Hi, Could anyone please assist o 2

[dannywallace]

Hi,

Could anyone please assist on the following;-

I have a naturally cooled 2.5MVA (11kv/400v) power transformer with an impedance value of 6%. The question is I need to understand the short circuit on the secondary under 'forced' cooling conditions. The primary short circuit is 250MVA. Under normally rated conditions I would expect a short circuit of 60 KA, my issue is under forced ventilation conditions we obtain an extra 40% of power taking the transformer from a 2.5 MVA to a 3.5 MVA. Question does the secondary short circuit remain at 60 kA considering that the cooling maintains the winding temperature relatively constant and therefore impedance remains 6%? Your response will be most welcome. kind Regards, Danny.

 

[Skogsgurra]

Assuming that the Isc is calculated from the non-forced cooling rated power, it will stay the same when you improve cooling.

The fact that you (when transformer better cooled) can load the transfomer 40 % more does not influence the scort-circuit current. It will stay at 60 kA.



Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

Impedance is quoted on a per unit basis, so for your transformer the 6% impedance quoted on a 2.5MVA base equates to 8.4% impedance when calculated on a 3.5MVA base. Your fault level will not change.

If you think through the underlying principles, the transformer core and windings don't change due to the ONAF rating, only the means of extracting heat from the transformer. The ohmic value of the transformer impedance will not change because it is determined by winding resistance and leakage reactance inherent in the design of the core and windings.


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Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[dannywallace]

Thanks Gunner, Scotty much appreciated.