Can anyone explain how to calculate the individual primary 3phase currents with unbalanced secondary load currents on a DY transformer neutral grounded.
The simple way of doing it is to use the star/delta equivalent and then "ignore it altogether". If, for instance, you have a 130 kV primary and a 20 kV secondary with secondary line currents being I1, I2 and I3. Then the primary line currents will be I1x20/130. I2x20/130 and I3x20/130.
The more complicated way of doing it is to keep the delta primary and calculate the primary currents for each winding and then vector sum the three current pairs to get line current. Both ways of doing it produce the same result.
It works only for non-distorted currents. If you have lots of triplenes, a primary delta and a primary star will yield different results (triplenes cancel in a delta winding).
The "simple" way of doing it won't work with unbalanced currents. For instance, for the 130 kV - 20 kV example, if the secondary line currents are I1=100@0°, I2=150@-120°, I3=200@120°, then the primary line currents will be 19.36@36.6°, 27.01@-85.3°, 23.5@139.1°
Yes, you are right. You get a much more complicated expression than I thought. The "side vectors" do not cancel. Not at all. Do you have the formula in "pretty" form?
One trick which I've found useful is to use the ampere-turn balance of each phase winding HV/LV pair. Regardless of the connection of the windings, if you have, for example, 1000 amp-turns on the secondary of the windings on a specific limb of the core, you will also have 1000 amp-turns on the primary winding of that core.
Once you know the three primary winding currents it's just ol' fashioned vector addition to find the line currents. It is worth sketching the current vectors to help you keep track.
I think that your Mathcad sheet is on a pretty "pretty" form. But it is not the simple formula I thought existed. I shall dig into it and see what gives.
Big difference in thread238-156846 with the delta secondary. Here, if the unbalanced loads have the same pf, the 120 degree displacement assumption is appropriate. Zero sequence current will be non-zero in the secondary and (only) in the primary windings.
