ECE 432
Homework Set 2 Transformer
Note: unless stated otherwise, all values of voltages and currents are rms values
1.
A 10 kVA, 480/120 V, 60 Hz, single phase, transformer has the following open‐circuit and short‐circuit test data: Test Voltage ( V)
Current Power (A)
(W) Open‐Circuit : measurements taken on the 120 V side 120
12
102 Short‐Circuit: measurements taken on the 480V side 35
20.833
153 Note, all voltages and currents are rms values. a) Ignoring magnetizing and core losses and using just the short‐circuit test data, determine the total series resistive and leakage reactance values referred to the 480V side of its equivalent circuit representation. What would be the corresponding values when referred to the 120V side? b) Again, ignoring magnetizing and core losses as in part a) above, determine the high side supply voltage needed to deliver rated 120 V to a 10 kVA load at 0.8 power factor lagging. Holding the high‐side supply voltage fixed at that value, what voltage regulation in percent will the terminal voltage at the low voltage side experience when the load is disconnected? c) Estimate the efficiency of the transformer operating at rated output voltage delivering rated kVA at 0.8 power factor lagging using both open and short‐circuit test data. 2.
The above single-line diagram shows a factory load supplied via a step-down transformer from a
distribution network. The distribution network is represented by a simple Thevenin equivalent: a
3-phase, 13.2 kV voltage source and a source impedance Zs = j20 . The transformer, with its
reactances in per unit on its own rating, and the three-phase load are as follows:
3-phase transformer T:
1500 kVA, 13.2kV/600V, leakage reactance of 0.1 per unit
on its own rating.
3-phase factory load:
1200 kW, 0.8 power factor lagging at 600 V
Unless stated otherwise, with 3-phase equipment, the power values are total 3-phase and the
voltages are line-to-line rms.
a) Using a common Sbase = 1500 kVA and a Vbas e of 600V on the load side of the transformer,
determine the base impedance on each side of the transformer.
b) Sketch an equivalent single-phase circuit representation of the distribution network and
factory load showing all parameters on the chosen per unit system.
c) For a balanced operating condition where the line-to-line voltage on the factory side is
600V, determine

the per unit value of the Thevenin voltage and it’s corresponding
line-to-neutral voltage in Volts;

the current supplied by the Thevenin source in Amps at the given
load condition.