Diesel Generator Overload Response

Greetings Plainsmith and Welcome to CR4,

In the case of a single, standalone DG tied into the grid (non-power utility plant) based on the unit size and scenerio you provided:

1. The DG main would sense an overload and open to prevent damage to the DG.

2. The engine may momentarily sag but should not stall if the main is calibrated correctly.

What if the overload relay was set so high that it would not trip the engine?

If the main did not trip, most likely the engine would stall. There is also the possibility of damage to the generator, regulator control or prime mover prior to the engine stall.

That is what we believe as well. I was hoping someone might have a story of an actual overload event and be able to explain how the system responded with some detail on how long it took to pull the engine down, etc...

dear,

good explanation.

Infact, to prevent the possible damages, there are so many back up relays for grid protection.

1. Infact, one stand alone dg is there, so that, during such a heavy load shedding, 200% loading does not come on an individual dg set.

2. Under frequency relay / Under voltage relay / Over load relay in KW as well as in KVAR / reverse power relay / df/dt relay for grid islanding / vector surge relay - dv/dt that aleways tries to prevent the possibility of heavy over loading conditions.

So, question of WHAt IF OVER LOAD RELAY IS SET TO HIGH SHOLD NOT COME.

Who can tell?

What is the point of over-riding trip systems designed to prevent such an occurrence [rhetorical question]?

dear,

this is not a theoritical question.

In practice, you will find the damages / faults occurred due to by p[assing of relays only. Even in substations of 80 MVA or such large ones, I have seen Instrument team doing this in order to continue supply, & relay being faulty. Later, under healthy supply conditions, thing remain un attended.

So, the main responsibility of an engineer is to prevent such occurrances, & try to keep spares / stand by arrangements , so, this situations does not arise.

Independent of any under voltage or under frequency shut downs, it depends on the makeup of the generator, if it doesn't have field boost or a permanent magnet exciter, it may not trip the main breaker because the voltage will decay, engine may stall, field may collapse. Depends on system sophistication.