Ac Apparatus and Devices Finals

AC APPARATUS AND DEVICES FINAL EXAMINATION Name:

Date:

I. True or False. Write T if the statement is false and F if the statement is true. STRICTLY NO ERASURE. F 1. The transformer operates at power factor depending on the power factor of the load. T 2. Transformer is used to change the values of frequency. F 3. In three phase connection of transformer, Delta to wye connection is used to step up the voltage. F 4. In any transformer, the voltage per turn in primary and secondary remains always the same. T 5. The efficiency of a transformer is normally in the range of 80% to 90% II. Solve the following WITH COMPLETE SOLUTION. ROUND OFF ONLY YOUR FINAL ANSWER. 1. A 150 kVA 4800/240 V single phase transformer has impedance referred to the secondary 0f 0.0175 + j0.0986 ohms. If the total core loss is 3.395 kW, determine the all-day efficiency of the transformer when it is operated as follows: 92.95% a) 78.564% load at 0.853 pf lagging for 8 hours b) 65.435% load at 0.862 pf lagging for 7 hours c) 68.307 kVA at 0.891 pf lagging for 5 hours d) 9853.437323 VAR at 0.987 pf lagging for 4 hours 2. The 3MVA, 13,800/480 V three phase transformer has impedance referred to the secondary of 0.00264 + j0.0104 ohms. If the total core loss is 25.691 kW, determine the all-day efficiency when the transformer is operated at: 97.654% a) 89.346% load at 0.862 pf lagging for 9 hours b) 73.914% load at 0.817 pf lagging for 6 hours c) 1.95372MVA at 0.835 pf lagging for 5 hours d) 644931.3445 VARat 0.895 pf lagging for 4 hours 3. A three-phase transformer is connected in a delta to wye configuration. A load is connected as a wye, and each phase has an impedance of 3 ohms. The primary is connected to a line voltage of 23,000 V and the secondary line voltage is 208 V. Find the primary and secondary line current. 4. Two single phase having equal turns ratio and ratings are operated in parallel to supply a load of 280 kW at 80% pf lagging. Transformer A has 2% resistance and 8% reactance while transformer B has 1% resistance and 6% reactance. What is the power delivered by each of the transformers to the load? 123 kW, 157 kW 5. A single phase 4kVA, 250/125 V, 50 Hz transformer has the following parameters: Resistance in the primary and secondary are 0.9 and 0.94 ohms respectively Leakage reactance in the primary and secondary are 0.4 and 0.11 ohms respectively. Determine the full load efficiency of the transformer if the iron loss is 89.25 W. 94.13% 6. Three single phase, 30 kVA 2400/240 V, 50 Hz transformers are connected to form three phase, 2400/416 V transformer bank. The equivalent impedance of each transformer referred to the high voltage side is 1.5 + j2 ohms. The transformer delivers 60 kW at 0.75 power factor leading. Determine the percent voltage regulation. -0.0875%

7. Two transformers connected in open delta supply a 400 kVA balanced load operating at 0.866 pf lagging. The load voltage is 440 V. What is the kW supplied by each transformer? 231kW, 115.5kW