Contact Behavior in Vacuum under Capacitive ... - IEEE Xplore

IEEE Transactions on Dielectrics and Electrical Insulation

Vol. 14, No. 3; June 2007

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Contact Behavior in Vacuum under Capacitive Switching Duty Florian Körner, Manfred Lindmayer, Michael Kurrat Technische Universität Braunschweig Institut für Hochspannungstechnik und Elektrische Energieanlagen Schleinitzstr. 23 38106 Braunschweig, Germany and Dietmar Gentsch ABB AG Calor Emag Mittelspannungsprodukte Oberhausener Str. 33 40472 Ratingen, Germany

ABSTRACT According to the relevant IEC standards vacuum circuit-breakers have to meet various needs, e.g. the interruption capability, making operations, and dielectric strength. Besides the interruption of short-circuit currents, switching of capacitive currents causes high stress of the circuit-breaker. Switching of capacitor banks, overhead lines, or cables leads to very small currents in comparison with short circuit currents. After current interruption the circuit-breaker must withstand twice the peak value of the system voltage. Furthermore, restrikes can lead to voltage multiplication. This conjunction of relatively small breaking currents with high voltage stress must be considered in detail. This work introduces a test arrangement for combined tests of making operation, current interruption, and dielectric stress of a vacuum gap under capacitive switching condition. A test vessel permits investigations of various contact materials and designs. It is connected to a synthetic test circuit which provides the appropriate test currents and capacitive voltage. During the test sequence the contacts are stressed by inrush-currents up to 4.5 kA peak, followed by a breaking operation at 500 A peak and a subsequent capacitive voltage up to 50 kV peak. Both the appearance of pre-ignitions at contact closing and the behavior under capacitive voltage stress after breaking are indications of the contact surface conditions. Index Terms - Capacitive switching, inrush-current, pre-arcing, restrike, welding force, vacuum circuit-breaker.

1 INTRODUCTION CIRCUIT-BREAKERS have to fulfill diverse requirements concerning interruption capabilities and dielectric strength. Typically a high switching duty and a distinctive dielectric strength are in the main focus of design, construction, and testing of circuit-breakers. This meets with the needs of short-circuit currents and overvoltage stress respectively. Additionally switching of capacitive loads i.e. capacitor banks, cable loads or overhead lines, represents a specific operating condition that requires extensive Manuscript received on 12 July 2006, in final form 5 March 2007.

performance. The test specifications given by IEC standards [1] correspond to these requirements. The connection of a capacitive load to the system leads to inrush-currents of up to several kiloamperes at frequencies significantly higher than the power frequency [2]. Typical currents at the interruption of capacitive loads are in the range of some tens of amperes to hundreds of amperes [1]. Subsequently the recovery voltage across the circuit-breaker rises up to twice the system voltage. Furthermore, in the event of a restrike after current interruption the capacitor can be reloaded causing an increase of the trapped charge and following an even higher voltage stress to the circuit-breaker [3,4].

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F. Körner et al.: Contact Behavior in Vacuum under Capacitive Switching Duty

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A few milliseconds after current zero the high current circuit is disconnected by opening the making switch SM1. Shortly after, the high voltage circuit is connected to the test switch by closing the making switch SM2. It provides a capacitive voltage up to 50 kV peak value (50 Hz). This voltage is created by the connection of pre-charged capacitor bank C2 and transformer T1 in series, supplying up to 25 kV dc voltage and a.c. peak value respectively. Added together they form a “1-cos” voltage shape. The capacitor bank is charged through the branch consisting of rectifier V1 and resistor R2. The resistor R1 limits the current in case of a breakdown at the test switch.

Figure 1. Basic synthetic test circuit.

Considering these operational conditions, the properties, the behavior, and the alteration of the contacts and their surface are taking centre stage. Accordingly a test arrangement was developed to examine various contact designs and materials.

2 EXPERIMENTAL SET-UP 2.1 SYNTHETIC TEST CIRCUIT The synthetic test circuit as shown in Figure 1 includes the test switch STEST connected to the high current circuit (left) and the high voltage circuit (right). The former is supplying the test current for both the making and the breaking operation by discharging the capacitor bank C1 over either inductance LC or LO respectively. Therewith the appropriate test currents of up to 4.5 kA at a frequency of 250 Hz for making tests and some 500 A at 50 Hz frequency for breaking tests are generated. The high voltage circuit supplies the capacitive recovery voltage up to 50 kV (50 Hz) which is applied to the test switch STEST subsequent to a current interruption. For making tests the capacitor bank C1 is charged using a separate charging system (not shown) while earthing switch SG1 is in open position. The selector switch SL connects the associated inductance LC providing a frequency of 250 Hz. The high current circuit is connected to the test switch STEST by closing of making switch SM1. Closing of STEST leads to a damped inrush-current flow accompanied by discharging of the capacitor bank C1. The maximum peak value of inrushcurrent is 4.5 kA at 20 kV charging voltage across C1. This voltage also represents the voltage stress to the test switch at contact closing. It is referred to the peak voltage at 24 kV power systems. After the decay of inrush-current the making switch SM1 is opened. The capacitor bank C1 is charged again along with changing selector switch SL to inductance LO in order to generate a breaking current of some 500 A at 50 Hz frequency. This current is applied to the test switch by closing the making switch SM1. During its first half cycle STEST is opened followed by the current interruption.

