltage. No certain acoustic signals could be seenfrom these discharges. At higher voltage levels very largedischarges occurred occasionally at uneven intervals up tosome 20 econds. These signals could he detected acousticallyand had a clear correlation to the power cycle.Some investigations have been made at CESI [l3]. Theyreport that discharges produced by metallic particles andcarbonised tracks located at the outside of conical spacers fora 420 kV GIS could be easily detected. A recent study atSINTEF Energy Research states that the sensitivity also forsmall partial discharges due to metallic particles on spacers isreasonable [16]. In a 300 kV test set-up the sensitivity wasfound to be some 2 pC for pa dc le s located at the outside theconical spacer and some 5 pC for those located inside thecone.

W IELD EXPERIENCEThe Acoustic Insulation Analyzer shown in Figure 1 hasbeen used since 1996 with good results both by m anufacturersof GIs, utilities, industry companies and universitieshsearchinstitutes. The acoustic method has found several ways ofapplication on GIs:In factory tests.As a stand-alone tool for checking the GIS insulationsystem during normal operation, for instance- on a per iodic basis.

- to follow UD alreadv detected flaws.

3 3 1

90 4 8 0 oPhse -le [degmes]

Fig. 1 Partial discharges due to a protrusiodscratch on the enclosure of a275 kV busbar detected during pretesting before commissioning.Figure 8 shows the acoustic signature due to an electricallyfloating shield in a 500 kV disconnector. The utility hadearlier experienced a flashover in a disconnector of the samemake and wanted to make sure that the other disconnectorswere in good condition. The signature shows relatively highpulse amplitudes (indicating large partial discharges) with adistinct 100Hz nvelope modulation, which are characteristicfor an electrically floating shield. The disconnector was

opened for insp ection and one floating shield was found. Afterreplacement of a new shield, the acoustic signal level wasclose to the acoustic background noise level (wh ich was about1 mv).

Phase a g l c [degrees]Fig. 8. Elechically floating shield in a 500 kV diswanector measured duringnormal service.

during pre-testing before comm issioning.- during com missioning. An example of a mechanical vibration in a 275 kV circuitbreaker is shown in Figure 9.A ro utine test was performed onthe breakers, and an acoustic signal deviating significantlyfrom the background noise was detected in one of the phasesof a circuit breaker. The signature shows a very distinct 100Hz enveloue modulation with oronounced vertical lines and

pn'or to revisions to give guidelinesfor inspection.after revision as a quality after inspection,. s a complimentary ool for continuous uHF.monitoringin order to pinpoint and characterize the flaw [17].Some typical examples of acoustic signatures from differenttypes of flaws detected during normal service are shown anddiscussed in the following.Figure 7 shows the acoustic signature from a partialdischarge due to a scratch (protrusion) on he enclosure of a

275 kV husbar. The protrusion is recognized by itspredominant and stable 50 Hz modulated envelope and therather l ow acoustic amplitude and l ow ratio betweenamplitude and rms-value. The partial discharge was detectedduring pretes ting before commissioning, and the scratch wasco nf m ed by inspection.

symmetry in the two half cycles. This indicates that a ~ t u r a lfrequency of the breaker has been excited and thereby that amechanical problem m a y he present. Since the utility earlierhad experienced that a bearing in a circuit breaker of the sam emake had collapsed, they decided to open the breaker forinspection. A shield was found to be loose and in closemechanical contact with another metallic part of the breaker.When the shield was replaced into its original position, theacoustic signal disappeared.

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90 180 zm 360Phase angle [degrees]

Fig. 9. Mechanical vibration of a loose shield in a 275 kV circuit breakermeasuredduringnormalservice.

In many cases inductive potential transformers (with ironcore) have been found to generate acoustic noise m odulated to100 Hz sually with low amplitudes. Similar signals may bedetected also on single-phase steel encapsulated GIS. In thelatter case, the noise increases with the load current. In bothcases the noise is usually due to magnetostriction in the steeland is thereby not related t the GIS insulation system. Theoperator of the acoustic system should keep this in mindduring measurements.

VII. CONCLUSIONDuring several years of field experience, the acousticmethod has proven to be a sensitive and reliable tool forcondition assessment of the GIS insulation system. Themethod is non-invasive, and the developed system makes iteasy to pinpoint and identify the most comm on types of flawssuch asmoving particles and partial discharges during normalservice. Potential week points found during service may bechecked more often and repaired in time to reduce the risk offailure.

[ I I] A.Bargagia, W.Koltunowicm, A.Pigini, Detection of Partial Dischargesin gas Insulated Switchgear IEEE Trans Pa.Del., Vol . 7 No. 3, July1992, pp 1239.1249.[I21 H.-D. Schlemper, R.Kurrer, K.Ferer. Sensitivity of &-Site PartialDischarge Detection in GIS. 8th ISH Conference, Paper no. 66-04,Yokohama. 1993.[ 31 W.Kolhm owicz. Personal commun ication.(141 IEC 270 Partial disch arge m e a w e m e n t s .[IS] L. E. Lundgaard, Paltieles in GIS. Characterization from AcousticSignatures , IEEE Trans. Dielectrics and Electrical Insulation, Vol. 8,No. , Dec. 2001.[I61 L. Lundgaard, D. Linhjell, B. Skyberg, Metallic particles an GISspacers: Elecmc and acoustic PD-measurements from Short TermTests , ISH conference 2001. paper 4-49. Bangalore, India.[I71 M. Foata, R. Beauchemin, C. Vincent. M. Roy, S. Talbot, CISEquipment Sounds Off... Are You Listening? , Transm ission andDistribution World Magazine, March 2001.

