Third Auxiliary Transformer in CC 2x1 Plant 1

[autobot_C]

We have a one line of a 2x1 Plant (2 GTGS and 1 STG) where two Auxiliary transformers are fed from a tapbox from the GTG trains to a MV switchgear. What would be the benefit of adding a third auxiliary transformer in this case and tying into the MV switchgear? Wouldn't there be a benefit had it been another gas turbine instead?

 

[rcw retired EE]

The biggest benefit is companies like my employer make more money on the extra equipment.

There is little benefit to adding a third Unit Auxiliary Transformer (UAT) on the STG bus in a 2 CTG on 1 STG (2 on 1) Combined Cycle configuration.

Reliability or availability don't improve much compared to feeding the STG auxiliary loads from two CTG UAT's. If both CTG UAT's are down, the CTG's can't run, so there is no steam to operate the STG. The third transformer keeps lights on, but doesn't make power.

With 3 AT's, the three busses are tied together with tie breakers and MV cables or bus for redundancy. Besides costing a lot more for the extra bus or cable plus a main and two tie breakers, the extra complexity tends to reduce reliability.

Three AT’s does make it easy to distribute the loads with each turbine feeding its own loads. Also, one CTG tripping may not dump the STG auxiliary loads. But a Main-Tie-Main with automatic fast transfer between two UAT’s can get close to the same availability.

Another cost savings with only two AT's is deleting the STG generator breaker, unit connecting the GSU and synchronize using the HV circuit breaker.

Even with three or four CTG's, many plants only use two AT's. Some designs use one AT per generator with no cross ties so the AT can be sized just to handle one unit's loads, saving cost on the AT, the tie breakers and bus. But then there are challenges in feeding any common BOP loads without affecting reliability.

 

[autobot_C]

Thanks. Your explanation was to the point and helpful.

 

[prc]

wilson, what is CTG?

One trend coming up with combined cycle plants where one GTG + one STG are used is to use only one GSU with two LV windings drawing power from the two generators.

 

[davidbeach]

CTG = Combustion Turbine Generator.