2.2 VACUUM TEST VESSEL The test switch STEST comprises a demountable vacuum chamber providing the opportunity to install various types of contacts for testing. The fixed contact is connected to making switches SM1 and SM2, thus to the high current circuit or the high voltage circuit. The moving contact is connected to ground and moved by the electro-magnetic drive from a commercial vacuum circuit-breaker. This provides the appropriate contact closing and opening speed. The contact gap is adjusted to 12 mm. The contacts are surrounded by a vapor shield mounted insulated from both the moving and the fixed contact (floating potential). The arrangement is designed for contacts of 45 mm in diameter. 2.3 FORCE MEASUREMENT A force measurement device enables the evaluation of contact separation force during opening operation subsequent to unavoidable contact welding. It is attached between the test switch and the electro-magnetic drive. For this purpose the drive and the test switch can be moved manually to determine the steady increase in force up to the moment of contact separation. This rising force is detected by a force sensor comprising a strain gauge.

3 MEASUREMENTS AND EXPERIMENTAL RESULTS A test series carried out with each of the contact types comprises 100 operations consisting of a making and a breaking test. Thus the changes of switching behavior during this period can be observed and evaluated statistically. Moreover the number of tests is referred to the requirements of the relevant IEC standard [1]. Special attention is given to the alteration of contact surface condition and hence the dielectric strength of the contact gap. 3.1 MAKING OPERATION At making operation the contact cap is stressed by 20 kV dc voltage and the contact gap decreases continually as the moving contact approaches the fixed one. At a certain moment the electric field strength exceeds the dielectric strength of the contact system and a breakdown occurs. Figure 2 shows a typical oscillogram of a making operation, indicating the


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the arc period anymore. Furthermore, the breaking current heating-up of the contact gap in this case is considerably low. So it can be assumed a cold vacuum gap is stressed by the recovery voltage. Even though a reignition is more likely to appear at the still increasing contact gap, a breakdown during the first quarter cycle after current zero is less critical in comparison to later restrikes with regard to voltage escalation [2].

Figure 2. Typical making operation oscillogram.

inrush-current, the contact travel, and the testing voltage. Beginning at the moment of breakdown, the damped inrush-current flows. Despite the appearance of contact bouncing the current flow continues until the discharge of the current sourcing capacitor bank is completed. 3.2 BREAKING OPERATION The present contact surface during the test series is affected by the previous switching cycles. Thus the making operations are observed in turn with the subsequent breaking operation and vice versa. The contacts are opened implicating the breaking up of contact welding. Due to the relatively low current conducted by the evolving arc, the smoothing effect of the arc is considerably reduced, ending up with the possibility of reignitions or restrikes after rising of the recovery voltage following current interruption. The sequence of a breaking operation is exemplified by the oscillogram in Figure 3. As the current flows, the contact opening starts and the current is interrupted at the first current zero. At this moment the contact gap is still increasing. Though the current is supplied by a capacitor bank, the voltage at the contact gap after current interruption rises to the residual voltage of the capacitor bank until the high current circuit is disconnected from the test switch. Afterwards the capacitive recovery voltage is applied to the contact gap as it has reached its final distance and the bulk of the contact bouncing has decayed. Due to the time interval between current zero and the rising capacitive voltage, the following dielectric behavior is not determined by

Figure 3. Typical breaking operation oscillogram.

