Simulated faults on directional, ground, overcurrent relays with emphasis on the operational impact on mutually coupled, intact lines

It is common practice to build two unrelated circuits on the same physical pole structures. When a ground fault occurs in one line of the mutually coupled pair, it can induce sufficient current in the intact line to cause a false trip. This is an effect of the zero sequence current. From an active transmission system, three pairs of 120 kV transmission lines were selected for investigation. Both double and single line-to-ground faults were simulated with and without mutual coupling. The resulting ground currents reported at the directional, ground, overcurrent relays that protect these lines were compared and the time-over-current settings of these relays were examined. The maximum current on the intact line does not occur at the common bus. It occurs at a point where the mutually coupled lines separate. This phenomenon must be considered in the design of the protection scheme