Reliability implications of increased fault currents and breaker failures

Increased power system fault currents impose stresses that may exceed the design rating of circuit breakers installed decades ago. Excessive stresses on the breakers increase their probability of failure. Breaker failures result in multiple simultaneous outages and are major contributors to system unreliability. As a result, it is desirable to quantify the contribution of increased fault currents to breaker failure. We propose a methodology that quantifies the effect of the relative statistical frequency of the different types of faults in the power system (from single-phase to three-phase faults) on the distribution of fault currents for a given circuit breaker and the implications on system reliability. The proposed work is illustrated with the IEEE 24-bus reliability test system where circuit breaker arrangements are explicitly modeled. The proposed methodology provides accurate fault current distributions through breakers (to closely reflect the expected fault conditions in the field) that help identifying common mode failures in cases of breaker failure. Applications of the proposed work include (a) identification of probabilistic models of common mode outages in real time and (b) reliability assessment of overdutied circuit breakers.