EE2005: Electrical Machines
Tutorial 4
1. A 10 kVA, 2200V/220V, 50 Hz single phase transformer has the equivalent circuit parameters as follows:
r1 = 4 Ω, r2 = 0.04 Ω, x1 = 5 Ω, x2 = 0.05 Ω
By choosing the transformer ratings at the respective sides as the base values, determine
(a) the per-unit impedance of the transformer at the low voltage side
(b) the per-unit impedance of the transformer at the high voltage side
2. Consider a 2 kVA, 230V/110V, 50 Hz transformer. The normalized values of the equivalent resistance and reactance taking the transformer ratings as the base values are 0.03
pu and 0.05 pu respectively. A load connected to this unit draws half the rated current at
UPF. Determine the percentage voltage regulation of the transformer at this condition?
3. For the transformer given in question (2), find out the actual values of the equivalent
resistance and reactance as referred to HV side. If the base value of power is chosen as
5 kVA instead of 2 kVA, what would be the corresponding normalized values of resistance
and reactance.
4. Consider two transformer units as follows:
T1 : 5 kVA, 2200V/220V, 50 Hz with r1 = 6 Ω, r2 = 0.05 Ω, x1 = 10 Ω, x2 = 0.09 Ω
T2 : 10 kVA, 2200V/220V, 50 Hz with r1 = 7 Ω, r2 = 0.04 Ω, x1 = 20 Ω, x2 = 0.1 Ω
(a) Determine the p.u impedances of each unit by choosing the ratings as the base
values.
(b) If the units T1 and T2 are intended to operate at rated load and 0.8 pf lag, determine
the percentage voltage regulation of each unit. [Do the calculations in per-unit]
Which transformer will provide a better regulation ?
5. The following data were obtained for a 20 kVA, 50 Hz, 2400 V/240 V distribution transformer when tested:
With HV winding open circuited: 240 V, 1.006 A, 126.6 W
With LV side short circuited: 57.5 V, 8.34 A, 284 W
Take the ratings of the transformer as the base values.
(a) Determine
(i) the p.u values of the equivalent resistance and leakage reactance.
(ii) the p.u values of iron loss and full-load copper loss.
(b) Suppose that a resistive load connected across the transformer secondary draws
half the full load current. Find the percentage voltage regulation of the unit.
(c) Find out the per-unit output power in case (b). Also determine the efficiency of
the unit.