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A Summary of Non-Sustained Disruptive Discharges (NSDD) in Vacuum Switchgear R.P.P. Smeets, Senior Member, IEEE, A.G.A. Lathouwers, L.T. Falkingham, Senior Member, IEEE, G.F. Montillet Senior Member, IEEE
Abstract-- This document described the phenomenon, the interpretation and the assessment of Non-Sustained Disruptive Discharges (NSDD). The first approach is to analyze the phenomenon, based on available tests standards and on real testing results. We will look at NSDD in models for capacitance switching, their theoretical relations to overvoltages in capacitive circuits, then to tests for certification of equipment. A list of experimental results is given, and a proposal for the assessment of NSDD is suggested. Index Terms--Circuit breakers, vacuum interrupters, vacuum interruption, electrical breakdown, Non-Sustained Disruptive Discharges (NSDD), reignitions, restrikes, testing.
I. INTRODUCTION
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HE occurrence of a breakdown after short circuit interruption sometimes happens shortly after the current zero or sometimes up to one second after the interruption. Usually the insulation is restored immediately after the breakdown. This type of event is relatively rare, but tests have shown that there is a definite probability of the occurrence of these phenomena. Usually such a 'late' self-restoring breakdown is associated with vacuum switchgear. This paper deals only with vacuum based technology but some have said that undocumented SF6 breakdowns also occurred. In the English literature, such self-restoring late breakdowns are called "Non-Sustained Disruptive Discharges" or (NSDD) reflecting the inherent characteristic of vacuum interrupters to restore the insulating medium almost immediately after the start of a NSDD. It is said that this is due to the particular good capability of vacuum chambers to interrupt currents of very high frequencies. The breakdowns are thought to be initiated by a combination of several mechanisms, the two main are: mechanical vibrations leading to a release of particles [1], or to a sudden increase of field emission level leading to breakdown [2]. This work was supported in part by KEMA High Power Laboratories and in part by AREVA T&D. R.P.P. Smeets is with KEMA High Power Laboratories, Utrechtseweg 310, 6812 AR Arnhem, Netherlands (e-mail:
[email protected]). A.G.A. Lathouwers is with KEMA High Power Laboratories, Utrechtseweg 310, 6812 AR Arnhem, Netherlands (e-mail:
[email protected]). L.T. Falkingham is with AREVA T&D, Areva T&D - Paris La Defense 92084 - France (e-mail:
[email protected]). G. F. Montillet is with AREVA T&D, One Power Lane, Charleroi, PA 15022 USA (e-mail:
[email protected])
The interpretation and assessment of NSDD have led to considerable discussions; particularly concerning the consequences of NSDD in real life circuits. It must be recalled that vacuum interrupters are not a new technology. The industry has more than 35 years of experience in medium voltages and millions of interrupter-years of service experience. Based on this huge installed park, there is today no hard evidence that NSDD have any detrimental effects in service. In this paper, the authors would like to give conclusive summary of the electrical phenomena related to NSDD, to assess the consequences and significance of NSDD, and put forward a new proposal to greatly simplify the assessment procedure of NSDD in tests laboratories. II. INTERPRETATION, TESTS ISSUES AND APPLICATIONS TO STANDARDS. A. Difficulties of Interpretation NSDD are difficult to detect in laboratories and even more so in real life ANSI/IEEE and IEC both recommend that medium voltage circuit breakers up to 52 kV be tested in a three-phase ungrounded circuit. This is because the highest recovery voltage is obtained in this configuration. In an ungrounded three-phase system, two phases that interrupt the current, will interrupt a three phases fault, because the third phase has no return path. This implies that if a NSDD occurred in one phase and the others are already open, the ungrounded circuit will prevent a measurable current to flow. And this makes it impossible to distinguish between a selfrestoring NSDD and a non self-restoring NSDD, which is called a restrike. The NSDD has usually no significant effect on the external circuit. The current following a NSDD is too small and too short in time to be measured normally. To identify the occurrence of a NSDD, one looks at the voltage to ground at one terminal (either load or source side, depending on whether the neutral or source is grounded or not). Then all three phases exhibit a steep jump that is recorded with laboratory instrumentation. Figure 1 shows an example of both a NSDD and a restrike. Note that here the NSDD is the cause of to the restrike: initially an NSDD in phase 2 occurs, raising the voltage in phase 3 to a value too high to be withstood by the circuit breaker. Then, a two-phase fault arises, which is interrupted after a full loop at power frequency.
