The influence of zero sequence X/R relation on circuit breaker ratings

The asymmetric short-circuit current is a determining factor in the selection of circuit breakers. The asymmetry in the current is directly dependent on the X/R ratio at the point of failure. ANSI and IEC standards present the methodologies for determining the X/R ratio at different voltage levels. The standards propose multiplying factors depending on the X/R ratio. The multiplying factor is used to calculate the capacity of the circuit breaker during the "contact parting time". In the methods used to estimate the X/R ratio and the multiplying factors, a three-phase fault is assumed as the worst-case scenario, neglecting the asymmetry that occurs during a single-phase fault. This article presents a study on the impact of the zero sequence X/R ratio on asymmetric current based on theoretical and computational framework. Scenarios including single-phase short-circuits shown important differences with legacy multiplying factors. Finally, it is identified the situations where these new multiplying factors could be used.