A stochastic study of hidden failures in power system protection

Recent studies have shown that power systems protection mechanisms have played a major role in propagating disturbances. All of the last five major Western Systems Coordinating Council (WSCC) events (the North Ridge earthquake, December 14, 1994, July 2 and 3, 1996, and August 10, 1996), involved either false trips of line protection relays or generator protection equipment. Using an importance sampling based algorithm on the 179-bus WSCC equivalent system, we modeled the sequence of rare events involving generator trips and incorrect zone 3 relay operations. Due to possible loss of generation and load, frequencies were monitored for the necessity of load shedding. The algorithm uses the Newton–Raphson method for accuracy but reverts to DC load flow when the Newton–Raphson algorithm fails to converge within three iterations. The resulting sequence of rare events and its corresponding probability are used to detect weak links in the power system. This information can be beneficial not only in areas such as service and maintenance scheduling and in planning, but also in determining locations where an investment in improving the protection system is warranted.