Relay Performance During Major System Disturbances

Power systems in the United States and abroad experienced several wide-area disturbances in the last 15 years including the largest blackout on August 14, 2003, in the Midwest and Northeast U.S. and Ontario, Canada, which impacted millions of customers. On September 28, 2003, the Italian network was separated from the rest of Europe, and the whole country of Italy fell into darkness. The July 2, 1996, and August 10, 1996, major system disturbances in the western U.S. impacted millions of customers. All of these disturbances caused considerable loss of generation and loads and had a tremendous impact on customers and the economy in general. Typically, these disturbances take place when power systems are heavily loaded, are operated outside their intended design limits, and experience multiple outages within a short period of time. These wide-area disturbances are typically characterized by large power oscillations between neighboring utility systems, low network voltages, and consequent voltage or angular instability. The aim of this paper is to explain which relay systems are most prone to operate during stressed system conditions, and why relay systems operate, to share experiences and lessons learned from the past, and to suggest protection system improvements to lessen the impact of blackouts and hopefully lead us toward their prevention in the future