capacitive current switching
TRANSCRIPT
Synthetic Tests of Capacitive Current Switching Using a Test Vessel
Florian Körner1, Manfred Lindmayer1, Michael Kurrat1, Dietmar Gentsch2 1Institut für Hochspannungstechnik
und Elektrische Energieanlagen, Technische Universität Braunschweig,
Schleinitzstraße 23, 38106 Braunschweig, Germany
2ABB AG, Calor Emag Mittelspannungsprodukte,
Oberhausener Straße 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. 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. The 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. Both the appearance of pre-ignitions at contact closing and the behavior under capacitive voltage stress after breaking are observed as indications of the contact surface conditions.
I. 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 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].
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.
II. EXPERIMENTAL SET-UP The synthetic test circuit as shown in Fig. 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 inrush-current is 4.5 kA at 20 kV charging voltage across C1. This voltage also represents the voltage stress to the
Fig. 1. Basic synthetic test circuit.
XXIInd Int. Symp. on Discharges and Electrical Insulation in Vacuum-Matsue-2006
1-4244-0192-5/06/$20.00 ©2006 IEEE.
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 wave STEST is opened followed by the current interruption. 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 d.c. 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. 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.
III. 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. At making operation the contact cap is stressed by 20 kV d.c. 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. Fig. 2 shows a typical oscillogram of a making operation, indicating the contact travel, the inrush-current, 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. 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 Fig. 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 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
Fig. 2. Typical making operation oscillogram.
Fig. 3. Typical breaking operation oscillogram.
Breaking Current
Contact Travel
Voltage
Contact Travel
Inrush-Current
Test Voltage
first quarter cycle after current zero is less critical in comparison to later restrikes with regard to voltage escalation [2]. 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-state-sintered 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 Fig. 4. It shows the tendency of a slightly decreasing field strength. However, comparative investigations under the same conditions but breaking tests at zero current resulted in a stronger deconditioning effect during the test series. Moreover the latter series showed a lower mean pre-arcing field strength of 7.6 kV/mm. At the last couple of operations the mean pre-arcing field strength fell 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. 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 (Fig. 4). 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. This can partly be contributed to the absence of an evident deconditioning effect over the 100 operations, accompanied by the distinguishable higher scattering. 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 Fig. 5.
Even though the pre-arcing behavior of the flat contacts shows a degradation of contact gaps dielectric strength resulting in partially very low pre-arcing field
strengths, restrikes after current interruption can rarely be seen. Fig. 6 shows exemplarily the oscillogram of a breaking operation followed by multiple restrikes. The first of them is occurring during the fourth voltage cycle (nearly 90 ms after 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 Fig. 6). However, a majority is detected within the period of rising voltage at the first cycle.
IV. 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 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
Fig. 5. Cumulative frequency (squares) and approx. Weibull distribution function (lines) of pre-arcing field strength for both contact types.
Fig. 6. Breaking operation oscillogram showing several restrikes.
Fig. 4. Pre-arcing field strength (test series using flat contacts).
Breaking Current
Voltage
Restrikes
Flat Contacts
Spiral Contacts
cause tips on the contact surface effecting deconditioning. As shown in Fig. 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 inrush-current 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. Even lower values between 5 kV/mm and 7 kV/mm were detected during similar investigations published in [6, 7], but with an inrush-current of 4.7 kA (270 Hz) and a larger number of tests (> 250). Ref. [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. 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 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. Ref. [9] introduces contact surface investigations after a single making operation. After applying an inrush-current of 5.1 kA a height of the resulting protrusion of up to 0.7 mm was measured. Microscopic investigations of the transfer of material between the contacts are described in [8]. 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.
V. 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.
REFERENCES [1] IEC 62271-100:2003-05, “High-voltage switchgear and
controlgear – Part 100: High-voltage alternating-current circuit-breakers”, Edition 1.1, 2003
[2] I. Bonfanti et al., “Shunt Capacitor Bank Switching – Stresses and Test Methods”, Électra No. 182, pp. 165-189, 1999
[3] C. Sölver et al., “Capacitive Current Switching – State of the Art”, Électra No. 155, pp. 33-63, 1994
[4] R. P. P. Smeets, A. G. A. Lathouwers, “Capacitive Current Switching Duties of High-Voltage Circuit Breakers: Background and Practice of New IEC Requirements”, Proc. IEEE Power Engineering Society Winter Meeting, Singapore, Vol. 3, pp. 2123-2128, 2000
[5] E. Slamecka et al., “Requirements for Capacitive Current Switching Tests Employing Synthetic Test Circuits for Circuit-Breakers Without Shunt Resistors”, Électra No. 87, pp. 25-39, 1983
[6] E. Dullni, D. Gentsch, I. Kleberg, K. Niayesh, W. Shang, “Switching Capacitive Currents”, Proc. 21st ISDEIV, Yalta, pp. 407-410, 2004
[7] E. Dullni, D. Gentsch, I. Kleberg, K. Niayesh, W. Shang, “Switching of Capacitive Currents and the Correlation of Restrike and Pre-ignition Behavior”, IEEE Trans. on Dielectrics and Electrical Insulation, Vol. 13, No. 1, pp. 65-71, 2006
[8] Z. Zalucki and J. Kutzner, “Dielectric Strength of a Vacuum Interrupter Contact Gap After Making Current Operations”, IEEE Trans. on Dielectrics and Electrical Insulation, Vol. 10, No. 4, pp. 583-589, 2003
[9] T. Kamikawaji et al., ”An Investigation into Major Factors in Shunt Capacitor Switching Performances by Vacuum Circuit Breakers with Copper-Chromium Contacts”, IEEE Trans. on Power Delivery, Vol. 8, No. 4, pp. 1789-1795, 1993
E-mail of authors: [email protected]