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Partial Discharge - IMS indicates remaining Decomposition Productsin Gas Insulated Substations and Circuit Breakers

P. Pilzecker1, J.I. Baumbach2

1 G.A.S. Gesellschaft für analytische Sensorsysteme mbH, TechnologieZentrumDortmund, Emil-Figge-Str. 76-80,D-44227 Dortmund, Germany

2 Institut für Spektrochemie und angewandte Spektroskopie (ISAS), Bunsen-Kirchhoff-Str. 11, D-44139 Dortmund,Germany

Abstract

Although gas insulated switchgear and substations (GIS) belong to the most reliablecomponents of electric power networks, failures still occur in their long term operation. Some ofthese failures may be attributed to small variations of gas compositions within the shieldedequipment. Thus the prediction of a breakdown should be attainable on the basis of sensitiveanalytical monitoring of the insulating gas. Such a monitoring system for GIS and SF6-filledcircuit breakers is realised on the basis of continuous sensing of the insulating gas using ionmobility spectrometers (IMS). Especially, partial discharge IMS provides power utilities adiagnostic tool able to perform on-site monitoring and event controlled inspections of the SF6

gas.

The results of on-site monitoring and on-line analysis of SF6 contained in a circuit breaker of aGIS in Germany, before and after operation are described. Conclusions of Investigations ofdifferent disconnecting switches in a substation in Germany are presented, indicating that a fullrecombination of decomposition products was not achieve in all cases. Finally, the structure of asystem for automatic on site sensing of SF6-decomposition in the hydroelectric power plant ofFoz do Areia, State of Paraná, Brazil, is discussed.

IntroductionDuring the past 30 years the use of sulphur hexafluoride in insulated switchgears in high voltagesubstations increased considerably. This due to their enhanced lifetime, reduced required areaand volume and longer maintenance cycles. Very high standards in engineering andmanufacturing provide a high reliability of these substations. However, during long termoperation failures still occur which can cause severe damages and be costly to repair. A closerview shows that partial discharge (over a long period), sparcs and power arcs which belong tothe operation of high voltage switches lead to decomposition of the insulating gas SF6 [1-3]. Dueto long maintenence cycles, decomposition products may accumulate over long times within theswitchgear. Thus, a relatively small partial discharge activity over a long period may lead to anoticeable amount of corrosive and toxic by-products. Accordingly, insulating spacers and othercomponents could be exposed to a corrosive medium for long time periods.

Hence, the establishment of a diagnostic tool, which allows the on-site and on-line analysis ofthe insulating gas, is of considerable interest for the user of SF6 filled equipment. Ion mobilityspectrometry is a technique designed for sensitive environmental monitoring by using a low cost,rugged and highly mobile instrument. Using this technique for supervision of the insulating gas inGIS during operation offers the opportunity to achieve an event-based maintenance and detectpossible failure. In the following paper results of an super vision of a substation will besummarised with respect to the question that full recombination of decomposition products in aswitchgear will be achieved or additional maintenance will be necessary.

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Ion Mobility SpectrometryIon mobility spectrometry is a technique which was designed for the detection of tracecompounds within a gas, for example gaseous pollutions in air [4-5]. It combines high sensitivityand relatively low technical expenditure with a high speed data acquisition [6]. The mainadvantage with respect to other common detection principles is the fact, that the instrument canwork on-line, continuously and even without operator [7].

Such an instrument is based on the drift of ions at ambient pressure under the influence of anelectric field. The ions experience a separation process that is based on different drift velocitiesdue to different masses and charges or different geometrical structures. Collecting these ions ona Faraday plate delivers a time dependent signal corresponding to the mobility of the arrivingions. Such an ion mobility spectrum contains information about the nature of the different tracecompounds present in the sampled gas. For generation of ionic species some additionalcomponents are required, such as an ionisation source (In the case of SF6 a Partial-DischargeIonisation source is needed.) and an ion shutter grid. Digital signal processing may provide abetter readability of the obtained spectra or may automatically derive key characteristics from thespectrum as the peak position or signal amplitude [5].