3.3 DIELECTRIC CONDITION The momentary dielectric condition of the contact system can be evaluated by both the pre-arcing behavior during making operation and the occurrence of restrikes after current interruption, always in consideration of its statistical distribution. During the test series of 100 operations with 500 A breaking current using flat contacts of solid-statesintered CuCr 75/25 wt% material, pre-arcing field strengths yield between 4 kV/mm and 22 kV/mm with mean value of about 10.6 kV/mm. The development of the pre-arcing field strength in the course of the test series is indicated in Figure 4. The circles mark values obtained subsequent to a no-load breaking operation in order to measure the contact separation force (see chapter 3.4). In view of the stress on the contacts due to the burning arc early pre-arcing corresponding to low pre-arcing field strengths is most critical. The lower edge of the scatter spread shows decreasing field strengths during the test series using flat contacts in the course of the first 80 operations. After reaching the minimum field strength of some 4 kV/mm the dielectric behavior of the contact system recovers and results in a conditioning during the rest of the test series. In the case of the spiral contacts the values at the lower edge of the scatter spread decline during the first 46 operations – neglecting those values obtained after a no-load opening operation and the very first operations during the test series. The second half of the test series shows a recovery of the dielectric condition. Comparative investigations under the same conditions but all breaking operations at zero current and using flat contacts resulted in a stronger deconditioning effect during the test series. Moreover the latter series showed

Figure 4. Pre-arcing field strength.

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Figure 5. Cumulative frequency (squares) and approx. Weibull distribution function (lines) of pre-arcing field strength for both contact types.

a lower mean pre-arcing field strength of 7.6 kV/mm. At the last couple of operations the average pre-arcing field strength fell well below 4 kV/mm. This clarifies the significant influence of the – even though low – breaking current on the smoothing of contact roughness originating from the contact separation. The cumulative frequencies of pre-arcing field strength and the approximated Weibull distribution functions of the test series using both types of contacts are shown in Figure 5. Noticeable higher values of pre-arcing field strength are achieved using spiral contacts instead of the flat contacts, equally made of CuCr 75/25 wt% material. The mean field strength reaches 12.2 kV/mm during this series of tests and is therefore 1.6 kV/mm higher than in the case of the flat contacts. The five percent value obtained testing the spiral contacts amounts to 8.0 kV/mm compared to 6.8 kV/mm regarding the flat contacts. This results from the stronger decrease of the pre-arcing field strength and later recovery in the course of the test series using flat contacts (Figure 4). Even though the pre-arcing behavior of the flat contacts shows a degradation of the contact gaps dielectric strength resulting in partially very low pre-arcing field strengths, restrikes after current interruption can rarely be seen. Figure 6 shows exemplarily the oscillogram of a breaking operation followed by multiple restrikes. The first of them is occurring here during the fourth voltage cycle (nearly 90 ms after

Figure 6. Breaking operation oscillogram showing several restrikes.

Figure 7. Cumulative frequency of restrike moments.

current zero), followed by the others two cycles later at frequent intervals. Considering the first restrike after current breaking, dielectric breakdowns occur at the first half wave of recovery voltage as well as some cycles after. Moreover they can develop at voltage rise, near the peak value or at declining voltage (see Figure 6). However, a majority is detected within the period of rising voltage at the first cycle. Figure 7 indicates the time distribution of the restrikes, comprising the results of several test series. A large proportion of the restrikes occurs during the first voltage cycle and mostly as the voltage rises. But some breakdowns are detected as late as several hundreds of milliseconds after current zero. 3.4 SEPARATION FORCE Individual samples of opening operations during the test series are carried out including the determination of contact separation force. At four different stages of the test series and thus differently prestressed contact surfaces the standard making operation is succeeded by a manual no-load opening operation. These individual tests are marked by circles in Figure 4. After releasing the prestress of the contact force spring, the electro-magnetic drive is moved slowly resulting in a tractive force applied to the contact system. This force rises steadily until it approaches the welding force of the contact surfaces and the contacts are separated. Subsequently the force detected by the strain gauge drops to nearly zero, except

Figure 8. Force progression during contact opening.


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for the remaining tractive force on the moving contact due to the atmospheric pressure on the vacuum vessel. The development of the force during this period shows exemplarily the sensor signal and the contact travel in Figure 8. In the course of the tests the detected signal peaks correspond to welding forces in the range of 0.5 kN to 2 kN in consequence of 4.5 kA inrush-current (250 Hz). But the welding forces show no steady rise with an increasing number of previously performed switching operations.

