Using a Mobile Substation Transformer As a Temporary GSU

[constantlylearning]

I would like to get some opinions on the following scenario:

We have small generation unit that has a GSU (Generator Step-Up transformer) that is delta-connected on the 34.5 kV generator side and grounded-wye-connected on the 138 kV transmission side. The generator is a connected in a grounded-wye configuration. In addition we have a mobile substation transformer that has an MVA rating that is not too dissimilar from the MVA rating of the GSU. The winding connections for the mobile unit are delta on the 138 kV side and grounded-wye (normally) on the 34.5 kV side.

We are currently considering whether we can use the mobile transformer as a temporary substitute for the GSU, should the GSU ever fail. In this scenario we would connect the wye side of the mobile to the generation bus and leave the neutral bushing of the mobile ungrounded. (FYI: X1, X2, X3 and X0 all have the same BIL level.) Disregarding the limitation that the generator will not be able to "sense" ground faults on the transmission system, is there any reason we can't do this as a temporary solution?

Internally we've discussed the aforementioned problem with ground faults, which we think we can solve with some additional instrument transformer and voltage-sensitive relaying. We've also discussed the dissimilar vector relationships between the two transformers and possible problems with phase rotation, which we don't believe will be a hurdle. There is also some concern about neutral shifts, but we believe this is mitigated by the presence of the delta on the HV (138 kV) side. From a power flow perspective, we don't see any major problems with this, even though it's not ideal.

Is there anything else we aren't considering or are possibly overlooking?

I welcome any input or feedback.

 

[davidbeach]

Not being able to source ground current into the 138kV side may be a significant problem, or may be no problem at all. What does the owner of the 138kV system say?

 

[constantlylearning]

We are the grid owner as well as the generation owner. While the lack of a zero sequence link between the two systems is certainly less than ideal, we believe we can mitigate this by selective use of undervoltage relays or zero sequence voltage relays on the 138 kV side.

 

[electricpete]

is the generator wye point solidly grounded or impedance grounded?

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(2B)+(2B)' ?

 

[constantlylearning]

Good question. I forgot to include that. It is grounded through a resistor.