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At this point a distinction must be made between short circuit current interruption tests and capacitive current interruption tests. In short circuit tests, the circuit breaker must demonstrate its ability to interrupt a fault current. Any circuit breaker from any technology is allowed to cause a disturbance to the circuit for half power frequency loop duration as part of the interruption process-reignitions. The pole will interrupt the current at the next zero passage without problem. The primary difference between a reignition and a restrike is the time of occurrence of the discharge. Capacitive tests (which are much larger in numbers than
The test laboratories faced the problem of how to deal with what they were detecting, and termed it NSDD. As the switchgear standards did not refer to these phenomena, there were uncertainties as to how to interpret the new information. The STL (Short-circuit Testing Liaison) – an organization of manufacturers and independent operated test laboratories started this study, and discussed the interpretation of NSDD. As soon as 1985, the STL Committee decided that “Certification will not be allowed if there are four or more occurrences of the NSDD phenomenon during a complete test series as required by the standard, including any test duties which are repeated or re-started for whatever raison.” This statement has evolved further during the following 20 years, and this idea was incorporated within the IEC Standard, but not with the ANSI/IEEE Standard. This limitation of four NSDD was arbitrary and not based on any science. This has resulted in considerable debates and discussions between test stations, manufacturers and users.
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short circuit tests, but at a lower current) are intended to demonstrate the absence of restrikes. These restrikes lead to the release of a significant amount of capacitive energy that might damage the circuit breaker or/and generate overvoltages in the system. B. Standardization history The discussion on NSDD as a phenomenon that possible might affect users of switchgear started in the mid-1980. At the time digital data recorders in laboratories just reached speed that allowed these very fast phenomena to be detected. CIGRE did not study these phenomena as soon as they were discovered, and vacuum technology was limited to distribution voltages of less than 40 kV. The IEEE Switchgear Committee did not set up a task force or a working group as it was left to test laboratories to deal with this phenomenon.
C. Actual Status Today, during tests in accordance with the widely used standards IEC 62271-100 [3]or the series of ANSI/IEEE C37.09 [4], latest printing (for tests procedures), the recommendations of the STL organization are followed. STL insists that the absence of multiple (which means less than 4) NSDD must be verified by subjecting the interrupters to the rated power frequency voltage for at least 300 ms after interruption. If there are more than three occurrences of NSDD during the entire series of test duties, tests are not acceptable. Restoration of power frequency current is never allowed. In capacitive current switching tests, the number of NSDD, allowed for certification is one ninth (1/9th) of the number of interrupting operations in a test series. In order to simplify the interpretation of a breakdown, IEC decided to designate every breakdown occurring later than half a power frequency cycle after current zero as a NSDD. The consequence of this simplification is that potentially harmful restrikes leading to discharge and possibly capacitive voltage escalation [5] are not discriminated from normally harmless NSDD. Since this can be considered technically unacceptable, discussion on how to deal with late breakdown is continuing and generating new proposals, one of which is being put forward in the present paper. After the publication of IEC 62271-100 a number of parties have continued to work on the subject of NSDD and in particular their potential consequences. The new knowledge generated has resulted in a completely different assessment of the NSDD phenomena. As a consequence, NSDD is not considered to be important, and certainly not a reason to refuse certification of circuit breaker. NSDD were not specifically addressed by ANSI/IEEE and as a result the relevant standard did not prohibit NSDD, tacitly implying that these phenomena did not pose a significant problem. The members of the IEEE Switchgear Committee
