480V Generator Protection

[WoodrowJWeen]

For a typical 480V, 3 Ph, generator, say under 2 MW, is the output circuit breaker(s) really protecting the generator winding? Obviously the NEC says you need protection for the feeder conductors but not sure if the breaker does anything for the generator. I've been under the impresssion that the generator output CTs and controls internally protect the stator winding from damage.

For a typical genset I would have the output breaker rated 125% of full load current but the NEC doesn't give this number as a requirement.

 

[rbulsara]

NO. Generator protection is much more than just overcurrent. You protect the windings using thermal sensors. Breakers essentially becomes a tripping device against short circuits, in conjunction with other protective relays such as backup overcurrent 51V and other functions such as loss of field and reverse power, etc.

Rafiq Bulsara

http://www.srengineersct.com