Ground Fault Current Return Path 3

[JulesBeland]

Have a situation were a genset (600V detla connected) is used to provide emergency power. The system voltage is 600v, normaly supplied from a 1mva delta / wye transformer that is resistance grounded via a 69ohm NGR. The questions is, with the high side of the 1mva transformer open, would the NGR on its secondary provide the return path to the genset if a ground fault were to occure when the genset is in use.

 

[jghrist]

Yes

 

[davidbeach]

If the generator is connected in delta, then it doesn't source ground fault current; with no source there is no return path.

 

[waross]

Generally the transfer switch will disconnect the transformer from the loads fed from the emergency generator.
Emergency generators are generally the same size or smaller than the service transformer. Unless the generator is much larger than the transformer it will not be able to energize a back-fed transformer.
There are some exceptions.
First the transformer must be reliably and provably disconnected on the primary.
Then the generator may be connected to the secondary and then the field energized. or
The generator may be connected and allowed to pull down while Under Frequency Roll Off assists in recovery. or
The generator may be over-sized to start a large motor and so able to energize the transformer with not too much drama.
Connected loads may not react well to the voltage dips occasioned by energizing a transformer.
Now if by some unusual but not impossible method the emergency generator is back feeding the transformer there are several conditions where the wye winding may supply ground current.
If the transformer primary is delta wound, the delta will force a stable neutral wye point. This is a variation of a wye:delta circuit which is used at times to provide an artificial ground, and at other times as a ground detection circuit or both.
Second in order of effectiveness is the three legged core transformer. A three legged core transformer has a "Phantom delta" effect. The phantom delta will act to stabilize the neutral point but the effect may not be as strong as a wound delta.
Third, if one phase is grounded the voltage across the other two legs of the wye raise towards line voltage. However saturation of the unfaulted phase windings increases the current in the unfaulted windings and tends to stabilize the neutral. I don't believe that the saturation effect is as good as either of the delta effects. Further, with a 69 Ohm NGR I doubt that the unfaulted voltages would rise to saturation.
Yes
The NGR may supply ground current but from the delta:wye transformer. David Beach is correct in that there will be no ground return path through the generator.
BUT
It is highly unlikely that the transformer will be in the circuit when the emergency generator is running.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

I'll have to qualify my yes answer. The transformer secondary has to be connected to provide the ground source. The generator will furnish all of the positive- and negative-sequence current. The delta-grd wye transformer will supply all of the zero-sequence current.

If the transformer secondary breaker trips, or if anything else (like a transfer switch) disconnects the generator from the transformer, then there will be no ground source.