EEET4072 Design and Integration of Renewable Energy Systems Page 1 of 25 Engineering EXAMINATION EEET4072 Design and Integration of Renewable Energy Systems This paper is for Mawson Lakes students. Examination Duration: 150 minutes Reading Time: 10 minutes Exam Conditions: Extra time for reading or writing and the use of an English language or bilingual print dictionary (without annotations) for ENTEXT eligible students This exam is closed book Materials Permitted In The Exam Venue: (No electronic aids are permitted e.g. laptops, phones) Any calculator permitted Materials To Be Supplied To Students: none Instructions To Students: The exam consists of three sections: A) Multiple Choice Questions, B) Numerical Problems, C) Conceptual Questions. A formula sheet is provided with the exam. It is recommended that the ten-minute reading time is spent focussing on Sections B and C. Each section and question shows the marks allocated, which should indicate how much time you should spend on each question. Please underline your answers and provide SI units. Show ALL steps in your solutions (working out) in the exam paper provided. The use of DIAGRAMS in showing understanding may attract marks, where applicable. Answers to questions should be expressed clearly and written legibly. These aspects of presentation will be taken into account in assessment. Student Number |__|__|__|__|__|__|__|__|__| Last Name _______________________ First Name _______________________ Calculator Details Make _________________ Model _________________ For Examiner Use Only Question Mark 1 (10) 2a (4) 2b (6) 3a (8) 3b (6) 3c (6) 4a (5) 4b (8) 5a (6) 5b (7) 5c (11) 5d (3) 6a (5) 6b (6) 6c (4) 6d (5) Total ________ Study Period 5 Exams, 2017 This exam paper must not be removed from the venue EEET4072 Design and Integration of Renewable Energy Systems Page 2 of 25 Section A: Multiple Choice Question 1 [10 marks] Circle the correct answer for each question [1 mark per question] i. A car alternator uses what type of machine to generate electrical power? a) Induction machine. b) Synchronous machine. c) DC machine. ii. A transmission line reactance absorbs equal amounts of reactive power from the sending and receiving end voltages, V1 and V2, respectively. This indicates that: a) |V1| > |V2|. b) |V1| = |V2|. c) |V1| < |V2|. iii. Which statement below best describes the conditions that a photovoltaic panel would be subjected to under standard test conditions (STC)? a) Irradiance of 8000W/m2, ambient temperature of 20°C. b) Irradiance of 1,000W/m2, cell temperature of 25°C. c) None of the above. iv. Which of the following types of energy storage is best suited for short bursts (a few seconds) of power? a) Battery. b) Ultra-capacitor. c) Pumped hydro. v. The synchronous speed of an AC machine is inversely proportional to: a) The number of pole pairs. b) The supply frequency. c) None of the above. vi. An AC machine operates from a 50 Hz supply, which is its most likely synchronous speed? a) 1,800 rpm. b) 3,000 rpm. c) 3,600 rpm. EEET4072 Design and Integration of Renewable Energy Systems Page 3 of 25 vii. The synchronous speed of an induction machine can be changed by: a) Adding and varying external rotor resistance. b) Increasing the load (opposing torque) on the machine. c) Using a variable speed drive that adjusts the supply frequency and voltage magnitude, in proportion to each other. viii. Excluding machine friction and windage losses, the rotor power of a doubly-fed induction generator is: a) Only positive. b) Only negative. c) Either positive, negative or zero. ix. Remote area power systems that use a backup generator can typically fully charge a battery in: a) 4 hours. b) 10 hours. c) 24 hours. x. Which of these economics terms is the regarded as the least convincing measure of the value of an investment? a) Simple payback period. b) Net present value. c) Internal rate of return. EEET4072 Design and Integration of Renewable Energy Systems Page 4 of 25 Section B: Numerical Problems Question 2: Electric Vehicles and Economics [10 marks] 2a) Electric Vehicles [4 marks] Consider an electric vehicle with a battery