4 DISCUSSION The variation of the contact surfaces condition owing to numerous capacitive switching operations can reduce the dielectric strength of the contact system in a critical range. Besides statistical distribution, the moment of a breakdown is determined by the previous history of the contact system. Hence the current contact surface condition must be taken into account [3, 5]. The measured time span between pre-arcing and contact touch predominantly lies between 1 ms and 3 ms. During this period the arc foot points are heating up the contact and are able to melt local areas on the surface. After contact closing both contacts are pressed by a spring force of about 1.8 kN in order to reduce transition resistance. This supports the formation of micro welds. During contact separation these welds are broken and can cause tips on the contact surface effecting deconditioning. As shown in Figure 4 breaking currents of 500 A associated with an arc leads to pre-arcing during the subsequent contact closing at field strength values down to 4 kV/mm. After a series of tests applying an inrushcurrent of 4.5 kA and opening the contacts at zero current, a pre-arc was struck between the contacts at a field-strength as low as 2 kV/mm during individual tests. As mentioned above the mean value of a series of 100 operations under these conditions met 7.6 kV/mm. This corresponds to the results obtained during similar investigations published in [6, 7]. Reference [8] compares breakdown field strengths following breaking tests with and without breaking current (2.0 kA, 1500 Hz inrush-current, 3 mm contact gap). At zero breaking current the breakdown field strengths were in the range of 3.2 kV/mm to 22.7 kV/mm but rising to between 5.0 kV/mm and 32.3 kV/mm after interrupting a current of 1.0 kA. The notable impact of the breaking current on the subsequent prearcing becomes obvious not only after a complete series of operations but also immediately after a single test. This becomes clear from the notably low pre-arcing field strengths observed at making operations subsequent to a single no-load contact opening (Figure 4). Besides the emergence of micro tips, rupturing of the partly melted contacts results in material displacement on the contact surfaces. After a complete test series an extensive tip is formed on one of the contact surfaces, whilst on the opposite contact surface a crater has developed. Predominantly the contact material is detached from the moving contact forming

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the cathode and deposited on the fixed contact, but the merged formation of both a tip and a crater within a melted area on the surface can be observed. After starting of material transfer at certain locations on the contact surface, pre-arcs or restrikes primarily stress these areas, supporting the formation of a growing material deposit or removal respectively. Thus the damage of surface is centered on a particular area on the contact, leading to a reduction of the effective contact gap. A tip height of around 2.4 mm could be observed after a series of 100 operations at 4.5 kA inrush-current and zero breaking current. This resulted in a reduction of the effective clearance to less than 10 mm between the contacts at full 12 mm contact stroke. Optical evaluation of the contact surfaces after the test series revealed a wider spread of the surface damage on spiral contacts in comparison with flat contacts. Due to a small arc movement the spiral shape leads to an extension of the damaged area and hence it is less centered on a limited sector on the contact surface. This proves the higher pre-arcing field strengths obtained during the test series using spiral contacts compared to flat contacts (Figure 5). Taking the event of a restrike into account, the very low quantity of restrikes allows no clear rating of the state of contact erosion at a certain moment during the test series. Furthermore, the restrikes can not clearly be linked with the present value of pre-arcing field strength. Restrikes do usually not coincide with low pre-arcing field strength at the preceded making operation and do not result in an exceptionally value at the proximate one. The moments of restrikes can be divided in three different time segments (Figure 7). Nearly 30% of the restrikes occur around 20 ms after current zero, i.e. during the first voltage cycle, in consideration of the short time interval between current zero and the rising voltage. Most of these restrikes develop at rising voltage slope. More than half of the restrikes are distributed almost uniformly over the second segment. It includes the time span between the second and the eighth voltage cycle. Finally a lower number of restrikes develops more than 150 ms after current zero. The cumulative frequency of restrikes in each of these time segments can be approximated by the straight lines plotted in Figure 7. While the restrikes during the first time segment are predominantly caused by the increasing field strength at the contact gap during the first voltage rise the second segment can be attributed to released particles initiating the breakdown [7]. During this period of time the vibrations of the test switch resulting from the preceding opening operation decay. Both breakdown initiating effects are superposed by a statistical breakdown probability arising from the continuous voltage stress on the contact gap. This provokes a smaller number of restrikes during the third time interval occurring even hundreds of milliseconds after current interruption. The observation of the contact welding behavior resulted in force sensor signals as shown in Figure 8. At the moment of contact separation the signal approaches the peak value of 1.5 kN. But the intensity of contact welding value is subject to


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a considerable wide scatter. A variation of welding forces by a factor of 15 was observed during investigations published in [9]. Hence the exemplarily measured peak value of the welding force does not rise steadily with an increasing prestress on the contact system by previous switching operations. The individual tests allow no statistical discussion of the welding behavior and show no distinguishable behavior of flat contacts and spiral contacts.

