1 Assuming that the transformer terminals have polarity marks 111, H2, X1, X2, make schematic drawings of the following connections:
a. Delta-wye
b. Open-delta
2 Three single-phase transformers rated at 250 kVA, 7200 V/600 V, 60 Hz, are con nected in wye-delta on a 12 470 V, 3-phase
line. If the load is 450 kVA, calculate the following currents:
a. In the incoming and outgoing transmission lines
b. In the primary and secondary windings
3 The transformer in Fig. 12.9 has a rating of 36 MVA, 13.8 kV/320 V. Calculate the nominal currents in the primary and sec ondary lines.
4 Calculate the nominal currents in the primary and secondary windings of the transformer shown in Fig. 10.18, knowing that the windings are connected in delta-wye.
Intermediate level
5 The transformer shown in Fig. 10.19 operates in the forced-air mode during the morning peaks.
a. Calculate the currents in the secondary lines if the primary line voltage is 225 kV and the primary line current is 150 A.
b. Is the transformer overloaded?
6 The transformers in Problem 2 are used to raise the voltage of a 3-phase, 600 V line to 7.2 kV.
a. How must they be connected?
b. Calculate the line currents for a 600 kVA load.
c. Calculate the corresponding primary and secondary currents. 7 In order to meet an emergency, three single-phase transformers rated 100 WA,13.2 kV/2.4 kV are connected in wye-delta on a 3-phase, 18 kV line.
a. What is the maximum load that can be connected to the transformer bank?
b. What is the outgoing line voltage? 8 Two transformers rated at 250 kVA, 2.4 kV/600 V are connected in open-delta to supply a load of 400 kVA.
a. Are the transformers overloaded?
b. What is the maximum load the bank can carry on a continuous basis?
9 Referring to Figs. 12.3 and 12.4, the line voltage between phases A-B-C is 6.9 kV and the voltage between lines 1, 2, and 3 is balanced and equal to 600 V. Then, in a similar installation the secondary windings of transformer P are by mistake connected in reverse.
a. Determine the voltages measured between lines 1-2, 2-3, and 3-1.
b. Draw the new phasor diagram. Industrial application
10 Three 150 WA, 480 V/4000 V, 60 Hz single-phase transformers are to be in stalled on a 4000 V, 3-phase line. The ex citing current has a value of 0.02 pu. Calculate the line current when the trans
formers are operating at no-load.
11 The core loss in a 300 kVA, 3-phase distri bution transformer is estimated to be 0.003 pu. The copper losses are 0.0015 pu. If thetransformer operates effectively at no-load 50 percent of the time, and the cost of electricity is 4.5 cents per kWh, calculate the cost of the no-load operation in the course of one year.
12 The bulletin of a transformer manufacturer indicates that a 150 kVA, 230 V/208 V, 60 Hz, 3-phase autotransformer weighs