11kV Test transformer

[EkWeetNie]

We have a client that requires a 415V/11kV transformer for testing 11kV substations under no load conditions in his factory before shipping the product to customers.

So we need a 415V to 11kV transformer for powering a 1.5MVA 11kV to 1kV substations.

I suspect because this is being done under no load conditions our main concern would be to have a transformer big enough to deal with the inrush current requirements for the 11kV/1kV transformer.

So for example : the 1.5MVA 11kV/1kV transformer and assuming 12 x the rated current as the inrush current for 100msec -> 1.5MVA rated current is 78A on primary side -> 12 x 78A = 936A.

I then assume the 415V/11kV transformer can deal with 40 times the rated current for a short period of time without damage (the average short circuit withstand rating). So 936A/40 = 23.4A. So by this logic and calculation the 415V/11KV transformer needs to be at least a 11kV*23.4A = 257,400kVA or 260kVA.

So here is my question: Am I calculating this correctly ? (The calculated size this seems a little BIG to me.)

 

[ausphil]

Don't forget, when you do a no load test on a transformer, you don't switch the bastard on at full volts. You raise the volts slowly until you get to the required voltage, so you don't really get the big inrush as you would when you energise it straight from the system with a switch.

I would hazard a guess that it is a bit big.

Ausphil

 

[ScottyUK]

The inrush current can only be as large as the source can deliver. The impedance of the small test transformer will act to limit the inrush; you need to factor in the source impedance to your calcs.


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