Hidden failures as potential faults in relay protection are lethal for power systems. Multi-grounding fault in the secondary circuit of a potential transformer (PT) is a typical hidden failure. When this hidden fault happens, zero-sequence voltage of computer relaying will shift. Then it will result in the mis-operation of zero-sequence directional protection, which may cause a cascading failure. In this paper, we analyze the impact on zero-sequence directional protection caused by multi-grounding fault first. Then in order to solve the problem, we analyze the characteristic of the affected voltage and current. We find that the zero-sequence voltage of the special phase is in phase with its negative sequence voltage under normal circumstance, but this is not true in this hidden failure. As a consequence, we propose an identification method of multi-grounding fault in a PT and a new zero-sequence direction algorithm that can avoid the mis-operation of the zero-sequence directional protection. Finally, the results of simulation prove the correctness of the theoretical analysis and the effectiveness of the identification method and the new zero-sequence direction algorithm. (c) 2016 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.
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