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Transformer Short Circuit Current Calculations

Transformer Short Circuit Current Calculations

Transformer Short Circuit Current Calculations

(OP)
Hello everyone,

Does anyone have experience using the Point-to-Point 3-Phase Short Circuit Current Calculations published by Cooper Bussmann in either their SPD or Electrical Plan Review Documents?

I am trying to get a handle on the bolted Isc at the secondary of a second transformer in a system (480primary/208 secondary) including motor contribution downstream.

Since there are no sold examples of this, I have run through calculations from one of our projects and I am coming up with a secondary Isc of about half of that which is listed in the typical transformer characteristic charts.

Since I have a definite primary current and not an Infinite primary I am assuming that my calculations are correct as the Secondary Isc would be limited by the Primary Isc if it is defined.

Secondly, with respect to motor contribution for these calculations, would the motor contribution provided after the secondary of the transformer transpose its full value to the 480v components ahead to the primary side or would the transformer limit some of the motor contribution due to the windings and turns ratio?

Thanks in advance for any thoughts on this matter.

RE: Transformer Short Circuit Current Calculations

No way to tell from what you posted. No values, no calcs!

Yes if you have some primary source impedance the SCC at the transformer secondary may be less than what would be with an infinite bus. By how much? It depends on the actual values involved.

Motor contribution to a bus will be affected by the transformer impedance, if the transformer is between the motor source and the bus in question, same way as any other short circuit current source would be. Turn ratio is also applicable at the same time.

You need to draw the impedance diagram with applicable voltage sources and solve the network. Refer to IEEE Red Book for a better reference and method of SCC calculations.


 

Rafiq Bulsara
http://www.srengineersct.com

RE: Transformer Short Circuit Current Calculations

And, are you trying to calculate the available short circuit current for switchgear selection or actual maximum transient fault current?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Transformer Short Circuit Current Calculations

I found a link to the calculation that you referenced and you can upload it.

In addition to utility source impedance which can be included, I noticed their example uses a fairly low impedance transformer.  For a 1500 kVA transformer, 5.75% impedance is more common and this will knock the fault current values down considerably from the ones based on 3.5% impedance.

While the source impedance will also have an effect on the results, it won't be that dramatic.

This might explain why your results are half of what the Bussmann results show.  Again, what type of system are you modelling and what impedance values did you use?

RE: Transformer Short Circuit Current Calculations

(OP)
Thank you for the intital responses.

I will upload a sample calculation when I get back into the office.

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