It is possible to examine the change in the SF6 gascharacteristics by comparing the change in the ionmobility spectra with the reference spectrum ofpure SF6 gas. Figure 1 shows the results obtainedby laboratory analysis of a sparked sample and asample aged through partial discharges comparedwith a reference spectrum. The shift of the peaktowards shorter or longer drift times can be seenchanges in the spectrum that comply with theknown formation of a variety of decompositionproducts. The peak shift is caused by ions withdifferent mobilities, while a broadening of the peakis the result of a greater variety of ions formed byimpurities produced during the discharges. [8-9]

Results and conclusionFor investigations on SF6, a portable IMS for inspections and an IMS for continuous monitoringwith partial discharge as ionisation source was developed (More details are summarised atWWW.GAS-Dortmund.de.) [10-12]. In initiated inspection programs these PD-IMS operated inseveral SF6-insulated substations in Germany during normal operation. The IMS was connectedto the SF6-filled gas compartments with a thin polyethylene tube, providing a small flux of SF6

passing through the instrument, adjustable with a needle valve. To ensure comparableconditions, also reference spectra of pure SF6 were taken on site, which exhibited no significantdifference to the spectra taken in the laboratory. Of special interest for a possible user is thedata acquisition time of some minutes (including data processing), which was achieved in thisexperiment under real conditions. The flux was adjusted at about 2 L/h at ambient pressure. Thetotal measurement time was about 5 min. The connection to the IMS was installed at the fillingvalve with an adapter and a Teflon® line of 1 mm inner diameter. A needle valve made theadjustment of a constant small flux through the IMS possible. Several circuit breakers, currentdivider, voltage divider and other SF6-filled compartments of different substations areinvestigated by the procedure mentioned. In Figure 2 results from disconnecting switches arepresented. The figure 2 a) shows the results of measurements at 3 phases of a disconnectingswitch with a small peakshift of around 0.5 ms correlating to a low concentration ofdecomposition products inside. The length of the peakshift is correlated to the meanconcentration of the decomposition products. In opposite to a) the results of investigations atanother disconnecting switch shown in figure b) indicates a peakshift of 5.4 ms in one phase

Figure 1: Ion Mobility Spectra for SF6 aged gas.

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Graph2 27.05.1999 12:43© J.I. Baumbach, Institut für Spektrochemie und Angewandte Spektroskopie, ISAS, Dortmund

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correlating to a very high concentration of decomposition products. The identity and origin of theby-products may be investigated using more selective analytical methods like FTIR and GC-IMS.The performance of the monitoring system, however, is demonstrated through detection of thepeak shifts. Directly after the measurements the disconnecting switch was opened by the user ofthe GIS and a heavily damaged spacer inside the switch was identified as possible error source.Accordingly the IMS-Measurement prevent a failure in the disconnecting switch.

Inside a different GIS in Germany measurements has been carried out at a circuit breaker underoperation before, while and after switching with load. Some seconds after switching a peakshiftof 0.4 ms was detected as result of the switching (Figure 3). A

comparison of this result with measurements at the other circuit breakers in the same GISshows, like displayed in Figure 4, that the detected peakshift are as wide as it would be switchedwith load several times. Some of the measured circuit breakers have a peakshift as it would beswitched more then 4 times (See Figure 4). This leads to the conclusion that despite of themolecular sieves installed in the compartments a significant amount of by-products is stillpresent in the gas of the circuit breakers after switching. The presumption of the manufacturer ofcircuit breakers that the decomposition products formed by the arc while switching recombinecompletely after switching is not true in all cases. Although molecular sieves are used, a totalrecombination of the decomposition products inside the circuit breaker was not achievedsometimes.

Figure 2: IMS spectra obtained from two different disconnecting switches a) with small amountof decomposition products b) with a high amount of decomposition products (failure)

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Figure 4: Classification of all circuitbreaker of a GIS depending onthe peakshift (virtuel amount ofswitchings)

E01 E02 E03 E04 E05 E06 E07 E08 E09 E10

Phase A 0,4 0,9 1,1 1 1,5 1,8 1,5 1,2 1,4 1,4

Phase B 0,7 1 1,1 1 1,6 1,6 1,7 1,3 1,5 1,3

Phase C 0,7 0,9 1,2 0,9 1,8 1,6 1,6 1,4 1,6 1,3

Peakshift of 10 different circuit breakers (3 phases)

Colour Switchings

1 - 2 2 - 3 3 - 4 > 4

Remaining decomposition products in terms of products by a single switch with load

Peakshift for switching once is0,4 ms !!

