EE380 Midterm II

A 2,400-V to 208-V, 300-kVA, three phase transformer has a core loss at rated voltage of 2.70 kW. If its equivalent resistance is 1.40%, find the efficency of this transformer for a load power factor of 0.9.

5.

A 200-MVA, 345-Kv delta/34.5-kV Y, substation transformer has an 8% leakage reactance. The transformer acts as a connecting link between 345-kV transmission and 34.5-kV distribution. Transformer winding resistances and exciting current are neglected. The high-voltage bus connected to the transformer is assumed to be an ideal 345-kV positive-sequence source with negligable source impedance. Using the transformer ratings as base values, determine the per-unit magnitudes of transformer voltage drop and voltage at the low-voltage terminals when rated transformer current at 0.8 pf lagging enters the high-voltage terminals.

6

Three single-transformers are used to form a three phase transformer bank rated 18MVA, 13.8Delta-120Y kV. One side of the transformer bank is connected to a 120-kv transmission line, and the other side is connected to a three-phase load of 10MVA at 13.2 kV and 0.8 lagging power factor

1.

A single-line diagram of a three-phase transformer bank connected to a load is given, Find the magnitudes of the line-to-line voltages, line currents, phase voltages, and phase currents on either side of the transformer bank. Determine the primary to secondary ratio of the line-to-line voltages and the line currents.

2.

A single-line diagram of part of a three-phase transmission system is given. Choose a base of 20MVA, 12.5 kV in the load circuit, and draw the equivalent line-to-neutral impedance diagram in per-unit quantities. Show the base kV in the three parts of the system.

3.

Three single-phase transformers, each rated 15MVA, 13.2:66 kV, with a leakage reactance of 0.1 pu, are connected to form a three-phase Y-Delta bank, one side of which is connected to a 120-kV transmission line and the other side, to three 12-ohm resistors in delta. The star point of the transformer bank is grounded. The series reactance per phase of the transmission line is 80ohms. Draw a single line diagram of the system and obtain the line-to-neutral impedance diagram with per-unit values, for a base voltage of 120 kV in the transmission line and a three-phase voltampere base of 50 MVA.

4

Three single-phase transformers, each rated 10 MVA, 66.4/12.5 kV, 60 Hz, with an equivalent series reactance of 0.1 per unit divided equally between primary and secondary, are connected in a three-phase bank. The high-voltage windings are Y-connected, and their terminals are directly connected to a 115-kV three-phase bus. The secondary terminals are all shorted together. Find the currents entering the high-voltage terminals and leaving the low-voltage terminals if the low-voltage windings are: a. Y connected, b. delta connected

7

A 30-MVA, 13.2-kV, three-phase generator, which has a positive-sequence reactance of 1.5 per unit on the generator base, is connected to a 35-MVA, 13.2 delta/115Y-kV step-up transformer with a series impedance of (0.005+j0.1) per unit on its base.

8