Current-limiting inductors used in capacitor bank applications and their impact on fault current interruption

Utilities often use in-line current-limiting inductors for capacitor bank installations to limit the severity of outrush currents from the bank(s) into close-in line or bus faults. Because of the very high inherent frequency of the current-limiting inductors used in these applications, circuit breakers used for protecting the capacitor bank bus can be subjected to very severe transient recovery voltages after the interruption of faults limited by the inductors. The impact of current-limiting inductors on circuit breaker interrupting performance was demonstrated by an incident involving a phase-to-ground fault in one of four 138 kV, 57.6 Mvar capacitor banks at ComEd's Silver Lake Substation. A 138 kV, 2000 A circuit breaker used for protecting two capacitor banks was unable to interrupt the fault current because of the severe transient recovery voltage produced by a 1 ohm current-limiting inductor in the circuit. This paper describes the details of the analysis involved in identifying the sequence of events that led to the circuit breaker being unable to interrupt successfully and highlights the impact of current-limiting inductors on fault current interruption in capacitor bank applications.