Upon loss of field excitation of a generator operating in parallel with others, what must be done?

Correct Answer: Option A — The parallel-connected generator which has suffered the loss of field excitation must be immediately disconnected from the bus by tripping its circuit breaker.

Loss of field excitation turns a parallel-connected alternator into an induction generator drawing massive reactive power from the bus. Per IEEE 45, the affected unit must be tripped immediately via its circuit breaker to prevent severe system voltage collapse and generator damage.