Capacitor bank switching with vacuum circuit breakers

Abstract

After interruption of a capacitive current sometimes Non Sustained Disruptive Discharges (NSDDpsilas) are observed when using vacuum circuit breakers. According to network calculations, NSDDpsilas generate significant over-voltage on the terminal of the capacitor bank to earth and across the circuit breaker (CB) terminals. In a full scale experiment at 12 kV on an 8 MVAr capacitor bank artificial NSDDpsilas are produced using a TVG. The over-voltage depends on the momentary voltage difference across the breaker and, hence, varies with the phase angle. The over-voltage on the terminal of the capacitor bank to earth varies from 1.5 to 4.2 pu, and to maximum 5.2 pu. across the CB terminals. The effectiveness of surge arresters on the limitation of the over-voltages is measured. The over-voltages are limited to 2.2 pu. on the terminals of the capacitor bank to earth and 3.2 pu. across the CB terminals. A single surge arrester on the neutral point of the capacitor bank yields the same level of protection and, therefore, is a cost effective alternative.