Acoustic Insulation Analyzer for Periodic Condition Assessment of Gas Insulated

Substations Asle Schei, Fellow, IEEE, Stig Kyrkjeeide and Vegard Larsen

Allvoltages I

Abstrad-A new instrument based on acoustic methods has been designed which is offering non-invasive condition assessment of Gas Insulated Substations (GIs) during normal service. The instrument will be able to reveal many of the defects resulting in breakdown of GIs and will in general have a significant cost benefit. Based on results from several years of field experience it has been confirmed that the recordings generated by this instrument make it possible to detect, locate and recognise the most common defects such as moving particles, particles on spaeers, partial discharges from protrusions and loose shields. The detected signatures from the defects may also be used for risk assessment of the CIS.

The paper describes the technical background of the instrument and illustrates how acoustic signals from different types of flaws are presented by and can be recognized from the instrument. Thereby, the instrument can be used eficiently by maintenance people without being experts in acoustics.

Index Terms-Acoustic, condition assessment, diagnosis, discharge, failure, GIS, insulation, maintenance, monitoring, SF6.

I. INTRODUCTION IS are often located in important nodes in the grid and at G power stations. Therefore the demands on availability are

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

0.9

TABLE 1. FAILURE RATES OF CIS.

125-145

300 3.4

420 I 2.2 I 1.8

550 I I 3.9

The general trend is that the failure rate becomes lower with time as mor5 mature designs are introduced. According to Norwegian experience the mean time to repair is in the order of 14 days, but may be much longer. The time to repair and the outage time after a flashover depends of course on the availability of spare parts. Typically, the cost of a repair in a 420 kV GIS after a flashover will be in the order of 100 - 300 thousand US$. In Norway the typical GIS is of 5 bays. On this voltage level, where the failure rate is 2 failures per 100 bay year, the failure probability per year for a 5 bay GIS will be 10 %. On average the yearly cost will therefore be 20000 US$. The value of non-produced or non-delivered energy is not included, but may be more important than the costs of the repair alone. Cigre [I] estimates that about 55 % of the defects leading to breakdown may be detected by suitable diagnostics. This means that the potential savings by succeeding with a condition based maintenance plan is 10000 US$ per year plus the value of avoided production or supply losses. In Norway unplanned supply losses to industry have to be valued according to a social-economic price of 5.5 US$ per kWh.

Defects leading to breakdown may either be introduced during manufacturing, erection or operation, e.g. particles from fast earthing switches. In Norway there is good experience with acoustic measurement of the dielectric integrity of GIS, which in most cases can be directly related to the performance. Such measurements are sensitive to a majority of the relevant defects. Based on an annual condition control the costs of a maintenance plan based on condition monitoring may be estimated. Usually one 5 bay substation may be thoroughly checked within one day, resulting in labour cost of about 400 US$. A correction of a severe defect will cost less than repair after a breakdown. An estimate is about 15000 US$ per corrective action on the 420 kV level. If 50 % of the potential failures, of which there is a failure probability of IO %, may be detected, the average annually expenditures on corrective actions will be 750 US$. The condition monitoring and corrective actions will add up to 1150 US$ per year as an average. The annual cost of an instrument written off over 5 years is about 10000 US$. In case of a single GIS the annual cost for the maintenance will add up to about I1000 US$ (i.e. break-even).

A. Schei, S. Kyrkjeeide and V. h e n are all with TransiNor As, 7037 Tmndheh, Noway (e-mail: asle.sehei@bansinor.no, stig.kyrkjeeide@bansinarno and vegard.lanen@transinor.no).

0-7803-7525-4/02/$17.00 8 2002 IEEE. 919

11. ACOUSTIC MONITORING OF THE INSllLATlON SYSTEM

Acoustic monitoring can be performed during service with externally mounted sensors and a portable instrument. It is a non-invasive self supported technique [2,3]. Experience shows that the technique offers several benefits: . Good sensitivity to detection of most defect fypes.

Immunity to external noise. Defects may be localized. Defects may be recognized. Risk assessment based on source characterization.

The signals from the defects may vary widely from continuous signals from internal corona to pulse shaped signals from for example moving particles. There are only a limited number of parameters to describe such a signal: - Continuous signals:

- Peak value over one power cycle - rms value over one power cycle - Degree 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 for evaluation of results. Setting of threshold and time gating for discrimination of echoes in pulse shaped signals are also essential features for achieving good results.

