100 kVA, 11 kV (delta) : 400 V (wye), Xeq = 0.1 p.u
with a neutral to ground connected through Zn = j0.1 (ohm)

I have a question where the above transformer is fed by a balanced positive sequence three phase source, and parallel delta and wye loads are connected on the load side of the transformer. I am then told that the b-phase drops to zero (not an open circuit).

I see they specified in the answer that there is no zero-sequence current after the fault.

Vo = 1/3(Va + Vb + Vc) Which would not be zero.

My textbook states: "In a balanced Y-connected system, line currents have no zero-sequence component, since the neutral current is zero."
But after the fault we no longer have a balanced system.

1) What is the purpose of stating that it is not an open circuit, what is the difference in this context?
2) Why is the zero-sequence current zero?
3) Does Xeq refer to the leakage reactance only?


1 Answer 1


The zero sequence has no phase shift between voltages. Therefore on the delta side of the transformer, voltages will remain equal at all times between terminals, which means no current can flow through these terminals.

In general; no current can flow into a delta when fed by a zero sequence source.


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