IX. BIOGRAPHIESA s k S r h r i was born in Mosjeen in Now ay in 1936 and gat his MasterDegree in Electric Power Engineering in 1960 from The Norwegian Instituteof Technology (NTH). He was then 12 years in ASEA AB in Ludvika,Sweden, within research and management of research, the last f ive years asManager of the Surge Arrester Department with responsibility of researchand design of surge arresten. Then he went to the Norwegian Research

Institute of Electricity Supply (EFI) in Trondbeim, covering insulationsystems and insulation coordination of high voltage bansmission lines andstations including GIS.In 1986 he founded the company TransiNor As, a company specializingin hansient protection technology and related fields. together with twocol leagues f mm EFI.During I 5 years MI chei was staff member of he Division of ElectricalPower Engineering, NTH, University of Trondheim. His position was part-time professonhip, and he offered graduate come in Insulation Coordinationand worked as supervisor for doctorate shld mts in his field ofspeciality.Mr. Schei has been active during nearly 30 y ears within several ClGREand IEC committees an d working groups, where the o btained competence onhigh international level has resulted in positions as the prereot Convener ofIEC Working Group on Metal Oxide Surge Arrestm and the present

Convenerof CIGRE Worlung Group on Insulation Coordination Pmcedwesfor Overhead Lines and Stations.He is author and co-author of about one hundred technical publicationsand the inventor ofabout ten patents.Mr Sehei is Fellow o f IEEE, personal member of ClGRE and Chainnan ofme Norwegian Committee of IEC TC37 and exEha i rman of TC42. He ismember of h e Norwegian Academy ofTechnological Sciences (NTVA),member of h e Nolwegian Society of Chartered Engineers (NIF) and The

VIII. REFERENCESC E R E IWG 33123.12, Insulation Co-ordination of GI R e m ofExpctience, O n Site Tests and Diagnostic Techniques . ELECTRA No.176, 176Feb. 1998.L.Lundgaard, M.Runde. BSkyb erg, Acoustic Diagnosis of Gas ~~~~~i~~ Association ~ k ~ t r i ~ ~ i NEF).Insulated Substations; A Theoretical and Experimental Basis , EEETrans. PO. D ~ I . ,ol. 5 NO 4, 1990,pp 1751- 1760.IEEE Substations Com, Working gmup K4, GIS Diagnostic Methods,' Partial Discharge Testing of Gas nsulated Substations. IEEE Trans.Power Delively, Vo1.7, No.2 April 1992, pp 499-505.L. Lundgaard, Partial Discharge. P atl XIII: Acoustic P artial DischargeDetection Fundamental Considerations , EE Electrical Ins.Mag, Vol.8N o. 4 ,Ju l y IA ug 19 92 , pp2 5- 31 .L.Lundgaard, BSky berg, Acoustic Signals from Corona Discharges inGIS. IEEE CEIDP, Knoxwille, 1991. pp 44 945 6.Cigre SC 15, WGIS.03, Diagnostic Methods for G Is InsulatingSystems , ClGRE Session 1992, Paper no 15/23-01.E.Colombo, W.Kolhmo wicz. A.Pigini. Sensitivity of Electrical and

Stig Ky rkjreide was born in MAley, Now ay. in 1970. He graduated FromNorwegian Institute o f Technology (NTH). Trondheim, in 19 93 with aMaster De- in Electrical Power Engineering. Fm m 1994 to 1999 he wasengaged at the Department of High-Voltage Technology, NTHlNorwegianUniversity of Science and Technology (NTNU), where he mainly wasc ur y i ng out research related lo hansient analyses and modelling of powertransformers 8s well as teaching high-voltage laboratoly comes. rom 1999Mr. Kyrkjeeide has been with T m si Na r As and b now workiog mainly withdiagnostic techniques of high-voltage a ppa atu n an d syst-.

Vcgard LPrsrn was born in Som. Norway, in 1955. He received hisAcoustic Methods for GIS Diamostics with oarticular Reference to Master De- &om the Denartmen1 of Electrical Eneineerine. NTH. in~ -. .On-Site Testing. . Cigre Symp on Diagno stic and Maintenance 1979. From 1979-1985 he was engaged as a research officer at theTechniques,paperno 130.13, Berlin, 1993. Norwegian Electric Power Research Institute (EFI) in Tmndheim, where heM.Runde, T.Amd. K.Ljekelsray, L.E.Lundgaard, J.E.Nekleby. was mainly concerned with insulation coordina tion studies, transientB.Sk yW % Risk Assessmeof Basis of Moving Particks in Gas analyses, failure investigations and develapmeot of computer software. FromInsulated Substations , IEEWPES Transm. and D i m Caof, 1996. 1985.1989 he joined the No wegia n State Oil Compan y as a SeniorH.D. Schlemper, K.Feser, Estimation of Mass and Length Og Moving ~n gi ne er . eing responsible for market shldies and involved in gas sales andParticles in GIS by Combined Acoustical and Electrical PD Detection. , plaaning gas and instal~ations. In 1989 M ~ . enoined

TransiNor As, first as Manager for software products, later as President in theaper submitted for the CEIDP conference 1996.[IO] M.Leijon; I.Ming, P.Hoff, SF, Gas pressure Influence on Acoustical company, At present Mr, Lanen is Vice Pnsident and Salff Manager inSignals generated by Partial Discharges in GIS. , 7th ISH conference, Trans iNor Aspaper no 75.11, Dresden 1991.

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Acustic Insulation Analyser for Periodic Condition Assessment of Gas Insualted Substations