Question

120 Power System Analysis 3.12. A single-phase system similar to that shown in Figure 3.11 has two transformers A-B a B-C connected by a line B feeding a load at the receiving end C. The ratings and parame ter values of the components are 500 V/1.5 kV, 9.6 kVA. leakage reactance 5 % 1.2 kV/120 V, 7.2 kVA, leakage reactance 4 % series impedance (0.5 + j3.0) 120 V, 6 kVA at 0.8 power-factor lagging Transformer A-B Transformer B-C: Line B Load C a. Determine the value of the load impedance in ohms and the actual ohmic impedances of the two transformers referred to both their primary and secondary sides. b. Choosing 1.2 kV as the voltage base for circuit B and 10 kVA as the system-wide kVA base, express all system impedances in per unit. What value of sending-end voltage corresponds to the given loading conditions? 3.13. A balanced A-connected resistive load of 8000 kW is connected to the low-voltage A-connected side of a Y-A transformer rated 10,000 kVA, 138/13.8 kV. Find the load resistance in ohms in each phase as measured from line to neutral on the high-voltage side of the transformer. Neglect transformer impedance and assume rated voltage is applied to the transformer primary 3.14. Solve Prob, 3.13 if the same resistances are reconnected in Y. 3.15. Three transformers, each rated 5 kVA, 220 V on the secondary side, are connected A-A and have been supplying a balanced 15-kW purely resistive load at 220 V. A change is made which reduces the load to 10 kW, still purely resistive and balanced. Someone sug- gests that with two-thirds of the load, one transformer can be removed and the system can be operated open A. Balanced three-phase voltages will still be supplied to the load since two of the line voltages (and thus also the third) will be unchanged. To investigate the suggestion further, a. Find each of the line currents (magnitude and angle) with the 10-kW load and the transformer between a and c emoved. (Assume V 22020, sequence abc.) b. Find the kilovoltamperes supplied by each of the remaining transformers. c. What restriction must be placed on the load for open-A operation with these transformers? d. Explain why the individual transformer kilovoltampere values include a Q component when the load is purely resistive. 3.16. A transformer rated 200 MVA, 345Y/20.5A kV connects a balanced load rated 180 MVA 22.5 kV, 0.8 power-factor lag to a transmission line. Determine a. The rating of each of three single-phase transformers which when properly connected will be equivalent to the above three-phase transformer b. The complex impedance of the load in per unit in the impedance diagram if the base in the transmission line is 100 MVA, 345 kV. 3.17. A three-phase transformer rated 5 MVA, 115/13.2 kV has per-phase series impedance of (0.007j0.075) per unit. The transformer is connected to a short distribution line, which can be represented by a series impedance per phase of (0.02 + j0.10) per unit on a base of 10 MVA, 13.2 kV. The line supplies a balanced three-phase load rated 4 MVA, 13.2 kV with lagging power factor 0.85. a. Draw an equivalent circuit of the system indicating all impedances in per unit. Choose 10 MVA, 13.2 kV as the base at the load. b. With the voltage at the primary side of the transformer held constant at 115 kV. the load at the receiving end of the line is disconnected. Find the voltage regulation at the load. 3.18. Three identical single-phase transformers, each rated 1.2 kV/120 V, 7.2 kVA and bai a leakage reactance of 0.05 per unit, are connected together to form a three-phase ba A balanced Y-connected load of 5 2 per phase is connected across the secondary of bank. Determine the Y-equivalent per-phase impedance (in ohms and in from the primary side when the transformer bank is connected (a) Y-Y, ( and (d) A-A. Use Table 3.1.

solve no: 3.14 , 3.16, 3.19

please show each step and solve for beginners