Capacitor bank switching with vacuum circuit breakers

After interruption of a capacitive current sometimes Non Sustained Disruptive Discharges (NSDDs) are observed when using vacuum circuit breakers. According to network calculations, NSDDs generate significant over-voltage on the terminal of the capacitor bank to ground and across the circuit breaker (CB) terminals. In a full scale experiment at 12 kV on an 8 Mvar capacitor bank artificial NSDDs are produced using a Triggered Vacuum Gap. 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 ground varies from 1.5 to 4.2 pu, and to maximum 5.2 pu. across the CB terminals. The effectiveness of surge arrestors 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 ground and 3.2 pu. across the CB terminals. A single surge arrestor on the neutral point of the capacitor bank yields the same level of protection and, therefore, is a cost effective alternative. An application guide for switchgear and arrestors is presented.