The plant was operating at 92% power when a ground fault on a land steam exhauster caused the supply breaker to a motor control centre (MCC) to trip. This supply breaker could not be reclosed. The loss of the MCC resulted in a loss of the hydrogen side seal oil pump for the main generator that is powered from that MCC. The plant continued operation as the licensee procedures and technical manual of the turbine-generator manufacturer (Westinghouse) allow full-load operation with the hydrogen side seal oil pump shut down. When operators initiated implementation of a procedure to restore the seal oil system, a small hydrogen explosion occurred and a reactor power reduction was initiated. Shortly thereafter, with the plant at 85% power, a large explosion and fire occurred and the turbine was immediately tripped. The reactor was automatically tripped by the recently installed anticipatory trip circuit. Following the large explosion, the fire burned for several minutes and was extinguished by the CO2 system. The explosion resulted in the generator housing lower skirts being thrown into some neighbouring equipment. The missiles also broke two large hand wheels for valves and damaged some small test valves and non-safety-related level instruments.