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discussed NSDD and have continued with this policy up to now (the subject comes up from time to time). It has to be noted that the IEC is now coming around their position and tend toward the position of the IEEE Switchgear Committee. The new revisions of the ANSI/IEEE C37.09-1999 in the works, will not specifically address the issue of NSDD. III. EXPERIENCE WITH NSDD IN TESTS LABORATORIES A systematic investigation was made of the occurrence of NSDDs during mainly short circuit tests, for a period of one full year (1999) testing Vacuum Circuit Breakers (VCB) at the KEMA High-Power laboratories. In that year 133 test reports were produced on different VCBs that underwent current interruption tests. In 90 reports no NSDD were reported. In the remaining 43 reports (32%) a total of 134 occurrences of NSDD were mentioned. The test objects covering these 43 reports originate from 10 different manufacturers from Europe, Asia and North America. In 9 reports, more than 3 NSDD occurrences were
also in service conditions, NSDD must be quite a common phenomenon. The conclusion of all these tests is that no correlation between NSDD and circuit breaker performances can be established based on the statistic reported in [6] and others [7]. As there is no relationship, an NSDD is not a sign of distress of the vacuum circuit breaker. IV. NSDD AND OVERVOLTAGES IN CAPACITIVE CIRCUITS
In order to assess the potential consequence of NSDD and their effects it was decided to model the behavior of NSDD in a typical circuit. Failure of the interrupter to withstand the recovery voltage leads to the discharge of various capacitances and re-arrangement of the energy within the circuit. In capacitive circuits, this could generate undesired voltage transients in the system. In order to gain understanding of the characteristics of such transients and their origins, first a simulation study was performed, based on a "reference" capacitive bank short-circuit connection load circuit shown in figure 2. Later reactance to load on, measurements were VCB current performed as described in section V. The basic "reference" circuit represents a capacitor bank of load 200 µF connected with a cable multiple cables cable to side load at supply bus (approx. 100 m, having a voltages lumped element equivalent capacitance of 25 nF) to the circuit breaker. At the source Fig. 2: Reference capacitor bank circuit for simulation analysis side of the breaker a relatively large capacitance (100 nF), representing multiple cables is reported during the entire test series, 4 cases of which showed assumed. For a capacitive circuit, the most severe transients a variety of problems with current interruption and 5 were occur at breakdown of the breaker in the first phase-to-clear at refused tests certification purely on NSDD grounds (occurrence of more than 3) and further testing was 600 aborted. 400 It was observed that NSDD is not related only to interrupter current 200 high current phenomenon. This confirms the 0 finding published in 2000, by [7], in which it is shown that NSDD can actually occur even after -200 interruption of a very small current. This indicates -400 that NSDD are not an indication of being close to the maximum current interruption limitation of the -600 load voltage-to-ground vacuum bottle. This confirms that during capacitive current 0 interruption tests, with current less than 400 A, upper phase frequent occurrences of late restrikes are observed -2 in all voltage ranges. In the paper [6], one of the middle phase authors observed that less than 4 NSDDs occur in -4 most cases. lower phase 5 pu maximum value The numerical data show that approximately in -6 25% of the test certificates made by KEMA, circuit 0 20 40 60 80 100 120 time (us) D F breakers have exhibited NSDD, which implies that Fig. 3: Calculated current and load voltage-to-ground in reference circuit
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the peak recovery voltage. The voltage across the breaker is then at a level of 2.5 p.u. Given the full three-phase circuit of figure 2, the transient voltages that develop can now be calculated . The printed oscillogram of the interrupter current and load side voltage transient to ground is given in figure 3. Results shows that a maximum of –5 p.u. (relative to the ground) is reached after a time of some 35 µs in a healthy phase. It is assumed that the discharge is long enough to actually reach this value. Various frequency components can be recognized in current and voltage, each of which is associated with different parts of the reference circuit. More information on the model and analysis can be found in paper [10].
