Differential Relay Operation.

[kh2]

Does the Differential Relay operate with zero current from on side and no internal fault?

Three single phase Generator/Transformer differential relays with percentage and harmonic restraint. They are GE model 12BDD15B relays. Tap setting is 2.9 Amp, zero restraint pickup 0.87 amps, 25% slope setting. Current transformers 4000/5 wye on the 13.8 kV sides and 400/5 delta on the 230 kV side of the transformer.
The full load current of the generator/transformer is 4.3 amps on the secondary side. The Gnerator is 13.8 kV, three phase wye grounded, and connected to a stepup transformer delta/ wye, 13.8 kV/230 kV. The question is: Do you disable the differential relays when you open the generator beaker but still the 230 kV and 13.8 kV are energized to feed the station service (load 300 kVA or 0.015 amps on the secondary) from the stepup transformer? The generator curerent transformers in this case are supplying zero current but the unbalance is far below the pickup current of the relays. Appreciate your input on the subject.

 

[busbar]

 
While it may not be too common to use harmonic-restrained transformer-differential relays to protect rotating machinery, I don’t think there is a problem with the 2-winding BDD application. Empirically, relay producers and engineering/applications texts do not call for a 3-winding differential relay to ‘exclude’ a station-power transformer from the differential zone.

Else, it would be sort of a “killing ants with a sledgehammer” application.

 

[RRaghunath]

No, the Generator/transformer overall differential protection is still active though the generator is off and the load current (due to auxiliary power) is too low.

I am sure the protection will do its job if and when a fault occurs in the G/GT/UAT zone.

It is important to see that the CT wiring on neutral side of generator is not disturbed so as not to cause maloperation of the protection, even though the generator is down.