Controlled switching of shunt capacitors

During field measurements on shunt capacitor banks several characteristics were investigated. For these measurements Eskom utilised its mobile transient measuring facility which is equipped with a 12 bit, Nicolet 500 system, a fibre optic system for measurement signal transmission and resistive/capacitive voltage dividers. For inrush current limitation during shunt capacitor bank energising a series damping network or a pre-insertion impedance is common practice. Reduction of transient phenomena is determined by a suitable choice of damping network element parameters or pre-insertion impedance. By precisely controlling the instant of closing it is possible to almost entirely to eliminate or reduce to a minimum, the impact of the inrush current phenomena. The inrush currents are of a moderate nature, but the substation busbar voltage can exhibit significant transients. In the case of back-to-back capacitor bank energising the impact on the circuit breaker itself is of major interest. Inrush currents of significant nature can flow. In most instances this can lead to unexpected and abnormal wear of circuit breaker contact systems. It is recommended for each new shunt capacitor bank application to consider the combination of a series damping network and controlled closing. Such a combined scheme offers an effective solution to different switching situations occurring in practice. A consideration of what represents a critical or noncritical switching application for either the network or the circuit breaker itself then becomes the basis whether or not to apply controlled switching