Low Voltage Gen bus differential - Necessary?

[Live2learn]

In my industrial plant I have (4) 480V 4W+G 2000kW .8pf generators that all terminate in paralleling switchgear. From the switchgear we go to closed transition switches and that feeds the loads.

Question:
I went through the shop drawings which are about 2 years old and noticed know bus differential protection is provided in the gear. The last system I touched was Medium Voltage and it had it. Isn't this differential system utilized for stator faults? I am wrong to think that it isn't necessary at 480V and only really applicable in medium voltage HRG systems?

Thanks,

 

[dpc]

Some type of fault detection is required, but differential relaying is never "required" except maybe by an insurance company. It is provided because it provides much faster fault clearing for internal generator faults. The larger the generator, the more likely it is to have differential applied. It's a little like insurance - how much is enough?

87G relaying is fairly rare for 480 V generators, although I have done it for 480 V generators of this size. But if these don't have it, they don't have it. Likely they have voltage-restrained overcurrent relaying along with ground relaying and other protection for fault detection.



David Castor

http://www.cvoes.com

 

[rbulsara]

You say gen bus and also refer to stator. They are two separate and different kinds of differential protections.

For generator this size, generator differential protection is recommended (not required). For 480V switchgear bus, differential protection is generally a gross overkill. However if this indeed is a 16000A bus (4x 2MW at 480V), may be the designer felt compelled to "protect" all that bussing.

4-2MW gens in parallel at 480V does not sound like best of the design decisions, to begin with but that is a mute point at this time. Even at MV bus differential is difficult to justify.

Rafiq Bulsara

http://www.srengineersct.com

 

[rcw retired EE]

Some of our clients require bus differential on 2 MW units.

It's a toss up whether it is needed or not.

If there is a multifunction generator relay protecting the generator including the 87G feature only adds one set of CT's and wiring.

The differential will reduce the switchgear arc flash hazard on the switchgear side of the breaker. It does not help as much on the generator side of the generator breaker because fault current will continue to flow after the breaker opens and the generator is tripped.