Ok. here's the setup. I have an ungrounded wye-delta main transformer with a phase-phase step-down transformer connected to it. I am trying to reflect the load (step-down transformer) impedance and the delta secondary to the primary (wye) side of the main transformer. I know how to convert unbalanced delta impedances to wye using:

Za=Zab*Zac/(Zab+Zbc+Zca);Zb=Zab*Zbc/(Zab+Zbc+Zca);
and Zc=Zac*Zbc/(Zab+Zbc+Zca)

But I'm not sure how to combine the step-down transformer impedance? Do I just treat it as a parallel impedance with one of the main transformer low side windings using:

Zab=Zstep*Zab/(Zstep+Zab)

and then use the formulas above to convert them to wye and then use the turns ratio of the main transformer squared to reflect the secondary to the primary side of the main transformer?

 

Stevetatum... No, you don’t parallel the secondary load-impedance to the transformer’s secondary-leakage impedance. This procedure will result in a large error! Also, reflection into the primary using the square of the voltage-ratio will be wrong.

Are you familiar with the Method of Symmetrical Components, a mathematical technique for dealing with unbalanced loads?

Regards, Phil Corso

 

Thanks Phil for the quick response. Yes, I am familiar with symmetrical components and using the positive and negative sequence for unbalance. But I am trying to come up with the equivalent circuit model. I need the correct circuit model first so I can calculate the correct sequence impedances that can then be used in the unbalanced symmetrical component equations.

 

Stevetatum... to continue discussion contact me off-list!

Phil (cepsicon[at]aol[dot]com)