A portable instrument that supports all these features has been designed (Figure I) and used with good experience by both suppliers and users of GIs in several countries. The instrument can either be operated in continuous mode or in pulse mode. The instrument is also equipped with a loudspeaker that allows a skilled technician to immediately observe and recognize signals. The instrument performs a meaurement automatically. Afterwards, results and instrument settings can be downloaded to a PC for further analyses and transfer to a database. The instrument may be powered either from the mains or from an internal rechargeable battery offering four hours of continuous operation.

Fig. I, Acoustic Insulation Analyzer (AIA) with acoustic emission sensor.

111. ACOUSTIC PROPERTIES OF GIs The shape of the detected signal will depend on the type of

source, the propagation path of the signal and the sensor. The sound sources are generally wide banded (partial discharges in the range of 10-100 Wz, particles up to several MHz) [4]. When a signal propagating on the enclosure hits a discontinuity (i.e. flange), it will partly be reflected and partly transmitted. Because the materials used in most enclosures have a very low absorption, a signal will ring for long time due the multiple reflections for instance from flanges.

In the SF,-gas the signals propagate with a speed of about 140 d s . The gas acts as a low-pass filter [ 5 ] . When the signal hits a surfacelenclosure, only a fraction of the energy is transmitted into the enclosure. The coupling between gas and enclosure improves with increased gas pressure.

Some defects (e.g. particles, corona at enclosure surface) act as point sources directly on the enclosure. Defects close to the high voltage conductor (e.g. corona from sharp points), however, first excite a pressure wave in the gas, which then excites the enclosure before it finally is picked up by the acoustic emission sensor.

w. DEFECTS IN THE INSULATION SYSTEM

Defects in the insulation system of GIS may be left after the production and erection and may also be produced during operation (e.g particles produced by fast earthing switches). The most important defects [5 ] are shown in Figure 2.

Fig. 2. The most important defects in a GIS insulation system

A. Protrusions on earth and live parts A protrusion from live or earth parts will create a local field

enhancement. Such defects will have little influence on the ac withstand level, because the voltage varies slowly and corona at the tip will have time to build up a space charge that shields the tip. For impulses like lightning surges or very fast transients produced by disconnector operation, there is not enough 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 moving andfixed to spacers Free moving particles have little impact on the LIWL, while

the ac withstand level can be significantly reduced from their presence. The reduction will depend on their shape and position; the longer they are and the closer they get to the HV-conductor the more dangerous they become. If they move

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on to a spacer they become even more dangerous. A particle on a spacer may with time lead to deterioration of the spacer surface.

C. Voids and defects in spacers

electrical trees and eventually lead to breakdown.

D. Elecirically and mechanically loose shields If a field grading shield becomes mechanically loose it may

in the end become electrical floating. A floating shield adjacent to an electrode will give rise to large discharges between shield and electrode.

If these defects are activated (e.g. discharging), the lightning impulse withstand level will be heavily reduced. Moreover, a mechanically loose shield may fall off and create a flashover.

A defect within a spacer will give rise to discharges,

v. ACOUSTIC SIGNATURES AND RISK ASSESSMENT

The parameters detected in continuous and pulse mode together with the loudspeaker signal give clear indications of the type of flaw. Further details concerning the various defects are explained in the following.

A. Free moving pariicles Particles will jump around when the electric field is strong

enough so that coulomb forces exceed gravity. Each time the particle hits the enclosure it emits a wide banded transient acoustic pulse which travels back and forth on the section it is contained within. The sensitivity is sufficient to detect sub-millimetre particles with a good signal to noise ratio. The signals are easily recognized by large amplitudes, a high crest factor (amplitude-to-rms ratio) and a large scatter in the peak value in continuous mode measurement. The signals are pulse type signals, and if the instrument is switched to pulse mode and the source really is a particle, the patterns like those shown in Figure 3 will be observed [8][9]. The 50/100 Hz correlation is weak (Figure 4).

LEVEL I INTERVAL f” mV

f I

Caln : 3 200 mr

1500 mv LEVEL I SYNC.

