Lessons Learned: Protecting Lines Without Communication-Assisted Tripping

The power system is built around Generators, Transformers, Buses, Transmission and Distribution circuits, and Motors. Each of these must be protected by relay application. With so many unique elements, coordination is required and can become complicated. Coordination is typically simplified using communication-assisted tripping between relays to form a zone of protection with instantaneous clearing. This paper will cover how mutual coupling and apparent impedance can affect backup ground protective elements, and how to coordinate these elements in areas where communication-assisted tripping is not available. It will show lessons learned examples of time delayed sequential clearing for a fault near the remote terminal, overreaching for non-parallel mutually coupled lines, and the effects of line reach and current flow on a three-terminal line.