Question

A three-phase Yy0 connected transformer has a nominal rating of 250 kVA and a voltage ratio 15:0.4 kV. After conducting the short-circuit test, where the secondary winding is short-circuited, it is found that the primary series resistance is 0.08 pu and the primary series impedance magnitude is 0.1pu. The effect of the parallel branch in the transformer’s equivalent circuit may be neglected.

1. Explain what information ‘Yy0’ gives about the transformer.

2. Draw the per-phase equivalent circuit of the transformer during the short-circuit test. What are the required measuring devices? Include them in the diagram.

3. What is the current flowing through the secondary winding if a three-phase short circuit occurs at the transformer secondary terminals when nominal voltage is applied at the primary?

4. A three-phase delta-connected balanced load of 15∠60° Ω is connected into the transformer secondary. What is the required primary voltage to maintain 400 V across the secondary terminals? Draw the phasor diagram.

5. What is the efficiency of the transformer for the load conditions in 4?

6. To feed a load rated at 400 kW pf=0.8 lagging, another 350 kVA Yy0 transformer is connected in parallel with the existing transformer. The primary series impedance magnitude of the new transformer is 0.1 pu and the primary series reactance is 0.08 pu. What is the power-sharing between each transformer? Calculate the active, reactive and apparent power of eachtransformer. The effect of the parallel branch in the additional transformer’s equivalent circuitmay also be neglected.

Answer 1