Investigating A Transmission Line Misoperation Caused By Mutual Coupling Effect

Like many other utilities, Dominion Energy uses multiple lines to transport power through a narrow right-of-way line corridor in densely populated areas. Often, two or more lines share the same right of way, or two or more circuits share the same transmission towers. Magnetic mutual induction occurs in multiple-circuit lines that run near each other using the same right of way; resulting in an induced current flowing in the mutual conductor. This induced current results in a quantifiable zero sequence current that affects the reach of ground distance elements and complicates setting selection for distance elements. This paper examines the case of a 115 kV transmission line that experienced an A-phase-to-ground (AG) fault. During this event, the relay on the mutually coupled, unfaulted transmission line misidentified the fault and tripped on Zone 1 ground distance. Fault records of the event were used to determine the ways that mutual coupling impacts both the ground directional elements and the reach of ground distance elements. This paper presents the investigation of the event and provides recommendation for setting a ground distance element in the presence of mutually coupled transmission lines.