Assignment CO2 EEB2093 Electrical Machines Analyze Circuits and Performance of Power Transformers Course Outcome: Analyze circuits and performance of power transformers. Main Reference book: Stephen J. Chapman, Electric Machinery Fundamentals, Mc Graw-Hill. This assignment must be submitted for assessment. The assignment submitted must be handwritten. Due date will be informed during the lecture. Section 1: Construction & How it Works? Watch this video and answer the following questions (Q1 – Q6): >> https://youtu.be/vh_aCAHThTQ Q1: Explain the function of a transformer. Q2: Explain how a transformer help to improve efficiency in power system. Q3: Discuss the magnetic field concepts applicable in the transformer. Q4: Derive the relationship between number of turn and voltage at primary & secondary winding of the transformer Q5: List all main parts of the transformer and explain the function. Q6: Explain why the transformer core is made of thin laminated ferromagnetic material. Section 2: Ideal vs Real Transformer Q7: Referring to Figure 1 derive equation for: - input power, Pin - output power, Pout - Relationship between Pin & Pout 1 Figure 1 Q8: Derive ZL in the Figure 1 in term of VP and IP. Q9: Referring to Q8, comment on the impedance through the transformer. Q10: Evaluate and compare the losses for system in Figure 2 (a) and (b). Figure 2 Q11: Explain the following losses in a real transformer: - Copper loss - Leakage flux - Hysteresis loss - Eddy current loss 2 Q12: Simplify the circuit in Figure 3 by referring all impedance to primary side and sketch the simplified equivalent circuit. Figure 3 Q13: Assuming small current in excitation branch, draw an approximate equivalent circuit. Q14: Explain how to determine the parameters in the approximate equivalent circuit. Q15: Explain: - How to perform short circuit test (draw the circuit). - What happened during the short circuit test. - Which impedance parameters can be obtained during the test. Q16 - Explain: - How to perform open circuit test (draw the circuit). - What happened during the open circuit test. - Which impedance parameters can be obtained during the test. Q17: A 20 kVA, 8000/240-V, 50 Hz transformer has been tested. The open circuit and short circuit test result are obtained as follow: Determine the transformer equivalent circuit impedances. 3 Section 3: Transformer Analysis Q18: Evaluate the voltage regulation Q19 : Calculate the transformer efficiency Q20. For (a) Q18 & (b) Q19, perform the calculation using per unit system: 4 Section 4: Auto Transformer Q21: Compare an autotransformer with a conventional transformer. Q22: Draw the equivalent circuit of a step-up autotransformer and step down autotransformer. Q23: Derive the relationship between voltages and turn ratio of an autotransformer. Q24: Derive the relationship between currents and turn ratio of an autotransformer. Q25: Derive the apparent power rating advantage of the autotransformer. Q26: Discuss the advantages and disadvantages of the autotransformer. Q27: A 10-kVA, 480/120-V transformer is to be used as a step-up autotransformer as shown in Figure 4. It is connected to a 600-V distribution line. (a) Evaluate the output voltage of the transformer. (b) Determine the power rating advantage of this autotransformer over a conventional transformer with similar ratings. Figure 4 Section 5: Three-phase Transformer Q28: Discuss about four possible connection for a three-phase transformer 5 Section 6: Additional Exercises Q29: A 13.2-kV single-phase generator supplies power to a load through a transmission line. Figure 5 (a) If the generator is connected to the load as shown in Figure 5 (a) calculate the ratio of the load voltage to the generated voltage (b) Determine the transmission line losses for system in Figure 5 (a). (c) If a step-up transformer is placed at the output of the generator and a stepdown transformer is placed at the load end of the transmission line as shown in Figure 5 (b), calculate the ratio of the load voltage to the generated voltage. (d) Determine the transmission line losses for system in Figure 5 (b). (e) Compare and comment on the transmission line losses and voltage ratio of system in Figure 5 (a) and (b). 6 Q30: Two ideal transformers are connected to the system as shown in Figure 6, determine: i. the current supplied by the generator, ii. the transmission losses of the system, iii. the voltage across the load. iv. The voltage across the Zline. Figure 6 Q31: A single-phase, 10-kVA, 415/240-V single phase transformer is tested in the laboratory, and the results are shown in Table. All the values are measured at the primary side of the transformer. a. Open Circuit Test Results Short Circuit Test Results Voc=415 V Vsc=10.0 V Ioc=0.41 A Isc=10.5 A Poc=30 W Psc=25 W Evaluate all parameters in per unit and show in a transformer’s simplified equivalent circuit. b. Calculate the voltage regulation of the transformer at rated condition and 0.8 power factor lagging. c. Determine the efficiency of the transformer at rated condition and 0.6 power factor lagging. 7 d. Suggest a method to operate the transformer more efficiently and prove it. 8 9 10 11 12 13