that 31 kWh of accessible storage. i. Calculate the vehicle’s range, assuming an energy consumption of 109 Wh/km, and how many return trips you could complete without charging if driving to work daily. Assume work is 35 km away from your home. [2 marks] ii. How long would it take to recharge the battery after 3 return trips? Assume the battery was fully charged before setting off on any trips, and the charger is rated at 2.3 kW. [2 marks] EEET4072 Design and Integration of Renewable Energy Systems Page 5 of 25 2b) Energy Efficient Air Conditioner Economics [6 marks] When purchasing an air-conditioner, you are presented with two models. One costs $1,800 more than the other to purchase, however it will save you $250 / year on your electricity bill. a) Calculate the simple payback period in years. [1 mark] b) Comment on this investment if the new AC lasts for 7 years. [1 mark] c) Calculate the net present value (NPV) of purchasing the energy-efficient AC, if the discount rate, d, is 9%, the cost of electricity is expected to increases by 2.5% each year, and the air conditioner lasts for 15 years. [4 marks] EEET4072 Design and Integration of Renewable Energy Systems Page 6 of 25 Question 3: Power Systems [20 marks] 3a) Transmission Line Parameters [8 marks] A 298 km section of a 132 kV, 50 Hz, three-phase transmission line has a phase capacitance of 10.7 pF/m and a phase inductance of 1.4 µH/m. Assume a base of 100 MVA and that the transmission line resistance can be ignored. i. Sketch the nominal π (pi) equivalent circuit for the transmission line and calculate the circuit components as impedances and in pu. [6 marks] ii. Calculate the characteristic impedance and the surge impedance loading of the transmission line. [2 marks] EEET4072 Design and Integration of Renewable Energy Systems Page 7 of 25 3b) Power System Analysis [6 marks] The single-line diagram below represents a generator driving a load over a long transmission line with a synchronous compensator, SC, connected to the receiving end. The transmission line can be modelled as shown with an inductive reactance XL = 0.35 pu and two capacitive reactances XC = -5 pu on a base of 1,000 MVA. Under a particular operating condition the bus voltage magnitudes and angles are as shown. i. Calculate the reactive power (in MVAr) absorbed by the transmission line inductive reactance, XL. [5 marks] ii. Under a different condition, QC2 is 0.18 pu and Q2 is 0.06 pu. Calculate the reactive power absorbed by the synchronous compensator (QSC) in pu, if the load requires reactive power of 0.22 pu. [1 mark] PL , QLPS , QS jXL jXC jXC QC1 Q1 Q2 QC2 SC QSC 0.98 0pu1.03 7.1pu EEET4072 Design and Integration of Renewable Energy Systems Page 8 of 25 3c) Synchronous Machines [6 marks] A 21 kV, 275 MVA synchronous generator has a stator reactance of 1.6 pu. Stator resistance can be ignored. i. The machine is operating at 87% of its rated apparent power with a power factor of 0.84 lag, and a stator voltage of 1% below rated. Calculate the terminal voltage and the stator current phasors (in pu) under this condition. Assume a stator terminal voltage angle of 0°. [2 marks] ii. Using the results calculated above, determine the required excitation (in pu). [2 marks] iii. Under a different operating condition, the generator’s induced voltage E0 is equal to 1.9 59° pu, and the stator voltage V is equal to 0.99 0 pu. Calculate the synchronous machine’s power (in MW) under this condition. [2 marks] EEET4072 Design and Integration of Renewable Energy Systems Page 9 of 25 Question 4: Stand-alone and Hybrid Power Systems [13 marks] 4a) Hybrid (PV + Wind Turbine) Generating System [5 marks] Consider a stand-alone hybrid system (Wind + PV) that supplies power to a property that requires stored (battery) energy of 19 kWh per day. Calculate: i. The radius of the wind turbine, if it is to provide 60% of the energy to the battery. Assuming a constant wind speed of 7.5 m/s, the turbine has a peak cp of 0.23, the generator has an efficiency of 81%. [4 marks] ii. The size of the PV system assuming it is to provide 40% of the energy to the battery. Assume