Hi Tom I am afraid your comment about over supply to the grid does not ring true with my memory of electrical theory. Granted I probably finished my electrical trades course well before you were born but I don't think things have changed since. A power generation device produces a voltage and has a capacity to deliver a certain amount of power. If the load is below the capacity of the generating device then its Automatic Voltage Regulator reduces the excitation to maintain the voltage. The only reason for stopping them would be to reduce unnecessary wear and tear to extend the unit's service life. I should imagine a major overhaul on one of these things would be horrifically expensive. <br>I would be interested to know what sort of alternators they use. Whether they are shunt excited or PMG excited. I would assume they are synchronous alternators.