Abstract
According to the relevant IEC standards vacuumcircuit-breakers have to meet various needs, e.g. theinterruption capability, making operations, and dielectricstrength. Switching of capacitor banks, overhead lines, orcables leads to very small currents in comparison withshort-circuit currents. After current interruption thecircuit-breaker must withstand twice the peak value of thesystem voltage. Furthermore, restrikes can lead to voltagemultiplication. The conjunction of relatively smallbreaking currents with high voltage stress must beconsidered in detail.This work introduces a test arrangement for combinedtests of making operation, current interruption, anddielectric stress of a vacuum gap under capacitiveswitching condition. A test vessel permits investigations ofvarious contact materials and designs. It is connected to asynthetic test circuit which provides the appropriate testcurrents and capacitive voltage. Both the appearance ofpre-ignitions at contact closing and the behavior undercapacitive voltage stress after breaking are observed asindications of the contact surface conditions.
I. INTRODUCTION
Circuit-breakers have to fulfill diverse requirementsconcerning interruption capabilities and dielectricstrength. Typically a high switching duty and adistinctive dielectric strength are in the main focus ofdesign, construction, and testing of circuit-breakers. Thismeets with the needs of short-circuit currents andovervoltage stress respectively. Additionally switchingof capacitive loads i.e. capacitor banks, cable loads oroverhead lines, represents a specific operating conditionthat requires extensive performance. The testspecifications given by IEC standards [1] correspond tothese requirements.The connection of a capacitive load to the system leadsto inrush-currents of up to several kiloamperes atfrequencies significantly higher than the powerfrequency [2]. Typical currents at the interruption ofcapacitive loads are in the range of some tens of amperesto hundreds of amperes [1]. Subsequently the recoveryvoltage across the circuit-breaker rises up to twice thesystem voltage. Furthermore, in the event of a restrikeafter current interruption the capacitor can be reloadedcausing an increase of the trapped charge and followingan 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 theirsurface are taking centre stage. Accordingly a testarrangement was developed to examine various contactdesigns and materials.II. EXPERIMENTAL SET-UPThe synthetic test circuit as shown in Fig. 1 includes thetest switch STEST connected to the high current circuit(left) and the high voltage circuit (right). The former issupplying the test current for both the making and thebreaking operation by discharging the capacitor bank C1over either inductance LC or LO respectively. Therewiththe appropriate test currents of up to 4.5 kA at afrequency of 250 Hz for making tests and some 500 A at50 Hz frequency for breaking tests are generated. Thehigh voltage circuit supplies the capacitive recoveryvoltage up to 50 kV (50 Hz) which is applied to the testswitch STEST subsequent to a current interruption.For making tests the capacitor bank C1 is charged using aseparate charging system (not shown) while earthingswitch SG1 is in open position. The selector switch SLconnects the associated inductance LC providing afrequency of 250 Hz. The high current circuit isconnected to the test switch STEST by closing of makingswitch SM1. Closing of STEST leads to a dampedinrush-current flow accompanied by discharging of thecapacitor bank C1. The maximum peak value ofinrush-current is 4.5 kA at 20 kV charging voltage acrossC1.


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