Non uniform load sharing by generators

Q: We have 02 motor generator sets each of same KW rating and speed etc running in parallel and having pure inductive load. Thrice it happened that 01 set got tripped on reverse power protection. 01 set operates with 0.65PF lagging while other runs with <0.5PF leading. After tripping of MG set, remaining set's current decreases from 42A to 35A. If we want to synch again two MG sets while plant is running at 100% power, is there any danger of tripping?(if remaining set got tripped, plant will also trip).

A: If your load is truly 100% inductive then you are just likely seeing nuisance trips on reverse power (real power). When multiple gensets are paralleled together with no real load this can occur. It happens due to slight tuning differences amongst the controls of each genset. There are a few things you should do to improve the situation. The first and best solution would be to add some small amount of real load to the system. Care should be taken not to add too much additional load as you do not want to overload the system when the control rod drives need to operate. I would suggest 3-5% of the generator kW rating should be enough. If this is not possible you could slightly desensitize your reverse power trip protection. Generally this protection should protect the generator from motoring but it does not need to be set at 0kW import. You could adjust a few percent of rating in the import direction generally without causing problems with the engine portion of the generator.

Finally you may be able to adjust the gains of the throttle system to improve performance of both units and tune out any slow oscillations of power between units when running unloaded.

Regarding the resynchronization of gensets. If one genset remains online and you want to reconnect the second, you should be able to without causing a trip, provided your genset controllers are paralleling, of which I have little doubt. When the second generator reconnects you may again experience a trip of one or the other unit on reverse power merely due to the slight instabilities associated with operating without loads. The chances of losing both gensets together is relatively low, although not impossible. Generally one genset or the other will experience the reverse power condition you described and will trip, leaving the generator which was driving into the system online.

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