Figure 3: Spectra obtained from a circuitbreaker before and after switching(with load) in correlation to pureSF6

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Employing IMS as presented hereallows the set-up of an automatedmonitoring system. The structure ofsuch a system, installed in the SF6-insulated substation in thehydroelectric power plant of Foz doAreia (owned by COPEL), Brazil, isdrafted in Figure 5. Crucialcompartments of the substation areequipped with an IMS connected tothe gas inside the compartments asshown in Figure 5. The acquiredspectra were transferred by a localarea network to the control roomwhere the IMS server is placed. Herethe spectra are evaluated throughcomparison with reference data, sothat a database of decisions ofmaintenance becomes available.With an installed LAN-connection ofthe operating company the IMS canwork without supervision by servicepersonnel.

References[1] Baumgartner, R., Fruth, B., Lanz, W., Petterson, K.,Partial Discharge - Part X PD in gas-

insulated substations - measurements; practical considerations; IEEE Electrical InsulationMagazine 8 (1992) 16-27

[2] Kusumoto, S., Itoh, S., Tsuchiya, Y., Diagnostic technique of gas insulated substation bypartial discharge detection. IEEE Transactions on Power Apparatus and Systems PAS-99(1980) 1456-1463

[3] van Brunt, R.J.: Processes Leading to SF6 Decomposition in Glow-type Corona Discharges.SPIG 1988 (1988) 161

[4] Roehl, J.E., Environmental and Process Applications for Ion Mobility Spectrometry. -Appl.Spectrosc.Rev.26 (1991)1-57

[5] Eiceman, G.A., Advances in Ion Mobility Spectrometry: 1980-1990. -Crit.Rev.Anal.Chem.22(1991)17-36

[6] Eiceman, G.A., Karpas, Z.: Ion Mobility Spectrometry, CRC Press, Boca Raton, Ann Arbor,London, Tokyo, 1994

[7] St.Louis, R.H., Hill, H.H., Ion mobility spectrometry in analytical chemistry. -Crit.Rev.Anal.Chem.21 (1990)321-355

[8] Baumbach, J.I., Pilzecker, P., Trindade, E., Meinders, J.: Diagnosing the Health Of SF6

Switchgear – Transmission &Distribution Jan. 2000[9] Pilzecker, P., Baumbach, J.I., Trindade, E.: Proc. of the IEEE International Symposium on

Electrical Insulation – Anaheim, CA, USA – April 2-5, 2000; 400-403[10] Held, M., Kurrat, M., Rickert, F., Baumbach, J.I., Segundo, S.M.A., Soppart, O., Klockow,

D., Versuchsaufbau zur Analyse von Zersetzungsprozessen im Isoliergas SF6 durchTeilentladungen. - Elektrizitätswirtschaft 96(1997)290-293

[11] Soppart, O., Baumbach, J.I., Alberti, S.M., Klockow, D., On-site Quality Assessment of Sf6

using Ion Mobility Spectrometry. - Conference Proceedings 10th Internat.Symp. on HighVoltage Engineering, Montreal, Canada, August 25 - 29, 1997, vol. 4, p. 147-150

[12] Baumbach, J.I., Klockow, D., Kurrat, M., Segundo, S.M.A., Soppart, O., Verfahren zurÜberwachung des Qualitätszustandes des Füllgases Schwefelhexafluorid in gasgefülltenAnlagen. - Patent DE 195 28 290 (3.8.1995)

Figure 5: Installation and Structure of an IMS-MonitoringSystem for automatic on-site sensing ofSF6-decomposition in the hydroelectric power plant ofFoz do Areia.

GAS-00-P1

GAS-00-P1

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Data transfer to control room

A vailable Network connection for system extention

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Internet access for diagnosisand consulting

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Network connection