When the Load of a Generator is Suddenly Dropped

This is a very basic simplified example – generator main trips, facility PCL (Program Logic Control) senses trip (loss of demand) and within a millisecond 1) reduces the stator voltage (delivered via separate stator generator) at the prime generator, 2) Electro actuated steam supply valve begins to throttle down reducing the supply to generator, 3) Purge valve on steam loop opens on high line and vents to atmosphere, 4) Coal delivery to boiler is slowed to "idle" feed.

Once all of the functions above are in motion, the PLC senses and stabilizes the following 1) Maintain required stator voltage to maintain in plant operational systems, 2) adjust steam supply to prime generator to maintain in plant operational systems, 3) Close stream purge valve once balance between demand and supply of steam is stabilized, 4) Adjust coal feed to boiler(s) based on current steam demand.

Obviously much, much more goes on during a demand loss event but I thought this would be the best way to describe it without going into all of the nuts, bolts, springs and washers.

The simplified scenario above is based on an operating system used at an older (built 1929) coal/steam/generation plant that was decommissioned in 2004 that I had worked at.

I am also an electrical engineer but trying to reply the question based on my experience in the related field. Electrical protection is classified in three categories. Class A, where the fault is within the generator and immediate tripping the generator is required to minimise the damage. In this case over speeding of the turbine generator set will take place. Class B trip is the one where the fault in out side the generator and therefore first the turbine trip signal is given and all the stored energy is allowed to be spent before tripping the generator. fault is fed till such time. Class C trip is used where there is provision of Low Pressure bye pass. In this case when the fault is out side the generator (in the grid), only the main circuit breaker is tripped isolating from the faulty grid but continues to supply the local load. This also permits re-synchronization of the generator quickly after the fault is cleared or faulty section of the grid is taken out of grid. As rightly pointed out, LP bye pass system passes the extra steam to the condenser.