� � * � THE UNIVERSITY OF HONG KONG Bachelor of Engineering Department of Electrical & Electronic Engineering ELEC3143 Power Electronics Examination Date: 13 Dec 2019 Time: _2:30pm to 5:30 pm ___ _ Answer ALL questions. Write down your name and university number clearly on the front page of your answer scripts. This is an open book examination. You should have access to the internet from which you will be asked to retrieve information. Please upload your zipped files to OLEX before 6 pm. Late submission will not be accepted. If you have trouble in uploading to OLEX you may upload onto Moodie. You may also email your script to
[email protected] entitled "ELEC3143 exam" if you run into trouble. (Put all your answers on A4 paper, use a fresh page for each question) EEE/ELEC3 l 43/ 2019 Dec page 1 of4 Solar panels -- C l -- T - - F I - AC mains 220V 50Hz ' Nr@ L --41 The diagram above shows a system whereby solar energy is fed to the AC mains through a series of power converters. There is also a set of battery to store energy when there is excessive solar power. Q 1. Refer to the solar power system shown. a) At a certain operating point the solar panel produces 2A at 40V. If the battery is constant 1 OOV and the power converter C operates in the Continuous Conduction Mode (CCM), state what type of power converter should C be and what duty cycle should it operate at? (5 marks) b) Actually the solar power panel is better modelled by a constant current source which produces an average current subject to the sunlight it receives. The voltage may vary according to the external circuit. If it is desired that the solar panel voltage should vary between 35V and 75V while the solar panel average current remains to be 2A and battery voltage 1 OOV, design an inductor value for the converter C so that it operates in the CCM all the time. Assume ideal devices with no loss. Choose the switching frequency of converter C according to the 8th digit of your HKU University Number 8th digit of your HKU University Choose switching frequency number 0-3 120 kHz 4-6 160 kHz 7-9 220 kHz E.g. Univ number 3035280126 should choose 220 kHz EEE/ELEC3143/ 20 I 9 Dec (10 marks) page 2 of4 c) Hence choose a capacitor for converter C so that the output peak to peak voltage ripple is less than 5V all the time. Assume that the battery is removed when voltage ripple is measured. (5 marks) d) Estimate voltage and current ratings of the power devices for converter C. Find suitable MOSFET and diode from the internet. Download the device data sheets and state the parameters extracted. Upload the data sheets together with your answer script. You should zip your files into one file for proper upload to OLEX. (5 marks) e) Calculate the switching loss and conduction loss of the MOSFET for your power converter C. Choose the minimum duty cycle case for your calculation and neglect MOSFET voltage rise and fall times. (7 marks) f) Calculate the conduction loss of the diode at minimum duty cycle. Hence calculate the converter C efficiency assume that losses are consumed by the MOSFET & diode only. (8 marks) Q2. Refer to the solar power system shown. a) Converter F is a single switch forward converter. Suggest why an isolated DC-DC converter is employed for converter F? (5 marks) b) The input voltage to converter F is 1 OOV constant. The output voltage is 330V for the inverter I. The switching frequency of converter F is 100 kHz. Assume Continuous Conduction Mode (CCM), design the number of turns of the three transformer windings. Choose an operating duty cycle and state your reason(s). Choose the ferrite of converter F according to the 81h digit of your HKU University Number 8th digit of your HKU University Choose ferrite number 0-3 Ae= 40 mm2, Bsat = 300 mT 4-6 Ae = 45 mm2, Bsat = 280 mT 7-9 Ae = 50 mm2, Bsat = 250 mT E.g. Univ number 3035280126 should choose Ae = 50 mm2, Bsat = 250 mT (15 marks) c) Refer to your design in part b ), calculate the voltage ratings required for all the MOSFET and diodes. You may assume ideal devices and no voltage spike. (10 marks) EEE/ELEC3143/ 2019 Dec page 3 of4 d) If you want to reduce the voltage ratings of the MOSFET and diodes discuss what circuit design parameter you may alter for each device. Tabulate your answer with respect to each device. (10 marks) Q3. Refer to the solar power system shown. a) The output of converter F feeds an inverter which converts DC to AC. Through a filter the inverter feeds alternating current (AC) to the mains power grid which can be regarded as an AC voltage source with 220Vrms at 50Hz. Sketch a circuit for block I which contains a full bridge inverter and an LC filter which removes high order harmonics. (5 marks) b) The inverter is driven by SPWM. The following table provides harmonics content of SPWM. Work out the modulation index ma required so that the inverter produces a fundamental voltage with 220Vrms at 50Hz, same as the AC mains. Table 8-1 Generalized Harmonics of v Ao for a Large mf" � 0.2 0.4 0.6 0.8 I 0.2 0.4 0.6 0.8 Fundamental mf l.242 1.15 l.006 0.818 "!, ± 2 0.016 0.061 0.131 0.220 mf ± 4 2mf ± I 0.190 0.326 0.370 0.314 2mf± 3 0.024 0.071 0.139 2mf ± 5 0.013 3mf 0.335 0.123 0.083 0.171 3mf ± 2 0.044 0.139 0.203 0.176 3mf ± 4 0.012 0.047 0.104 3mf ± 6 0.016 4mf ± I 0.163 0.157 0.008 0.105 4mf ± 3 0.012 0.070 0.132 0.115 4mf ± 5 0.034 0.084 4mf ± 7 0.017 Note: (9A0hliVd (= (9 ANhliVdJ is tabulated as a function of m0• (5 marks) 1.0 1.0 0.601 0.318 0.018 0.181 0.212 0.033 0.113 0.062 0.157 0.044 0.068 0.009 0.119 0.050 c) Discuss whether you will use Bipolar PWM or Unipolar PWM in the inverter. (4 marks) d) It is suggested that in order to reduce the number of semiconductor switches a half bridge inverter should replace the full bridge inverter. Discuss whether you will agree with such proposal and the possible impact to the whole system. (6 marks) *** END OF PAPER*** EEE/ELEC3143/ 2019 Dec page 4 of4
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