Question

Three single-phase transformers are used in a Y-Y configuration to supply a load rated 4000 V, and drawing 80 kW at a power factor of 0.72 (lagging), independently of the voltage. Each transformer, rated 14500/2300 V, 40 kVA, has the following parameters in pu: series elements: 0.01 (resistive), 0.06 (inductive); magnetizing branch: 70 (resistive), 20 (inductive).

(a) Assume the transformer is ideal and the phase voltage at the primary is 13000 V. Draw the per phase equivalent circuit of the transformer and load. Give the phase voltage and current. Indicate if the transformer is overloaded. Compute the real and reactive power drawn from the feeder. Draw the per-phase V-I diagram and power diagram seen from the primary side.

(b) Draw the per phase equivalent circuit of the transformer, indicating parameters in pu and Ohm referred to the primary. Draw the equivalent circuit used under no-load and full load conditions seen from the primary side in pu and Ohm. For operation under load, draw the series impedance diagram and compute the impedance in Ohm and angle. Estimate the per phase transformer losses at rated voltage and current in pu and in kW. Find the efficiency for the given load. Indicate the impact on losses and efficiency of using a transformer with a 0.02 pu series reactance and a 100 pu core resistance.

(c) For rated voltage on the primary side and a short circuit across the load with an impedance of 0.10 pu resistive, compute the current in A as seen from the primary side. Draw the V-I diagram. Compute the current if the fault impedance is 0 and compare the 2 values. Indicate, for a 0.10 resistive short, the impact on the fault current of using a transformer with a 0.02 pu series reactance and a 100 pu core resistance. Compare the fault impedance and V-I diagrams.

Answer 1