# Homework Help: Star/delta transformer - single phase fault on secondary

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1. Feb 1, 2016

### acw260

1. The problem statement, all variables and given/known data
A three-phase distribution transformer is configured as follows:
-primary side = star with solidly grounded neutral
-secondary side = delta, grounded via zigzag earthing transformer

A single phase-earth fault occurs on the secondary (delta) side of the circuit.

2. Relevant equations
Will current flow in the neutral connection on the primary side of the transformer (solidly grounded star)?

3. The attempt at a solution
I believe that the answer is 'no'.

If I understand correctly, the earth-fault current on the secondary (delta) side will flow back into the secondary side of the circuit via the zigzag earthing transformer and be evenly distributed between the phases. The primary (star) side of the transformer will therefore 'see' the fault as a balanced three-phase fault, with equal current flowing in each of the three legs of the delta windings. Therefore no current will flow on primary side neutral.

Is my understanding above correct? Any help would be much appreciated.

2. Feb 6, 2016