Generator + GCB + GSU Transformer + 2 UAT reliability

[radug]

Hello,

I have seen a tender for a power plant where the single-line diagram shows Generator + GCB + GSU Transformer + 2 UAT.

The only way of feeding generator auxiliaries is through the UAT, as no station transformers of MV lines are available.

The connection between Generator + GCB + GSU Transformer + 2 UAT, is made using IPBs.

I never encountered a configuration like this, only with 1 UAT instead of two. When 2 UATs where present, was in Combined Cycle Plants where each UAT was fed from a different generator.

So my question is: Which would be the benefit of using 2 UATs instead of only 1?

As I see it, a fault in the IPB would cause both UATs to be offline (GCB trips, HV side of the GSU Transformer CB trips and CB at the secondaries of the 2 UATs trip).
Also, as each of the UATs does not have a CB in the IPB side, an internal fault in one of the UATs would also make both of them offline.
The only benefit I see is if the fault is at secondaries of UATs.

 

[slavag]

It's common configuartion for coal power plants ( 300-700MVA per unit ).
Where UAT is about 25-30MVA every one.
Benefit is less SC current on aux bus and separation of load to two ( or four ) aux buses.

 

[radug]

Thansk slavag,

So it is not for increased reliability reasons.

I will check if the SC decrease is worth the extra cost of 1 UAT + IPB.