0 90 inn no 380

Fig. 4. Acoustic amplitudes versus phase angle tiom 1000 impacts enclosure.

Gal” : 3 D.W..

,the CIS

From these patterns a lot about the particle characteristics and behaviour can be inferred. The dangerous particles are those which are elongated and give high jumps. Of course, the design of the GIS (i.e. insulation distances and possibility for the particle to arrive at a spacer) should also be considered. Not all moving particles in a GIS are dangerous. This is also shown by our return of experience, where substations with multiple particles has operated safely for years. However, a large percentage of major failures is caused by particles. It is therefore very essential to have good methods for risk assessment in case a particle is revealed, i.e. decide about particle size, movement and location.

The recorded patterns are tightly connected lo the particle characteristics and its behaviour. From the particle patterns (as the one in Figure 3) it can be possible to estimate the particle characteristics such as the length of the particle, its mass and its elevation height [IS]. In the phase plot (Figure 4) symmetrical patterns in the two half cycles will appear if the particle keeps its charge during flight, while an asymmetry will appear if the particle looses charge (i.e. discharges) during flight.

When measuring on a GIS, the raw signal contains both the direct incident wave at the sensor and the multiple reflections. Only the directly incident signal should if possible be measured as this signal will not be “polluted by the geometry of the enclosure. This can be done by gating, i.e. choosing appropriate live- and dead-times. Particle movement may be initiated either by a high voltage or a mechanical shock (e.g. circuit breaker operation). During measurement a hammer tap may be applied to activate the particle in order to detect it.

Compared to conventional partial discharge (PD) detection the acoustic method is very sensitive as can be seen from Figure 5. For the conventional method ( E C 270 [14]) a sensitivity of 5 pC is considered good, and will only be achievable in a screened set-up. From the figure it is seen that with such a sensitivity it will be difficult to detect 5 nun oarticles (and smaller) with the conventional method. For the

Fig. 3. Acoustic amplihldes versus elevation time fmm 1000 impacts to the CIS enclosure.

. acoustic system, however, the signal-to-noise ratio is very large even for 2 mm particles.

92 1

a.* 1.0 100 Awmmawrmthm Wl

Fig. 5. Acoustic sensitivity for detection of free, bouncing particles compared to conventional partial discharge measwments.

E. Protrusions A prohusion will create a field enhancement. If the ac field

exceeds a certain level, a discharge will first occur at the negative peak. When further raising the voltage, the number of discharges will increase and large discharges will occur also at the positive peak. These discharges create pressure waves that generate a continuous acoustic signal with a 50 Hz modulated envelope.

Figure 6 a) shows how the acoustic signal level varies with the peak level of discharges in a 300 kV CIS. The fact that much of the energy in the waves stays within the section with the defect, results in a drop in the signal level once a flange is crossed as shown in Figure 6 b).

~. ~ ~~~

0 . tb zd JO o n w m z m wur 1w Bq wnaton- Ern,

a) b) Fig. 6. a) Acoustic pulse amplitudes as a function of maximum electric (apparent) discbarge levels in a 300 kV CIS 121, b) dependence of sensor location on acoustic pulse ampliNdes (21.

Even if no good method presently is available for calibrating the acoustic method for corona type signals, several independent investigations using different types of enclosures demonstrate sensitivities in the 2-5 pC range [IO]- [12]. The standards (IEC 517) set a limit for acceptance of 10 pC at 110 % of system voltage, while Cigre SC 33 advises a level of 5 pC at 80% of test voltage [I] . As explained, the acoustic method will in most cases have a sensitivity better than this. However, for an acoustically detected protrusion with corona, it can be hard to make a fml judgement of the risk based on limits for the electrically measured PD level as given in the standards, due to uncertainties in the connection

between acoustic signal level and PD level. This far, it is only possible to give an estimate of the discharge level from the measured acoustic signal. However, the position may be taken that there should be no corona within the CIS at normal operating voltage, as a protrusion will reduce the flashover voltage for steep transients and over time decompose the gas and create products which may be harmful to insulator surfaces.