that there is an average of 3.9 peak sun hours per day, and an over-supply coefficient of 1.3 [1 mark] EEET4072 Design and Integration of Renewable Energy Systems Page 10 of 25 4b) Generator Only Stand-alone Power System [8 marks] Consider designing a remote area power system that uses only a generator and a battery. Assume the daily load requirement is 23 kWh/day and that all appliances are operated from AC; the inverter and generator are both 92% efficient. i. Calculate the daily stored energy of the battery to operate the AC appliances, and the daily charge required if the battery voltage is 48 V. [2 marks] ii. Calculate the battery capacity at a constant discharge rate if the maximum depth of discharge (DOD) is 73%. [1 mark] iii. Calculate the charging current required to fully charge the battery in 10 hours. [1 mark] EEET4072 Design and Integration of Renewable Energy Systems Page 11 of 25 4b) Generator Only Stand-alone Power System (continued) iv. Calculate the size (power rating, to the nearest 100W) of the generator required if the charging voltage is 20% higher than the nominal battery voltage, and if the generator should be operating at 89% of its rated power, when charging the battery. [2 marks] v. Explain whether a petrol or diesel generator would be cheaper to operate, briefly discussing the two main reasons why this is the case, assuming that the cost of diesel is slightly higher than the cost of petrol. [2 marks] EEET4072 Design and Integration of Renewable Energy Systems Page 12 of 25 Question 5: Induction Machine & Wind Turbine Operation [27 marks] 5a) Induction Machine Torque vs. Speed Characteristics [6 marks] i. Consider a wound-rotor induction machine, which has a variable external resistance, R2EXT’ that can increase the effective rotor resistance. The bold line (curve a) shown in the figure below represents the torque vs. speed characteristic of the machine when R2EXT’ is 0Ω, which has a maximum torque of 2pu. Knowing this, and that the slip corresponding to maximum torque of curves b and c are double and triple that of curve a (2sa, 3sa), respectively, complete the table below. Note that the external rotor resistance, R2EXT’, is to be expressed as a ratio of the actual rotor resistance, R2’. [3 marks] ii. Consider a 1,470 rpm squirrel-cage induction machine, that is to be used as part of a Type 4 wind turbine. Calculate the supply frequency and pu voltage required to generate rated torque if the turbine achieves peak power at a speed of 16 rpm and has a gearbox ratio of 98:1. [3 marks] s 1 T 0 c b a 3sa 2sa sa Curve sTMAX R2EXT’ / R2’ Tmax (pu) a 0 b 0.5800 c 2 pu EEET4072 Design and Integration of Renewable Energy Systems Page 13 of 25 5b) Wound-Rotor Induction Motor [7 marks] A star-connected three-phase, 1,760 rpm, wound-rotor induction motor has the following simplified equivalent circuit parameters: R1 = 0.24 Ω; R2’ = 0.22 Ω; X1 = 1.3 Ω; X2’ = 1.2 Ω. The motor is connected to a 3ph, 415V supply. Assuming that external rotor resistance is set to zero for part i), and using the simplified equivalent circuit, and the resulting Torque equation (shown on the formula sheet), calculate: i. The maximum torque of the machine and the speed (in rpm) at which this occurs. [5 marks] ii. The external resistance required such that the maximum torque occurs at a speed of 90 rpm? [2 marks] EEET4072 Design and Integration of Renewable Energy Systems Page 14 of 25 5c) Wind Turbine Characteristics [11 marks] Consider a three-bladed wind turbine, with blade radius of 50m and a maximum coefficient of performance (cp) of 0.44 at a tip-speed ratio (λ) of 7. i. Calculate the power in the wind for a rated wind speed of 12 m/s. [2 marks] ii. Calculate the operating speed of the turbine (in rpm), if the maximum cp occurs at the wind speed in part (i). [2 marks] iii. Calculate the theoretical maximum (rated) mechanical power captured by the wind turbine, given the conditions in parts (i) and (ii). [1 mark] EEET4072 Design and Integration of Renewable Energy Systems Page 15 of 25 5c) Wind Turbine Characteristics (continued) iv. Assuming the answer in part (iii), calculate