A New Guidance for Circuit Breaker Rate Selection Including the Impact of Zero-Sequence X/R Ratio

Abstract

The multiplying factor (MF) is critical for the correct design, sizing, and selection of protection devices in a power system. This factor depends on the X/R ratio at the point of failure. The standards establish an equivalent MF for the X/R ratios for both three-phase and single-phase fault accordingly for engineering practices. However, X/R ratios for single-phase faults ignore how much from the total X/R ratio comes from the positive-sequence or zero-sequence X/R ratio. This research demonstrates that the total X/R ratio at the point of failure is not the only variable that affects how the multiplying factor changes. The results demonstrate with dynamic short-circuit simulations using EMTP-ATPDraw, which it is also necessary to establish the MF considering the differences of the X/R ratios of each sequence, respectively. To support the engineering designer in the correct selection of the protection device, this article proposes a methodology complementary to the standards, which allows establishing the multiplying factor using the X/R ratios of positive and zero sequence separately. The results show that both oversized and undersized design situations can occur when ignoring the zero sequence in particular scenarios.