Two Improvements Related to Overcurrent Functions for Bus Protection in Distribution Systems

This paper shows two improvements related to the overcurrent functions (OCFs) for bus protection in distribution systems. The first topic is the use of two high-set definite-time OCF, in conjunction with the inverse-time OCF, for the main circuit breaker (MCB). A zone-interlocking signal can block one high-set definite-time OCF, while the other one is the fastest possible backup action. The second topic is the inclusion of a backup function at the MCB for cases of failure of a downstream relay, by using the OCF of this MCB to first trip the circuit breaker related to the corresponding downstream feeder. If the OCF of the MCB remains asserted, then this OCF trips the MCB after a suitable coordination time. Both improvements can be implemented by using the IEC-61850 standard.