the NSDD that will determine whether or not significant overvoltages can develop. Short duration NSDD (less than 10 µs) in VCBs with excellent capability to interrupt high frequency current, such as vacuum bottles, do not allow discharge of more distant and larger sources of capacitive energy and the NSDD phenomena remain confined to the vicinity of the circuit breaker. If the NSDD duration becomes longer (in the case of the interrupter cannot interrupt high frequencies) the entire circuit becomes involved and more distant capacitances start to discharge and significant overvoltages may arise. Only for capacitive loaded circuits such overvoltages reach significant values. Thus, in the circuit modeled, NSDD are a clear example for interaction of interrupter and circuit, and the duration of NSDD is the key parameter. V. EXPERIMENTAL RESULTS
In an attempt to get more quantitative information on the NSDD phenomena, a very large number of tests were performed [8, 9]. A test circuit was realized consisting of three Fig. 4: Multiple frequency experimental circuit for NSDD duration evaluation oscillatory circuits of 1.1 MHz, 430 kHz, and 72 kHz see figure 4. Upon breakdown of the test-breaker, As a conclusion of the present analysis, it is the duration of each circuit will discharge. Two commercially available 72 kHz 1.1 MHz 430 kHz VCBs at 12 kV rating were stressed with a Recovery Voltage 500 500 500 (RV) up to 40 kV after interruption of a very small current. This very high RV (approximately double the rated value for these devices) was applied in order to give a higher 0 0 0 occurrence probability of NSDD. The randomly happening late breakdowns were monitored with high frequency digitizers. -500 -500 -500 4
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A. Single frequency NSDD. In the first series, NSDDs were monitored in oscillatory circuits of only one single frequency (L1C1 or L2C2 or L3C3). A striking difference in duration of the NSDDs is observed. In figure 5 examples of the NSDD current (upper) and voltage across the interrupter (lower) are given, together with the cumulative fraction of duration of HF current flow for each of the two interrupter designs (solid, dotted curves respectively) and for each of the three discharge frequencies. B. Multiple frequencies NSDD. In the second series, all three circuits were connected to the VCB, so that current consisting of three frequencies components arises upon breakdown of the interrupter. A typical current is shown in figure 6, together with the statistical distribution of the three-frequency NSDD duration. As can be seen, by comparison with figure 6, the duration of thee NSDDs is determined by the highest frequency component. The exact reason for this needs further investigation to ascertain.
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C. Discussion of the experiments. The important information gained from the experiments with commercial VCBs under multi-frequency NSDD discharge current is that the duration of the discharge is below 10 µs for the circuit breakers tested. Combining the short NSDD duration information with the overvoltages analysis of section “IV - NSDD and Overvoltages in Capacitive Circuits” it may be concluded that the discharge, leading to the 5 pu overvoltage from figure 3 (peak F) cannot develop in the reference circuit of figure 2. Apparently, the NSDD remains localized in the circuit consisting of the VCBs and its connecting cables. As a consequence of this, the overvoltages generating capability of NSDD is very limited. However, in very special cases, it is possible that even very short lasting NSDD, could cause significant overvoltages. Such a case is for example a short cable connection to the load, which will shift the 5 pu absolute overvoltages peak (figure 4, peak F) to a time that may fall within the typical NSDD duration. VI. PROPOSAL FOR NSDD ASSESSMENT Based on the above tests and calculations presented, it is possible to make the following assertions: • The duration of the observed NSDD is normally too short to let the relevant parts of the circuit reach significant overvoltages. • NSDD must occur very frequently under service conditions but nevertheless reported overvoltages related problems are very rare. • It is generally accepted that NSDD are inherent to interruption in vacuum which is the most common technology used in medium voltage switchgear. • NSDD is very difficult to assess in three-phase ungrounded test circuits. There is a continuous search of circuit solutions in order to make easier distinction between restrike and NSDD. • It is difficult to explain the consequences of the phenomenon to users of switchgear.
