Hello Norm:

The turbine is connected to the generator. The spinning turbine causes part of the generator (the rotor) to also spin at 1,800 revolutions per minute inside the rest of the generator (the stator). No matter what power level the plant is at, the generator's rotor spins at 1,800 rpm and the generator puts out electricity at 22,000 kilovolts.

As power level is increased or decreased, the field on the generator is correspondingly vaired so as to keep the output voltage at 22,000 kilovolts.

 The control on the generator field is performed by the exciter.

 It's not the best analogy in the world, but it's a little like cruise control on a car that keeps the speed at the desired setting by adjusting the throttle to regulate the flow of gas to the engine.

In this event, a detector measuring the differential field across the generator (kinda like monitoring the difference between the car's actual speed and the cruise control setting) signaled a problem.

From full power, that signal caused an automatic trip of the turbine and an automatic trip of the reactor.

Had the power level been lower, that same signal might have produced a turbine trip without a reactor trip.

The reactor trip/turbine trip from full power caused a momentarily low level condition in the steam generators.

If you lift a boiling pan of water off the stove, the "froth" level drops because the heat reduction produces fewer steam bubbles.

The steam generator level dropped to the point where the auxiliary feedwater pumps automatically started to restore the level.

For this kind of trip, that's not unusual.

All safety systems (control rods, etc) worked as they should have.

The complication seems to have been in the switchyard.

The ongoing maintenance on one breaker caused one of the transmission lines between the plant and its electrical grid to de-energize.

But two other transmission lines (the minimum allowed) remained in service. From this report, it's not clear if the generator differential field signal is related to the ongoing work in the switchyard.

My hunch is that they are not connected, but it's possible.

Thanks, Dave