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Parallel XMRs PU calc

Parallel XMRs PU calc

Parallel XMRs PU calc

(OP)
Hi Ladies and Gents

happy Holidays

Can please somebody explain how per unit calculation  will be done  for two transformers in parallel ? one is off turn ratio .

Thanks
 

RE: Parallel XMRs PU calc

Off turn ratio, = the open circuit voltage difference will cause a circulating current, limited by the impedance of both transformers = smoking hot

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

RE: Parallel XMRs PU calc

Treat the voltage difference caused by the turn ratio difference as a voltage source in one leg of the parallel network.
 

RE: Parallel XMRs PU calc

Why would you want to do that?  It will omly cause you problems.

Alan

RE: Parallel XMRs PU calc

Is this a correct example, jghrist:
Two transformers @2%Z with a voltage unbalance of 4% will have a circulating current of 100% rated current? Not good.
I have corrected circulating current issues a number of times but I have never measured the current or voltage. It was always a case of recognizing the issue and then making the issue go away as soon as possible.  

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

RE: Parallel XMRs PU calc

Bill,

I believe that is a correct example.  I agree that the situation should be avoided.  Sometimes making the issue go away is not simple.

I have used this approach to calculate circulating currents when temporarily paralleling transformers that have different ratios and can get close but not exactly the same ratios with taps.  
 

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