certification reports as an explanation of the sudden jumps in the recovery voltage oscillograms, which may be seen. VII. CONCLUSIONS We can ascertain that late breakdown after current interruption in vacuum interrupters (termed NSDD) is a common phenomenon. Due to displacement of charges within the circuitry around the vacuum interrupter, transient voltages may occur. A. Short circuit current interruption. During short circuit current interruption, there is no risk that such transients could reach a high value. • In an ungrounded circuit: A failure can only occur if the voltage jump associated with the breakdown leads to high voltage across one of the other breaker poles. Leading to breakdown in this pole, and a power frequency current flow in two phases (as in figure 1). In healthy vacuum circuit breakers, the high frequency interruption capability of VCBs normally prevents power frequency flow. • In a grounded circuit: A failure can only occur in the case where the high frequency interruption capability is not sufficient to interrupt the current after breakdown, resulting in power frequency current in one phase. In a 600 400 200 0 -200 -400 -5
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It is then proposed that NSDD can no longer be seen as s sign of distress of a circuit breaker and have no significance to the performance of a circuit breaker. One cannot follow the determination of STL that allows only three-recorded NSDD. However, in capacitive circuits, it is important to differentiate from a harmless NSDD and a potential harmful restrike, therefore a very simple distinction between restrike and NSDD is proposed: • To define a restrike as a breakdown leading to at least one loop of discharge current of the main capacitive load: the appearance of an “inrush” current. • To define NSDD as any breakdown, not having the characteristics of a restrike as defined above. It is then necessary to retain the term NSDD in test- and
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healthy circuit breaker, the high frequency interruption capability of the interrupter will again prevent power frequency current flow. Therefore NSDD are not considered to be a sign of distress of the circuit breaker and are not detrimental to the short circuit current interruption performance.
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B. Capacitive current interruption. Due to the presence of pre-charged load in the circuit that is unfavorable for the transients in one of the healthy phases, a certain probability of significant voltage transients exists in theory. However, combining the experimental results with network analysis we conclude that the duration of the conductive period normally is too short to let severe transient voltages develop. For special circuits, allowing a very fast building up of transients, it is theoretically possible to develop significant transient voltages, and so special care must be exercised in these cases and the installation of surge arresters may be advisable. C. Laboratories Tests Certification It is proposed to redefine the term "restrike" with respect to the current definition. Restrike should cover all breakdowns leading to a discharge of a capacitive load and/or power frequency current. All other breakdown phenomena should be termed NSDD. The user of switchgear, together with the manufacturer of vacuum circuit breaker, should realize that NSDD is in principle a harmless phenomenon inherent to interruption in vacuum, but could in theory and in some special capacitive circuit lead to significant overvoltages. NSDD is not considered as a sign of distress in a circuit breaker. In the cases where a breakdown leads to power frequency current, the breakdown is not longer termed a NSDD but a restrike. The identification of NSDD is not a reason for a test laboratory to certify that the test was not successful. VIII. REFERENCES [1]
[2]
[3] [4] [5] [6]
[7]
[8]
[9]
R. Gebel, D. Falkenberg, "Mechanical Shocks as Cause of Lat Discharges in Vacuum Breakers" IEEE Transactions on Electrical Insulation, vol. 28, pp. 468-472, 1993. B. Jüttner, M. Lindmayer, G. Düning, "Instabilities of prebreakdown currents in vacuum I: late breakdowns" Journal of Physics D: Applied Physics, vol 32, pp 2537-2543, 1999. IEC Standard High Voltage Switchgear and Controlgear, No. 62271100 First Edition, 2001-05, Geneva, Switzerland. IEEE Standards collection, Power & Energy Switchgears, IEEE Standards 1998 Edition, New York, USA. A. Greenwood, Electrical Transients in Power Systems, New York, John Wiley & Sons, 1991, chapter 5.3. R.P.P. Smeets, A.G.A. Lathouvers, "Non-Sustained Disruptive Discharges, Test Experiences, Standardization Status and Network Consequences" IEEE Transaction on Dielectric and Electrical Insulation, vol. 9, pp 194-200, 2002 M. Schlaug, L.T. Falkingham, "Non-Sustained Disruptive Discharges (NSDD) - a New Investigation, Method Leading To Increased Understanding of this Phenomenon". Xi'an, China, 2000, pp 490 - 496. D.W. Thielens, "Circuit Influences on Non-Sustained Disruptive Discharges in Vacuum Switching Devices", Eindhoven University, MSc. Report S433649, October 2003. R.P.P. Smeets, D.W. Thielens, R.W.P. Kerkenaar, "Characteristics of Late Breakdown In Vacuum Interrupters", XXIth International Symposium on Discharges and Electrical Insulation in Vacuum- 2004.