the rated generator torque, assuming a gearbox with ratio 90:1. [1 mark] v. Calculate a realistic electrical (machine output) power, assuming a gear box and generator are used with peak efficiencies of 92% and 91%, respectively. [1 mark] vi. Calculate the cp needed at a wind speed of 17 m/s to maintain rated wind turbine power. [2 marks] vii. Calculate the average wind power if the average wind speed is 8m/s. [2 marks] EEET4072 Design and Integration of Renewable Energy Systems Page 16 of 25 5d) Doubly-Fed Induction Generator Wind Turbine [3 marks] Consider a 1,480 rpm doubly-fed induction generator whose stator is rated at 2.3 MW. Ignoring the stator and rotor copper losses and those from power electronic converters, calculate the rotor power, and net power fed to the grid (Pnet) for the following scenarios: i. 1,480 rpm, stator power (Ps) = 1.9 MW. [2 marks] ii. 1,500 rpm, Ps = 2.2 MW. [1 mark] EEET4072 Design and Integration of Renewable Energy Systems Page 17 of 25 Section C: Conceptual Questions Question 6. [20 marks] 6a) Power Electronic Converters [5 marks] i. Apart from the turn-on time, list the four other parameters that influence the switching power losses in a power converter. [2 marks] ii. Describe two methods that can be used to reduce the power losses of power electronic converters. [2 marks] iii. Identify these two pulse width modulation techniques. [1 mark] EEET4072 Design and Integration of Renewable Energy Systems Page 18 of 25 6b) Type 3 Wind Turbine Components [6 marks] Explain the components that make up a Type 3 wind turbine, and indicate how this differs from a Type 1 configuration. Your answer should indicate the type of generator and any other components, such as external resistances, power converters or safety devices etc. that are used. Discuss the typical (pu) power ratings of any power converters and the resulting generator variable speed range. [6 marks] EEET4072 Design and Integration of Renewable Energy Systems Page 19 of 25 6c) Grid Integration Issues [4 marks] List and briefly describe two issues associated with integrating renewable sources of energy to the network. One sentence about each issue is sufficient. [4 marks] EEET4072 Design and Integration of Renewable Energy Systems Page 20 of 25 6d) Battery Sizing for Stand-Alone and Hybrid Systems [5 marks] Explain how the battery storage requirement for a stand-alone PV system would change if a second source of renewable energy, e.g. wind, was introduced to form a hybrid system. How would this further change if a backup generator was introduced and explain how a typical generator would be sized and operated, and explain why? [5 marks] END OF EXAMINATION EEET4072 Design and Integration of Renewable Energy Systems Page 21 of 25 EXTRA SPACE FOR WRITING (please indicate the question you are continuing). EEET4072 Design and Integration of Renewable Energy Systems Page 22 of 25 EXTRA SPACE FOR WRITING (please indicate the question you are continuing). EEET4072 Design and Integration of Renewable Energy Systems Page 23 of 25 EXTRA SPACE FOR WRITING (please indicate the question you are continuing). EEET4072 Design and Integration of Renewable Energy Systems Page 24 of 25 EXTRA SPACE FOR WRITING (please indicate the question you are continuing). EEET4072 Design and Integration of Renewable Energy Systems Page 25 of 25 Formula Sheet Constants g = 9.81m/s2 ρ = 1.225kg/m3 Machine Speed synchronous speed = 60 angular (rotational) speed = 2 60 Induction Machines slip and related terms slip speed = − or − slip = − slip frequency = ideal IM efficiency = 1 − IM torque formula = 3ℎ 2(2 ′/) [(1 + 2 ′/)2 + (1 + 2 ′)2] slip for max torque in induction motor 2 ′ = |1 + (1 + 2 ′)| Synchronous Machine power and rotor angle relationship = 0 sin Power Systems characteristic impedance 0 = √ ⁄ surge impedance loading = 2 0 ⁄ lossless transmission line with reactance X = 12 sin 12 = 1 2 − 12 cos 12 Wind Turbines wind power = 1 2 23 wind Turbine Power = tip-speed ratio = Economics net present value = ∆. (′, ) − ∆ where (′, ) = (1+′) −1 ′(1+′) where ′ = − 1+
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