Probabilistic short-circuit analysis in unbalanced three-phase power systems

Short-circuit evaluation is a very important step in the design of circuit breakers, bus systems, grounding systems, and so on. Usually, the worst-case conditions are assumed as reference, avoiding the inherent random nature of some aspects (mainly fault location, system conditions and fault type) affecting the short-circuit current amplitudes. A probabilistic fault current analysis allows one to take into account the variability of these aspects. The probabilistic approaches up to date refer to normally balanced three-phase system; however, several transmission and distribution systems are characterized by line and load unbalances that can not always be neglected. In this paper a probabilistic approach based on a Monte Carlo simulation is proposed where the evaluation of the short-circuit currents is effected by the method of phase-coordinates. To reduce computational efforts, the system prefault conditions are calculated by using a linear form of the three-phase load flow equations. Several numerical applications are presented and discussed with reference to the three phase unbalanced IEEE 13-bus distribution test system.