Current Interruption Transients Calculation, 2nd Edition

7Capacitive Load Switching

7.1 General

The capacitive load switching cases to be considered are the switching of shunt capacitor banks, unloaded transmission lines and unloaded cables. Capacitive load currents are 90° leading and current interruption leaves a DC voltage on the load circuit. The recovery voltage across the circuit breaker will thus be the difference between the source AC voltage and the load side DC voltage as shown in Figure 7.1 for banks with earthed neutrals.

Similar to inductive load switching, there are a number of load circuit configurations of interest and these will be addressed as we cover the various cases. However, capacitive current switching differs from inductive load switching duties in that energization inrush currents are a major consideration. An extension to this are outrush currents, i.e. discharge of a capacitor bank through a circuit breaker into a fault. Normal practice is to apply some form of mitigation to limit inrush current magnitudes and frequencies to values specified in IEC 62271‐100.

For each of the capacitive load circuit configurations, we will discuss both energization and de‐energization and associated current or voltage transients and their mitigation, as applicable.

7.2 Shunt Capacitor Banks

Shunt capacitor banks are three‐phase lumped capacitive circuit elements with either earthed or unearthed neutral points. Two cases need to be considered, single capacitor banks where the energization is relative to the system and ...