Power swing relaying in the Texas Panhandle - an out-of-step Odyssey

Power swing block and out-of-step trip relaying has been used at Southwestern Public Service Company (SPS), an operating company subsidiary of Xcel Energy Inc., for more than 50 years. SPS´s unique geographic location in the extreme southwest corner of the Eastern Interconnect allows for islanding opportunities to occur due to weak interconnecting transmission lines with the rest of the Eastern Grid. Because of this, the SPS system utilizes over 600 power swing relays to allow for proper islanding to prevent complete system blackouts. Setting these relays has evolved from simplistic “rules of thumb” philosophies using electro-mechanical relays to sophisticated, logic based philosophies using modern microprocessor relays. This paper will discuss the evolution of power swing relay setting philosophies for the SPS system over a 50 year span. It will show how philosophies have changed due to technological innovations in protective relays and analysis tools. It will also show how philosophies have changed due to the implementation of North American Electric Reliability Corporation (NERC) Reliability Standards.