Unit Aux. Transformer (UAT)

[kssschsekhar]

Dear Folks,
I would like know the benefits, if UAT are connected at the terminals of GTGs (Gas turbine Generators ), instead of STGs (Steam Turbine G) in a combined cycle power plant.
Advanced Thanks...

 

[ScottyUK]

Normally a unit auxiliary transformer provides power for the unit it is connected to, so the GT would have one UAT and the ST would have one UAT. You might consider that the GT is essential for operation of the ST, but not vice-versa.


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If we learn from our mistakes I'm getting a great education!

 

[kssschsekhar]

One project has one GTG with HRSG, two STGs and two aux. boilers (coal based). In this case, which of the following options are better for feeding plant aux. loads
1. one UAT connected at GTG terminals + one UAT connected at STG terminals
(These UATs will feed unit loads as well as station loads)

2. one UAT connected at GTG terminals + One Station transformer

 

[ScottyUK]

Impossible to say without seeing the single line diagram, but instinctively I would prefer a dedicated station auxiliary transformer given the choice.


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If we learn from our mistakes I'm getting a great education!

 

[kssschsekhar]

Dear Scotty,
Thanks for Info. Pl. find the attached conceptual SLD which shows two options. Which option is better?

 

[TurbineGen]

With just the information you provided, I'd go with option #2. This way if STG-1 or it's GSU transformer is down for maintenance, STG-2 can run (assuming it gets it's power from the same UAT as STG-1). Of course, option #2 looks like it will be a bit more expensive.

Is there a reason why there is no breaker between STG-2 and T20?

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[slavag]

Hi.
From my point of view---> option#2 is better.

TurbineGen,
Good Q, but I would see it in another way:
or both steam turbine with GCB, or both w/o.
BTW, I think Kssschsekhar forgot delete GCB for the STG1 after revesion of option#1
Best Regards.
Slava

 

[ScottyUK]

Option 2 looks the better solution operationally. The Station transformer will be more expensive than a second UAT, and it costs another bay in the HV substation but it is a worthwhile cost.

With a 3km line between the transformer and the HV sub I'd very much prefer to see a local GCB to protect the generator.


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If we learn from our mistakes I'm getting a great education!

 

[slavag]

Scotty.
I fully agree with you.
GCB is better from protection/operation/maintanance and synchronization point of view.
Best Regards.
Slava

 

[kssschsekhar]

Thanks for discussion.
1. Yes, I forgot to delete the GCB for STG#1 in option-2. Good observation

2. With all your discussion, it seems reasonable to provide GCB at STG#2. I would like to think more of it. Thanks for this.

3. As two boilers (coal based) are there in this project apart from HRSG, Steam turbines are not exclusively depending on GT .
As both UATs in Option-1 will be sized for 2x100% plant load, still option-2 is recommended!!!

 

[slavag]

Hi.
I think if "Steam turbines are not exclusively depending on GT" , possible system w/o SAT. In case of GCB, possible feed plant load from the grid and after synchronazing generator.
In additional you wrote that size of every UAT enough for the whole plant load.
But in all cases a good style add GCB for the second STG.

Best Regards.
Slava