Three single phase transformer with different %Z's

[JWT]

Hello everyone,

I have a three phase circuit that consists of a three-phase transformer bank feeding a balanced three-phase delta load. The transformer bank consists of three single phase transformers connected in delta. Each transformer is rated at 333kVA, 2400V. Two of the transformers have percent impedances of 5.35%, while the third has a percent impedance of 4.04%. Also, each transformer has the same X/R ratio of 4.5 (if that matters), and assume the balanced load is 0 ohms, and the line impedance is 0 ohms.

My question- in this circuit, how would you calculate the three line currents?

I appreciate your time.

 

[JWT]

By the way, this is not a homework question. I just want to see the effects of difference in transformer impedances on the line currents, even if there are nothing but three-phase loads in the facility. VFD's and soft starts are tripping on current imbalance.

 

[jim hardy]

and assume the balanced load is 0 ohms, and the line impedance is 0 ohms.

So you're after short circuit currents?

 

[JWT]

In this example, yes. Really what I'm after is getting an idea of what the percentage deviation would be on the line currents if a three-phase delta secondary comprised of single phase transformers that had different percent impedances (which in my case, two cans have 5.35%Z while the third has 4.04%Z).

Thank you, sir, for your help.

 

[jim hardy]

i didn't set down with pencil and paper

but does this not look soluble by basic circuit analysis?

Shorting the 3 apexes sets each phase current trivially to V/Z ,
is each line current not the sum of the two phases to which it's attached ?

 

[JWT]

Yes, sir. That does make sense. For some reason I was looking into using sequence impedances and didn't think to just do a simple nodal analysis. Thanks again for your help.