Using a GSU transformer 3

[brianle]

Can a Generator Stepup transformer that has nearly the same ratio of the input to output voltage be used in place of a damage one if the MVA rating is high enough? The original one was 300 MVA, 15KV delta- 230KV Y. Can a 445 MVA, 22KV delta - 345KV Y transformer be used instead?

Thanks,

Brian

 

[electricpete]

seems ok to me fwiw.

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(2B)+(2B)' ?

 

[electricpete]

There will be analysis required before replacement. One item to consider might be fault current on the high side of the transformer.

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(2B)+(2B)' ?

 

[brianle]

Thanks electricpete.

Would the fault current issue just require different relay settings or some other minor modification, or would it make the new transformer unusable?

Thanks,

Brian

 

[ScottyUK]

You need to look at the impedance of the transformer - it is key to limiting the fault level. The basic premise of using this transformer seems reasonable.


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

 

[brianle]

Thanks ScottyUK.

Will it make much difference if the designed input voltage of the new one is 22.8KV instead of 22KV?

Thanks,

Brian

 

[waross]

At that ratio you would have to reduce the input voltage from 15 kV to 14.67 kV to achieve 230 kV out. This will reduce the kva capacity of your generator to 14.67/15 or 97.78%.
Dropping the voltage of the transformer from 345 kV to 230 kV will drop the capacity of the transformer to 230/345 or 66.7%

If you go with 22.8 kV instead of 22 kV you will be able to use the full KVA capacity of the generator.
Recalculate your PU impedance on the new KVA rating based on the reduced voltage before calculating the available fault current.
PU Impedance = X% at 345 kV.
PU impedance = X% x (230kV/345kV) at 230 kV.

Bill
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"Why not the best?"
Jimmy Carter

 

[ScottyUK]

I would expect that you can make up the difference on the tapchanger if you are required to maintain a constant generator terminal voltage for the benefit of auxiliary loads fed from the machine terminals.

Your machine may struggle if you need to meet a maximum reactive despatch at a time when the grid voltage is a little high, but this combination is quite uncommon. If this is a short-term problem until the GSU is repaired then you can probably apply to the TSO for a derogation if the machine capability is limited to the extent that it can't meet contractual obligations. If this is a permanent 'fix' then you may have to see how a limitation on reactive capability affects your contractual obligations to the TSO and let the lawyers argue it out.


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

 

[brianle]

Thanks to all. You've given me enough to offer it for further evaluation.