Mutual Coupling In Subtransmission Systems—a Real-World Experience

This paper presents a case study in which a single-line-to-ground fault on a 34.5 kV subtransmission line caused a misoperation of the adjacent subtransmission line protection. Investigation of the event revealed mutual coupling as the root cause behind the misoperation, which is often overlooked during the modeling stage by protection engineers for subtransmission systems. Where mutual coupling is suspected in overhead lines, using zero-sequence directional elements is not recommended, as they may cause false trips. In this case, the relay on the adjacent line had the best choice ground directional element algorithm and was correctly configured to declare directionality based only on negative-sequence voltage-polarized impedance-based elements. Nevertheless, the relay still operated on 51NT, which was supervised by directional control and was set to trip for forward faults.

The paper begins with a refresher on the concept of mutual coupling between overhead conductors, followed by a detailed event analysis that examines fault oscillography records, symmetrical components, and ground directional overcurrent elements in the relay. It also describes the subtransmission system modeling and mutual coupling studies that were performed to confirm the presence of mutual coupling.

By combining investigative efforts from both relay settings and system modeling standpoints, the paper offers valuable lessons learned and proposes a corrective action plan via relay setting changes to improve security against future misoperations. Finally, the paper shares practical challenges at Los Angeles Department of Water and Power at this time and their ongoing mitigation plans to identify lines prone to this misoperation and enhance security through subtransmission system modeling and relay settings development over the entire service territory.