The physical processes during the capacitive switching duty for vacumm circuit breakers have been studied by T. Kamakawaji, T. Shioiri, T. Funahashi, Y. Satoh, E. Kaneko and I. Oshima, “ An Investigation into Major Factors in Shunt Capacitor switching Performances By Vacuum CBs With Copper Chromium Contacts ”, IEEE Power Engeneering 1993 winter meeting Columbus OHIO
They found that the pre-ignition at contact closing and the subsequent inrush current heavily eroded the contacts leading to detached particles. These particles cause late breakdowns which appear as NSDD’s. The NSDD or non-sustained disruptive discharge is a disruptive discharge associated with current interruption that does not result in the resumption of power frequency current or, in the case of capacitive current interruption does not result in current in the main load circuit. Breakdown frequencies of typically 3/1000 where found under the experimental conditions studied. A restrike can generate an overvoltage of 3 pu. between the terminals of the capacitor bank, whereas a NSDD is accompanied by a sudden voltage shift of the neutral capacitor bank voltage, which leaves the voltage across the capacitor unchanged, but creates an overvoltage of between 1.5 and 5 pu. on the terminal of capacitor bank to earth. Due to the rare occurrence and the random nature of the phenomenon, the relation between NSDD and overvoltage is not well understood.
The occurrence of a Non Sustained Disruptive Discharge leads to a shift of the voltage of the neutral point of the bank. Although the phenomenon in itself is rare, the over-voltages produced are deterministic and depend on the recovery voltage of the phase in which the NSDD occurs and on the moment of breakdown. The experimental study shows that the overvoltage after NSDD is limited to about 4.2 pu. on the capacitor bank terminals. Surge arresters are shown to be an effective means to limit this overvoltage. As cost efficient solution it is proposed to use a single surge arrester on the neutral point of the capacitor bank.