Three winding Generator Step-up Transformer

[kssschsekhar]

Dear Folks,
Looking for expertise advice regarding three winding generator step-up transformer.

For connecting two generators to one 3-winding step-up transformer:

1. What is the criteria for selecting percentage of impedance between ST1 to ST2 (between secondary windings)?

2. If one generator is not available, and another generator is fully loaded, how to maintain voltage regulation at transformer terminals, where generator is not connected?
Thanking you,

 

[TurbineGen]

I'm having a hard time understanding your second question. Are we talking about synchronous generators or induction generators? You can increase the voltage of a sychronous generator by changing its field strength in the exciter. When you do this, you must stay within the "D" curve of the generator. Sometimes this means you need to sacrafice some kW in favor of KVAR to maintain voltage.

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

 

[slavag]

Hi.
It's a very hard Q.
I try understand this a few mounth.
In lot of new projects I see this configuration with few variants.two steam turbines, one steam and one gas turbines.
Only one UAT. Not same voltages on the LV side of trafo.
As far, as I understand, it's other logic of regulation.
Voltage regulated not bu AVR of generators, voltage regulated by OLTC on the HV side of three-winding trafo.
Best Regards.
Slava

 

[kssschsekhar]

Thanks for participation. It is clear for my second question. Please discuss for my first question.
Thanks...

 

[rcw retired EE]

Select ST1-ST2 impedance to limit fault current to within the generator breaker rating.

If high voltage winding is H, and LV windings are X & Y, and Zhx is impedance between H and X winding, select impedance so Zxy = Zhx + Zhy. (All Z on same base.) This keeps trasnformer costs down.

I would have to analyze it more to figure out what the best selection is for regulation.

Usually we are using a 3-winding transformer to save the cost of another HV breaker, minimize space, and to limit the short circuit levels on the generator busses.