Parallel XMRs PU calc 1

[Lenz81]

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

 

[davidbeach]

Refer to Blackburn's Protective Relaying: Principles and Applications or to his Symmetrical Components book. Many other sources in faq238-1287.

 

[waross]

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

 

[jghrist]

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

 

[racobb]

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

Alan

 

[waross]

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

 

[jghrist]

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.