Estimation of short circuit capability of Generator Circuit Breaker for a generator fed fault

The Generator Circuit Breaker (GCB) is a core component of a power plant, protecting both the generator and the power transformers. Currently, the only standard that addresses GCB ratings is IEEE C37.013. This standard defines the primary short circuit rating of a GCB based on a system fed fault. When attempting to determine the required ratings for a GCB, it is sometimes difficult to check the capability of a particular model of GCB with respect to the characteristics of the project specific HV system, connected generator, and the associated MV distribution system. The difficulty increases with the emergence of large generators contributing high symmetrical fault current with a very high degree of asymmetry. Many of today’s large MVA generators can produce a maximum degree of asymmetry up to 130% for generator fed faults. Currently there are no guidelines available to the end user to determine the suitability of the GCB tested at a certain degree of asymmetry when being applied to a system with a higher degree of asymmetry. This paper provides a discussion on selection of a GCB to interrupt generator-source faults with a high degree of asymmetry. Suitability for a given application may have to be confirmed by the GCB manufacturer.