Considerations for applying IEC standards in short circuit calculations and breaker selections in cogeneration plants

This paper discusses a recent experience with the utilization of IEC standards 909 and 56 for calculating short circuit currents and selecting medium and high voltage circuit breakers for cogeneration plants. The paper addresses the "near-to-generator" fault category as defined in these standards. Such a category involves decaying AC current components as well as DC current components with relatively large time constants. The paper depicts the relevance of these factors when applied to circuit breaker ratings and the engineering concerns associated with such exercises.