Determination of interrupting ratings of MV switchgear breakers for power plant applications by IEEE methods

The method of calculating symmetrical short circuit current is unambiguous. However, the process for calculating the asymmetrical short circuit current and selecting a circuit breaker to interrupt the required asymmetrical short circuit current of the system has several alternative methodologies that have been applied. There are differences in approach for selecting the multiplying factor to ascertain the effect of asymmetry among the existing IEEE standards and among the previous and latest version of certain IEEE standards. The approach of the main governing standard IEEE C37.010 has evolved over time. This leaves the end user with the question of whether a circuit breaker selected by the older approach is conservative enough. This paper will identify the differences in the approaches and the conservatisms inherent within these approaches for calculating the symmetrical interrupting duty of circuit breakers. It will also establish a realistic approach to estimate the adjusted symmetrical capability to facilitate optimization of the selected circuit breakers on ANSI/IEEE methods and used for medium voltage unit auxiliary switchgears of new and existing generating plants.