Electronic – How to calculate voltage regulation of three phase transformer correct in given case

three phasetransformer

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we both agreed on the value of series impedance which is (12.82[82] ) however what i think he did wrong is the value of secondary reflected on primary

since it is Y-delta
7967/480 = root(3) a hence a will be 9.6

so the vs* = 480 * 9.6
=4608

Is(phase) will be (200k/(3(480)) = 80,16[-25.84]
NOW vp(phase) = vs* + (Is/a) * Z(eq)
4608+107[56.16]
=4668[1.1]

so vp(line)=root 3 v(phase)
8086[1.1]

Vr in my calculation (4668-4608)/4608 = 1.3 %

Is my answer correct and the solution did a mistake if so ok but if not what did I miss in my calculation?
hope that you help me with this ! :/

Best Answer

Take a look at the first sentence of the question.

The bank is made by 3 identical transformers 7967/480V

When you connect the primary in wye configuration, then the primary line to line voltage is able to work with voltage until 7967V*root(3)=13.7kV.

What question doesn't say is the bus voltage. In that case, if you suppose that the bus voltage is 7.9kV then you are right. If you suppose that the bus voltage is 13.9kV, then the book is right. Thinking power grid, I'd say that 13.9kV is a line to line voltage to expect.

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