If the discharge source is located at the ground side, the absorption of the high frequency signals will be small as the propagation path through the gas is short. Consequently, the frequency content of the detected signals will be shifted upwards compared to the case where the source is further away from the enclosure like from corona at the centre conductor. This makes it possible to decide whether the source is on the high voltage or ground side. If the signal disappears when a high-pass filter is applied, it will be on the high voltage side, and vice versa.

To get a corona discharge from the ground side of a CIS a long and sharp particle/prohusion has to exist. A metallic object resting on a painted interior surface will act as a floating metallic object and create discharges towards the painted surface at the rising flanks of the voltage.

C. Floating shields Sometimes field grading shields will become loose and start

to vibrate. They may also loose the electrical contact and become electrically floating. A large floating metallic object will be capacitively charged, and when the withstand voltage between the object and its base is exceeded a large discharge/arc will occur. Such discharges occur at the rising flanks of the voltage, and will produce a large continuous signal with mainly a 100 Hz envelope. The signal level will usually be stable and have a low crest-factor.

D. Voidr and defects in spacers V o i d s and defects inside spacers wil l create dischatges once

the initiation voltage is exceeded. Usually such voids are found during the quality control in the factory. Because the sound absorption in filled epoxy is very high, the chance to detect them with acoustic measurement is small. Measurements indicate a sensitivity of some hundreds of pC for a discharge occurring inside the spacer close to the centre conductor.

E. Particles on spacers A particle that moves on to a spacer may behave in many

ways. It may move around on the spacer where it may discharge and deplete charges. This is particularly relevant for horizontal spacers (e.g. located at the bottom of a riser). It may also become fixed to the spacer and create discharges towards the spacer surface. The spacer surface is not a self healing insulation material and may during time be deteriorated (e.g. carbonized) and eventually break down.

The knowledge of acoustic signals generated from particles on spacer surfaces is relatively limited. Signals from mechanical impacts may occur and because they are very energetic they may be detected at the outside of the enclosm

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even if the absorption in the spacer is high. The particles can also generate sound waves in the gas that propagate like those from a protrusion. If the particle is located at the inside of a conical spacer, the spacer may act as a barrier for the pressure wave and reduce sensitivity.

Some early studies showed that signals from large particles on spacers could he detected. At low voltage levels small discharges (in the pC range) occurred regularly on the rising flank of the voltage. No certain acoustic signals could be seen from these discharges. At higher voltage levels very large discharges occurred occasionally at uneven intervals up to some 20 seconds. These signals could he detected acoustically and had a clear correlation to the power cycle.

Some investigations have been made at CESI [l3]. They report that discharges produced by metallic particles and carbonised tracks located at the outside of conical spacers for a 420 kV GIS could be easily detected. A recent study at SINTEF Energy Research states that the sensitivity also for small partial discharges due to metallic particles on spacers is reasonable [16]. In a 300 kV test set-up the sensitivity was found to be some 2 pC for padcles located at the outside the conical spacer and some 5 pC for those located inside the cone.

W. FIELD EXPERIENCE

The Acoustic Insulation Analyzer shown in Figure 1 has been used since 1996 with good results both by manufacturers of GIs, utilities, industry companies and universitieshsearch institutes. The acoustic method has found several ways of application on GIs:

In factory tests. As a stand-alone tool for checking the GIS insulation system during normal operation, for instance - on a periodic basis. - to follow UD alreadv detected flaws.

3 3 1

90 4 8 0 m %o Phse -le [degmes]

Fig. 1. Partial discharges due to a protrusiodscratch on the enclosure of a 275 kV busbar detected during pretesting before commissioning.

Figure 8 shows the acoustic signature due to an electrically floating shield in a 500 kV disconnector. The utility had earlier experienced a flashover in a disconnector of the same make and wanted to make sure that the other disconnectors were in good condition. The signature shows relatively high pulse amplitudes (indicating large partial discharges) with a distinct 100 Hz envelope modulation, which are characteristic for an electrically floating shield. The disconnector was opened for inspection and one floating shield was found. After replacement of a new shield, the acoustic signal level was close to the acoustic background noise level (which was about 1 mv).

Phase a g l c [degrees]

Fig. 8. Elechically floating shield in a 500 kV diswanector measured during normal service.

