Coordination Study: Double breaker size for parallel generators?

[Magic Gorge]

Doing a coordination study for a parallel generator system. There are two 800kW generators with 1200A breakers, and 1200A main breakers at the generator switchboard. When I look at coordination, should those 1200A breakers be 2400A in reference to the breakers on the load side of the switchboard? Or should it be looked at radially? Looking for opinions on how other engineers might view this, since the load sizes suggest both gens are always online. No other scenarios have been outlined for generator operation.

 

[ron]

Consider the load flow.

The two 1200A in series with each other will carry the same power and never need to carry more than 800kW.

 

[Magic Gorge]

From a coordination perspective, doesn't the load side equipment "see" 1600kW? For instance, a 1200A overload on the 1200A feeder circuit is only half the capacity of the switchboard breaker protection.

 

[ron]

The load side 1200A breaker (the generator input to the SWBD) is just acting as a disconnect for most cases. You will not coordinate it, just set it the same as the 1200A breaker at the generator, which you will try to coordinate with the generator characteristics.

It would be nice if there was a fault somewhere in the generator zone of each of the 1200A feeders when the generators were running, that one of those darn 1200A breakers would trip allowing the other generator to continue to feed the downstream load, rather than drag it down too and feed the fault, but usually it is too slow. :weeping:

 

[lielec11]

I agree with ron. The two input breakers will never see more than what their individual generators can deliver due to directional power relays that will likely be installed. Adjust them to the same settings as the generator breakers upstream based on the damage curves.