Analytical approach to study effect of mutual coupling on ground over-current relays

Zero-sequence mutual coupling between transmission lines has been known to adversely affect various facets of transmission line protection. In specific, the ground over-current and distance relay elements are most impacted by presence of mutual coupling between transmission lines. Mutual coupling causes spurious tripping of ground over-current relays and leads to over/under-reach issues in distance relays. The impact of mutual coupling on distance relays is well documented. However limited research and development has been witnessed in the use of analytical techniques to evaluate impact of mutual coupling on ground over-current protection elements. This paper presents an analytical approach to accurately and quickly determine whether ground relays on un-faulted lines will trip due to the mutual coupling effect. The proposed approach analytically identifies optimal fault locations to evaluate potential for mis-operation of ground over-current relays. It is tested on transmission lines which are a part of two asynchronous grids (ERCOT and SPP). It precisely identifies relay mis-operations on the mutually coupled lines. In addition to being highly accurate, it is amenable for implementation on conventional phasor-domain analysis tools used by most of the utilities.