- during pre-testing before commissioning. - during commissioning. An example of a mechanical vibration in a 275 kV circuit

breaker is shown in Figure 9. A routine test was performed on the breakers, and an acoustic signal deviating significantly from the background noise was detected in one of the phases of a circuit breaker. The signature shows a very distinct 100 Hz enveloue modulation with oronounced vertical lines and

pn'or to revisions to give guidelines for inspection. after revision as a quality after inspection, . As a complimentary tool for continuous uHF.monitoring

in order to pinpoint and characterize the flaw [17].

Some typical examples of acoustic signatures from different types of flaws detected during normal service are shown and discussed in the following.

Figure 7 shows the acoustic signature from a partial discharge due to a scratch (protrusion) on the enclosure of a 275 kV husbar. The protrusion is recognized by its predominant and stable 50 Hz modulated envelope and the rather low acoustic amplitude and low ratio between amplitude and rms-value. The partial discharge was detected during pretesting before commissioning, and the scratch was confmed by inspection.

symmetry in the two half cycles. This indicates that a ~ t u r a l frequency of the breaker has been excited and thereby that a mechanical problem may he present. Since the utility earlier had experienced that a bearing in a circuit breaker of the same make had collapsed, they decided to open the breaker for inspection. A shield was found to be loose and in close mechanical contact with another metallic part of the breaker. When the shield was replaced into its original position, the acoustic signal disappeared.

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i 90 180 zm 360

Phase angle [degrees]

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

In many cases inductive potential transformers (with iron core) have been found to generate acoustic noise modulated to 100 Hz, usually with low amplitudes. Similar signals may be detected also on single-phase steel encapsulated GIS. In the latter case, the noise increases with the load current. In both cases the noise is usually due to magnetostriction in the steel and is thereby not related to the GIS insulation system. The operator of the acoustic system should keep this in mind during measurements.

VII. CONCLUSION During several years of field experience, the acoustic

method has proven to be a sensitive and reliable tool for condition assessment of the GIS insulation system. The method is non-invasive, and the developed system makes it easy to pinpoint and identify the most common types of flaws such as moving particles and partial discharges during normal service. Potential week points found during service may be checked more often and repaired in time to reduce the risk of failure.

[ I I] A.Bargagia, W.Koltunowicm, A.Pigini, "Detection of Partial Discharges in gas Insulated Switchgear " IEEE Trans Pa.Del., Vol. 7 No. 3, July 1992, pp 1239.1249.

[I21 H.-D. Schlemper, R.Kurrer, K.Ferer. "Sensitivity of &-Site Partial Discharge Detection in GIS." 8th ISH Conference, Paper no. 66-04, Yokohama. 1993.

[ I 31 W.Kolhmowicz. Personal communication. (141 IEC 270 "Partial discharge meawements ". [IS] L. E. Lundgaard, "Paltieles in GIS. Characterization from Acoustic

Signatures", IEEE Trans. Dielectrics and Electrical Insulation, Vol. 8, No. 6, Dec. 2001.

[I61 L. Lundgaard, D. Linhjell, B. Skyberg, "Metallic particles an GIS spacers: Elecmc and acoustic PD-measurements from Short Term Tests", ISH conference 2001. paper 4-49. Bangalore, India.

[I71 M. Foata, R. Beauchemin, C. Vincent. M. Roy, S. Talbot, "CIS Equipment Sounds Off... Are You Listening?", Transmission and Distribution World Magazine, March 2001.

IX. BIOGRAPHIES Ask Srhri was born in Mosjeen in Noway in 1936 and gat his Master

Degree in Electric Power Engineering in 1960 from The Norwegian Institute of Technology (NTH). He was then 12 years in ASEA AB in Ludvika, Sweden, within research and management of research, the last five years as Manager of the Surge Arrester Department with responsibility of research and design of surge arresten. Then he went to the Norwegian Research Institute of Electricity Supply (EFI) in Trondbeim, covering insulation systems and insulation coordination of high voltage bansmission lines and stations including GIS.

In 1986 he founded the company TransiNor As, a company specializing in hansient protection technology and related fields. together with two colleagues fmm EFI.

During I 5 years MI. Schei was staff member of the Division of Electrical Power Engineering, NTH, University of Trondheim. His position was part- time professonhip, and he offered graduate come in Insulation Coordination and worked as supervisor for doctorate shldmts in his field ofspeciality.

