An assessment of the impact on the operations of directional, ground, overcurrent relays on an intact line from a fault on an adjacent, mutually coupled line

It is common practice for utilities to build two unrelated circuits on the same physical pole structures. The resulting mutual couplings of these lines raises concerns over zero sequence currents. When a ground fault occurs in one line of the coupled pair, it can induce sufficient current in the intact line to cause a false trip on the intact line. From an active transmission system, the author selected three pairs of 120 kV transmission lines to investigate. Both double and single line-to-ground faults were simulated with and without mutual coupling. The author compared the resulting ground currents reported at the directional, ground, overcurrent relays that protect these lines and the time-over-current characteristics of these relays. The maximum current on the intact line occurs when the fault is located where the mutually coupled lines separate rather than at the common bus. This phenomenon must be considered in the design of the protection scheme