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1 UNIVERSITY OF TORONTO FACULTY OF APPLIED SCIENCE AND ENGINEERING FINAL EXAMINATION: December 13, 2017 at 6:30PM DURATION: 2½ hrs. ECE314 - Fundamental of Electrical Energy Systems Exam Type: C (One letter-size, hand-written aid-sheet allowed) Calculator Type: 2 (non-programmable calculators) Examiner: Hamid S. Timorabadi LAST NAME: FIRST NAME: STUDENT NUMBER: INSTRUCTIONS: Answer all the five questions on this examination paper. The value of each question is indicated in the brackets. All work is to be done on these sheets. Use the back of the pages if you need more space. Show details of your solutions for all questions. Full marks are only applied to legible solutions with required details. MARKS Ql /13 Q2 /7 Q3 /10 Q4 /10 QS /10 Total /50 ECE3 l 4 Final Exam 2017 Page I ofl4

2 [4 Marks] QI: Part A: For the following questions clearly circle the appropriate answer. 1. Which of the following harmonics can be present in a single phase VSI (Voltage Source Inverter). a) 2 nd Harmonic b) 4 th Harmonic c) 8 th Harmonic d) 7 th Harmonic 2. In a full bridge VSI, which one of the following conditions should be avoided? a) Top and bottom switches of different legs can be turned on while the top and bottom switches of the other leg are turned off. b) Both top switches can be turned on while both bottom switches are turned off. c) Top and bottom switches of the same leg can be turned on while the top and bottom switches of the other leg are turned off. d) None of these 3. An engineer decides to design a low-power synchronous machine, which one of the followings is a better choice? a) A Salient rotor is chosen since it provides low efficiency despite of its high cost. b) A Salient rotor is chosen since it costs less despite its low efficiency. c) A Permanent magnet rotor is chosen since it allows its magnetic field to be controlled. d) Both b and c. 4. Suppose an engineer decides to use one slap of iron instead of laminated iron plates in designing a transformer. What component of the equivalent circuit would be the most affected? a) This has no effect on the transformer. b) This increases the secondary side current. c) This increases the secondary side voltage by 10%. d) This decreases magnetization inductance. e) This increases secondary side reactance (Xp). f) This increases the value of Re. 5. A transformer designed only for isolation: a) Provides Galvanic isolation and has a turn ratio of 1.5. b) Provides step-up voltage and has a turn ratio of 1.5. c) Has Galvanic isolation and unity turns ratio. d) None of the above. 6. Suppose a Buck-Boost Converter is operating under the steady state, then the voltage drop across the converters' inductor indicates a volt-second balance. a) True. b) False. ECE3 l 4 Final Exam 2017 Page 2 ofl4

3 7. It is possible, in any switch mode DC-DC converter, to achieve a discontinuous operation by reducing the load current. c) True. d) False. 8. The friction losses in a non-ideal transformer is: a) Negligible. b) 5% c) Zero. d) Depends on transformer power ratings. ECE3 l 4 Final Exam 2017 Page 3 of 14

4 [9 Marks] Ql: Part B: Answer the following questions. 1. Supposed a generator only provides 3-phases and no neutral. Propose a method for connecting this generator to a 3-phase load that requires a neutral connection. You do not need to provide calculations but just mention what should be done or provide a drawing. (1 mark) 2. Suppose a voltage is applied to the primary side of a non-ideal transformer but no load is connected to the secondary, is there any current flowing in the primary side? Justify. (1 mark) 3. Compared with a unipolar PWM, a bipolar PWM inverter results in reduced Total Harmonic Distortion of the output voltage. (1 mark) Ture False Justification: 4. For the same frequency modulation ratio (m1) the switching loss of bipolar PWM inverter is equal to that of a unipolar PWM inverter. Note identical switches are used in both cases. (1 mark) Ture False Justification: ECE314 Final Exam 2017 Page 4 of 14

5 5. Supposed an engineer purchases an off-shelf buck converter, when testing the converter the engineer finds out that even at no-load the converter is operating in continuous mode (CCM). Provide two reasons that can keep the buck converter in CCM. Justify. (2 marks) 6. How can one push harmonics_ in a Uniploar PWM to higher frequencies? Provide one advantage of pushing harmonics to higher frequencies. What are the limiting factors in doing so? (2 marks) 7. What aspect of safety requires extra attention when working with an auto-transformer rather than a transformer? Justify. (1 mark) ECE314 Final Exam 2017 Page 5 of 14

6 (7 Marks] Q2: A Boost converter with a switching frequency of 100-kHz. is designed to deliver 2.5-A at 300-V into a resistive load, with an input supply of 150-V. Under these conditions the inductor peak ripple current is 50% of the output current. Assume the capacitor is very large. a) Draw a circuit diagram and label all the components with their values as given above. (1 mark) b) Draw inductor voltage and current and clearly label all the values on the graphs then determine the inductance value. (3 marks) c) For the rated conditions, determine the boundary inductor value. Draw inductor current waveform and label. (2 marks) d) For the inductor value from Part (c) and rated values for input and output voltage, determine the continuous-discontinuous boundary load resistance value. (1 mark) ECE314 Final Exam 2017 Page 6 of 14

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8 [10 Marks] Q3: A step-down distribution transformer has name plate information as: 23-kVA, 2300:230-V, 60-Hz. This transformer has a primary resistance of 4-Q, primary reactance of 12-Q, secondary resistance of 0.04-Q, and secondary reactance of 0.12-Q. The no-load current of transformer is zero, i.e. the excitation current (Io) is negligible. If the transformer is connected to a load with a leading power factor of at 75% of the rated load then determine the efficiency. ECE3 l 4 Final Exam 2017 Page 8 of 14

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10 [10 Marks] Q4: A 20-MVA, 12.2-kV, 60-Hz., Y-connected synchronous generator has a negligible armature resistance and a synchronous reactance of A Y connected load is driven by this synchronous generator with a lagging power factor of0.8 drawing a current of 946-A. a) Draw the three-phase equivalent circuit that illustrates both Y-connected load and synchronous generator. (3 marks) b) Draw the single-phase equivalent circuit of the synchronous generator without the load. (1 mark) c) Draw phasor diagram for this generator and clearly label all the values on the diagram including angles. (4 marks) d) Suppose the internal generated voltage is decreased by 5 percent then what is the new armature current? (2 marks) ECE314 Final Exam 2017 Page 10 of 14

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12 [10 Marks] QS: For the converter shown below, derive the three following relations: + Vin Dz fin = (1 - D)2 R ECE314 Final Exam 2017 Page 12 ofl4

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14 Student Number: This Page Left Blank/or additional work space. You can tear off this page but you MUSTwirteyour student number. _Jfyou require the work on this page to be marked then it.is your responsibility to staple ii back securely. ECE314 Final Exam 2017 Page 14 of 14