Do we need to model delta-Y transformer 30 degree phase shift?

answered 2013-11-19 11:28:23 -0500

yfwing
426 ●2 ●3 ●8

Generally speaking, you can do either way (with or without phase shift) as long as you are consistent with all the transformers. Y-Delta transformers are mainly for generator step-up and nearly all the generators are connected by Y-delta step-up transformer. When we do the dynamic simulations, we are comparing relative angles, rather than absolutely angle. So the phase shifting effects are cancelled out already.

In terms of short-circuit calculation, there should not be any difference. You may see that the phasor value of short-circuit current is different (i.e., the real and imaginary part), but the magnitude should be the same.

So, the point is that no matter which practice to be followed, it should be consistent with all the transformers. Otherwise, wrong results will be expected.

answered 2016-10-07 18:10:57 -0500

oppossumX
40 ●2 ●4 ●7

Section 11.2.5.2 of the Program Application Guide: Volume 1 (for PSSE 33.9) gives an in-depth explanation of the effect of neglecting transformer phase shifts.

"Neglect of the inherent 30° phase shift of wye-delta transformers results in the following:

• Calculated sequence and phase currents that are correct in all branches on the fault side of the transformer.
• Calculated sequence currents that are correct in amplitude but erroneous in phase in all branches that are removed from the fault by a transformer.
• Erroneous values of phase current in all branches separated from the fault by a wye-delta transformer.

answered 2016-10-07 17:57:56 -0500

perolofl
3312 ●1 ●8 ●18

Yfwing is wrong in his answer about short circuit calculation. If 30 degree phase shift is not entered for delta-Y or Y-delta transformers the result will be wrong if you study the current at one side of the transformer and the short circuit is located at the other side of the transformer.