What is the effect of low frequency on Turbo- generator set if it runs for prolonged time?

Answer 1

If this is the only generator on line such as on a ship then you would not have to worry about motorization of the Gen. Low speed of parallelled generators causes the slower generator to give up its load to the other generators. If speed is reduced enough the generator will become motorized and draw current from the buss. A reverse power relay is supposed to trip a motorized generator off of the buss.

In any case it is possible to have additional generators trip off line due to being overloaded as the slow generator is not taking its share of the electrical load. The sudden loss of one or more generators off of a buss can result in a progressive failure ending in a blackout.

With a single generator on a buss....
Low freq operation due to the prime mover running slow could be caused by a mis set throttle/speed setting, governor fault, or a steam supply issue such as a boiler casualty.

Low speed operation (low freq) can cause over heating of the generator. Air circulation and cooling within the generator box is reduced. Low frequency operation of the electrical buss reduces the reactance of motors causing an increase in current demand. Lower speed also reduces the amount of voltage produced, the automatic regulator will increase drive current to the field winding in an attempt to maintain buss voltage. If the regulator cannot maintain voltage then low buss voltage results further increasing current demands of the connected loads

These overheating factors factors combine and the amount and rate of overheating is directly proportional to the amount and time of reduced speed operation.

Steam pressure casualty also impacts the turbine and condenser resulting in less vacuum and higher exhaust trunk temperatures further slowing the turbine speed. The close proximity of the generator to teh condenser and exhaust trunk mean that addition heat will be applied to the generator box.