Mr. Schei has been active during nearly 30 years within several ClGRE and IEC committees and working groups, where the obtained competence on high international level has resulted in positions as the prereot Convener of IEC Working Group on Metal Oxide Surge Arrestm and the present Convener of CIGRE Worlung Group on Insulation Coordination Pmcedwes for Overhead Lines and Stations.

He is author and co-author of about one hundred technical publications and the inventor ofabout ten patents.

Mr Sehei is Fellow o f IEEE, personal member of ClGRE and Chainnan of me Norwegian Committee of IEC TC37 and exEhairman of TC42. He is member of "he Norwegian Academy of Technological Sciences (NTVA), member of h e Nolwegian Society of Chartered Engineers (NIF) and The

VIII. REFERENCES

C E R E IWG 33123.12, "Insulation Co-ordination of GI% R e m of Expctience, On Site Tests and Diagnostic Techniques". ELECTRA No. 176, 176Feb. 1998. L.Lundgaard, M.Runde. BSkyberg, "Acoustic Diagnosis of Gas ~~~~~i~~ Association ~ k ~ t r i ~ ~ i (NEF). Insulated Substations; A Theoretical and Experimental Basis ", IEEE Trans. PO. D ~ I . , vol. 5 NO 4, 1990, pp 1751- 1760. IEEE Substations Com, Working gmup K4, GIS Diagnostic Methods, '"Partial Discharge Testing of Gas Insulated Substations." IEEE Trans. Power Delively, Vo1.7, No.2 April 1992, pp 499-505. L. Lundgaard, "Partial Discharge. Patl XIII: Acoustic Partial Discharge Detection Fundamental Considerations ", EEE Electrical Ins. Mag, Vol. 8No.4,JulyIAug 1992,pp25-31. L.Lundgaard, BSkyberg, "Acoustic Signals from Corona Discharges in GIS." IEEE CEIDP, Knoxwille, 1991. pp 449456. Cigre SC 15, WGIS.03, "Diagnostic Methods for GIs Insulating Systems", ClGRE Session 1992, Paper no 15/23-01. E.Colombo, W.Kolhmowicz. A.Pigini. "Sensitivity of Electrical and

Stig Kyrkjreide was born in MAley, Noway. in 1970. He graduated From Norwegian Institute of Technology (NTH). Trondheim, in 1993 with a Master De- in Electrical Power Engineering. Fmm 1994 to 1999 he was engaged at the Department of High-Voltage Technology, NTHlNorwegian University of Science and Technology (NTNU), where he mainly was curying out research related lo hansient analyses and modelling of power transformers 8s well as teaching high-voltage laboratoly comes. From 1999 Mr. Kyrkjeeide has been with TmsiNar As and b now workiog mainly with diagnostic techniques of high-voltage appaatun and syst-.

Vcgard LPrsrn was born in Som. Norway, in 1955. He received his Acoustic Methods for GIS Diamostics with oarticular Reference to Master De- &om the Denartmen1 of Electrical Eneineerine. NTH. in

~ ~ I -. . On-Site Testing.". Cigre Symp on Diagnostic and Maintenance 1979. From 1979-1985 he was engaged as a research officer at the Techniques,paperno 130.13, Berlin, 1993. Norwegian Electric Power Research Institute (EFI) in Tmndheim, where he M.Runde, T.Amd. K.Ljekelsray, L.E.Lundgaard, J.E.Nekleby. was mainly concerned with insulation coordination studies, transient B.SkyW% "Risk Assessmeof Basis of Moving Particks in Gas analyses, failure investigations and develapmeot of computer software. From Insulated Substations", IEEWPES Transm. and D i m Caof, 1996. 1985.1989 he joined the Nowegian State Oil Company as a Senior H.D. Schlemper, K.Feser, "Estimation of Mass and Length Og Moving ~ngineer. being responsible for market shldies and involved in gas sales and Particles in GIS by Combined Acoustical and Electrical PD Detection.", plaaning gas and instal~ations. In 1989 M ~ . ben joined

TransiNor As, first as Manager for software products, later as President in the Paper 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 in

Signals generated by Partial Discharges in GIS.", 7th ISH conference, TransiNor As. paper no 75.1 1, Dresden 1991.

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