[10] R.P.P. Smeets, A.G.A. Lathouvers, L.T. Falkingham, "Assessment of NonSustained Disruptive Discharges, (NSDD) in Switchgear, Test Experience and Standardization Status" Council on Large Electric Systems, Session No. 40, 29th August - 3rd September 2004, Paris, France.
IX. BIOGRAPHIES René Peter Paul Smeets (M 1995, SM 2002) received the M.Sc. degree in physics from the Eindhoven Univ. of Technology in 1981. He obtained a Ph.D. degree for research work on vacuum arcs. Until 1995, he was an assistant professor at Eindhoven University, dealing with high-frequency phenomena in power engineering. During 1991 he worked with Toshiba Corporation’s Heavy Apparatus Engineering Laboratory in Japan in the development of lowsurge vacuum interrupters. In 1995, he joined KEMA. At present, he manages the R&D activities of KEMA's High Power Laboratory. In 2001, he was appointed part-time professor at Eindhoven University of Technology in the field of high-power switching technology. He is convener of CIGRE TFA3.01, member of CIGRE WGA3.12, IEC MT32 of IEC 17A, the “Current Zero Club”, IEEE and CIGRE. André G.A. Lathouwers received his M.Sc degree in Electrical Eng. from the Eindhoven Univ. of Technology 1988. From 1988 to 1990 he worked at TNO's Physics and Electronics Laboratory on calibration procedures for EMP measurements. In 1990 he joined KEMA as a Test Engineer. He has gained extensive experience in performing type tests on all kind of components for electric power transmission and distribution systems. At present, he is marketing manager of the High-Power Laboratory. He is member of the Short-Circuit Testing Liaison (STL) and chairman of several taskgroups of STL. Leslie T. Falkingham (M 1998, SM 2003) was born in Leeds, England, on May 13, 1955, is married and lives in Rugby, England. He was made a Fellow of the Institution of Electrical Engineers (IEE) in 1988 and a Fellow of the Institution of Mechanical Engineers (IMechE) in 1987. He is a Chartered Engineer in the UK (C.Eng) and is also a European Engineer (Eur.Ing). He graduated from Lanchester Polytechnic, Coventry, England, in 1978 with BSc (Hons) in Combined Electrical and Mechanical Engineering, and obtained his PhD by part time study from Cranfield University (UK) in 2002. His thesis was on the subject of optimization of industrial R&D and R&D Strategy. He is currently a Visiting Fellow of Cranfield University. He is a member of the Permanent International Scientific Committee of the International Symposium for Discharges and Electrical Insulation in Vacuum (ISDEIV) and is also a member of the Current Zero Club, with an interest in electrical switching phenomena. He has spent most of his working life involved in the research, design, development, and manufacture of vacuum interrupters and switches for medium voltage vacuum switchgear. He holds 26 patents, and is an acknowledged expert internationally in this field. During his career he has produced papers and presented lectures on the technology worldwide. His special fields of interest include all aspects of vacuum interrupter design & manufacture; R&D Management & Strategy and in particular the optimization of industrial R&D.
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He received the Nelson Gold Medal from GEC 1n 1997 for “outstanding technical innovation”, and the J.J. Thomson Medal of the IEE in 2002 for “distinguished contribution to electronic engineering”.
Georges F. Montillet (M 1970, SM 2003) was born in Nice, France on December 8, 1944. He graduated from the Polytechnic Institute of Grenoble (Ecole Nationale Superieure d 'Ingenieur) in 1968 in Power Electrical Engineering (MSc) and in 1974 obtained a MBA in Finance/Operational Research from NYU-Stern School of Business in New York (Beta-Kappa-Sigma). He joined Cogenel, now ALSTOM, New York, in 1971 after working on several projects in France, Algeria and New York. He was Assistant to the President from 1976 to 1990, and he was Executive Vice President of GEC– ALSTHOM T&D from 1990 to 1997, Deputy General Manager of the ALSTOM US High Voltage Switchgear from 1997 to 2000, and is now Vice President Marketing for the High Voltage Switchgear. He is a member of IEEE since 1970, and a member of the High Voltage Switchgear Committee, and various Subcommittees, one of which is High Voltage Circuit Breakers. He is also a member of CIGRE and SEE.
