Neutral impedances in fault analysis

Object-oriented modeling of transformers and other Y-connected devices preserves neutral nodes and impedances, making them available as points to fault. This is in contrast to conventional modeling in which all neutral nodes are eliminated by modifying the parameters of the devices. Phase-domain expressions are developed for faults between an energized node and a neutral node, and between two neutral nodes. An example using a three-circuit transformer shows the effects of faulting various neutral points. The results show that fault currents may be either higher or lower than for standard phase-to-ground faults, depending on the interaction of source, transformer, and neutral device impedances. The modeling was verified by comparison with phase-to-ground faults on a conventional transformer model.