Interpretation of NSDD and Restrike in Capacitive Current Switching Test on Medium Voltage Vacuum Circuit Breaker

Abstract

Capacitor banks are installed in an increasing number to control power quality issues and improving the power factor tocounteract the reactive power in the transmission and distribution networks. Due to load fluctuations switching of capacitorbanks is a typical phenomenon. Capacitor bank is normally having concentrated capacitance in contrast with distributedcapacitance. It generally draws more current than unloaded cableor line in practical cases up to several hundred amperes.Hence, switching of capacitor bank causes a very high rate of rise of transient recovery voltage across circuit breakercontacts. This scenario can be simulated in testing laboratories by a voltage source connected to a circuit breaker whichagain is connected to a large capacitance in terms of IEC called capacitive current switching test. Non-sustained disruptivedischarge (NSDD) is a voltage breakdown after the vacuum circuit-breaker open the fault current which will not cause therecovery of power frequency current. Restrike is different from NSDD, the recovery of power frequency current betweenthe contacts will make the breaker failed. It’s not easy to show the distinction between NSDD and restrike during some typetests. The paper presents the study of performance evaluation of the circuit breaker during the capacitor current switchingduties. A statistical analysis of failure of circuit breaker during capacitor current switching tests are included in the paperto help the manufacturers & utilities, so that the care can be taken for trouble free services. The paper also discusses thechanges made in the IEC 62271 – 100, Amendment1:2012 for conducting the capacitive current switching tests.