5 CONCLUSION For circuit-breakers especially capacitive currents represent a definitely distinctive demand on its performance. The tendency of contact welding gains more relevance, since low breaking currents reduce smoothing of the contact surface. The severest stress appears at no load contact opening, as obvious from the about 40 % longer mean pre-arcing time after zero current interruption in comparison to 500 A breaking current. The significant development of local protrusions on the contact surface affects the microstructure but also the macroscopic surface condition. This leads to the reconsideration of an appropriate contact design for capacitive switching duties. In particular the choice of contact material plays a major role, though the high current interruption ability of copper-chromium contacts is opposed to its limited restrike performance. The latter becomes apparent from the appearance of restrikes during the test series and the considerable erosion on the contact surfaces. To rate the capacitive switching performance of a certain contact system layout the evaluation of a large number of operations is required. This corresponds to the demand of standards and the real operation conditions as well as the need of a notable number of restrikes to allow a comparative study of different layouts.

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IEC 62271-100:2003-05, “High-voltage switchgear and controlgearPart 100: High-voltage alternating-current circuit-breakers”, Edition 1.1, 2003. I. Bonfanti, “Shunt Capacitor Bank Switching-Stresses and Test Methods”, Électra No. 182, pp. 165-189, 1999. C. Sölver, “Capacitive Current Switching-State of the Art”, Électra No. 155, pp. 33-63, 1994. R. P. P. Smeets and A. G. A. Lathouwers, “Capacitive Current Switching Duties of High-Voltage Circuit Breakers: Background and Practice of New IEC Requirements”, IEEE Power Eng. Soc. Winter Meeting, Singapore, Vol. 3, pp. 2123-2128, 2000. E. Slamecka, “Requirements for Capacitive Current Switching Tests Employing Synthetic Test Circuits for Circuit-Breakers Without Shunt Resistors”, Électra No. 87, pp. 25-39, 1983. E. Dullni, D. Gentsch, I. Kleberg, K. Niayesh and W. Shang, “Switching Capacitive Currents”, 21st ISDEIV, Yalta, Ukraine, pp. 407-410, 2004. E. Dullni, D. Gentsch, I. Kleberg, K. Niayesh and W. Shang, “Switching of Capacitive Currents and the Correlation of Restrike and Pre-ignition

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Behavior”, IEEE Trans. Dielectri. Electr. Insul., Vol. 13, pp. 65-71, 2006. Z. Zalucki and J. Kutzner, “Dielectric Strength of a Vacuum Interrupter Contact Gap After Making Current Operations”, IEEE Trans. Dielectri. Electr. Insul., Vol. 10, pp. 583-589, 2003. G. Ludwar, Schweißen einschaltender Kontakte im Vakuum, Thesis, Vienna University of Technology, Vienna, Austria, 1985. Florian Körner was born in Helmstedt, Germany in 1977. He received the Dipl.-Ing. degree from the Technische Universität Braunschweig, Germany, in 2003. Since 2003, he has been Scientific Assistant at the Institut für Hochspannungstechnik und Elektrische Energieanlagen (High-Voltage Technology and Electric Power Systems Institute), Technische Universität Braunschweig, Germany. His research activities are in the field of vacuum switching technology

Michael Kurrat (M’02) was born in 1963. He graduated in electrical engineering (Dipl.-Ing.) in 1988 and received the Ph.D. degree in 1993 from the University of Dortmund, Germany. He was engaged at the Felten & Guilleaume switchgear division in Krefeld, Germany. Since 2001 he has been Professor of the Institut für Hochspannungstechnik und Elektrische Energieanlagen (High-Voltage Technology and Electric Power Systems Institute) at the Technische Universität Braunschweig, Germany. His group’s research activities lie in the fields of switching in power circuits, as well as plasma applications for materials processing and surface treatment. Manfred Lindmayer received the Dipl.-Ing. degree from the Technische Universität München, Germany, in 1966 and the Dr.-Ing. degree from the Technische Universität Braunschweig, Germany, in 1972. As an assistant at Technische Universität Braunschweig he was engaged in research work in the field of switchgear, contacts, and switching arcs from 1966 to 1975. Between 1975 and 1981 he worked for Degussa, Hanau, Germany, and was responsible for contact testing and application. In 1981 he was appointed Professor at Technische Universität Braunschweig, where he was the head of the Institut für Hochspannungstechnik und Elektrische Energieanlagen (High-Voltage Technology and Electric Power Systems Institute) until his retirement in 2005. Dietmar Gentsch received the Dipl.-Ing. degree in mechanical engineering at the Institute for Mechanical Science, Technical University of Hannover, in 1992 and the Dr.-Ing. from the Institute for Electrical Science, Technical University Braunschweig, Germany, in 2002. He has worked in the field of interruption performance of vacuum interrupters in medium-voltage switchgear and simulations in test devices. Since 1993 he is employed at ABB AG, Calor Emag Medium Voltage Products in the development of materials and in the design of vacuum interrupters. Since 2001 he is the team leader of research and development of vacuum interrupters.