w ~ * ~ THE UNIVERSITY OF HONG KONG Bachelor of Engineering Department of Electrical & Electronic Engineering ELEC3141 Power Transmission and Distribution Examination Date: 10 December, 2018 Time: 2:30 PM - 5 :30 PM Answer ALL questions. Use of Electronic Calculators: "Only approved calculators as announced by the Examinations Secretary can be used in this examination. It is candidates' responsibility to ensure that their calculator operates satisfactorily, and candidates must record the name and type of the calculator used on the front page of the examination script." EEE/ELEC3141 I 2018 Dec page 1 of 4 (Use a fresh page for each question) Ql. The one line diagram of a simple power system is shown as Fig. QI. Transmission Transmission Subswtlon ;I Primary Feeder I ~L.t!L I f J Subtrausmission Circuit- C ~ 1 j ~~ "'"-""'= Distribution Substation"" )' : Secondary Fig. Ql (a) Does Primary Feeder 1 shown in the diagram have a "radial" structure? __ (Yes/No) (3 marks) (b) Does the transmission system shown in the diagram have a radial structure if all circuit breakers are normally closed? (Yes/No) (3 marks) (c) If Primary Feeder 1 is serving 2 MWA load (3-Phase Load) in total at the rated voltage 12.47 kV (line-line voltage), what is the line current at the beginning of the feeder? __ (Amps) (5 marks) (d) If the actual voltage at the END of Primary Feeder 1 is 11 kV and the rated voltage (12.47kV line-line voltage) of the feeder is used as the per unit base value, what is the voltage at the end of the feeder? (Per Unit) (4 marks) EEE/ELEC314 l/ 2018 Dec page 2 of 4 Q2. A three-segment feeder is shown in Fig. Q2. The impedance of one mile of line is Z = 0.306 + j0.627 Q. Assuming all load power factors are 0.9 lagging and a nominal voltage is 12.47 kV (line-to-line). 300 kVA (a) Please find the Kdrop factor. 750kVA Fig. Q2 NS 0.5mile .l T 500 kVA (b) Please determine the percent voltage drop from NO to N3. (11 marks) (9 marks) Q3. A Transformer rated 200MV A, 345 wye/20.5 delta kV connects a balanced load rated 180MVA, 22.5kV, 0.8 power factor lag to a transmission line. Please determine (a) the rating of each of three single-phase transformers which when properly connected will be equivalent to the above three-phase transformer (7 marks) (b) the complex impedance of the load in per unit if the base value in the transmission line is IOOMVA, 345kV (8 marks) Q4. A 12.47kV (line-line voltage) feeder provides service to an unbalanced Y- connected load specified to be Phase a: 1000 kV A, 0.9 lagging power factor Phase b: 800 kVA, 0.95 lagging power factor Phase c: 1100 kVA, 0.85 lagging power factor Please compute the load currents (magnitudes and phase angles) assuming the loads are modeled as constant complex power. QS. Please draw the following substation layouts (a) Breaker-and-a-half (b) Double-bus double-breaker EEE/ELEC3 l 4 l/ 2018 Dec (15 marks) (7 marks) (8 marks) page 3 of 4 Q6. Refer to the Fig. Q6 below. The substation transformer is rated 24 MV A, 230 kV delta/13.8 kV wye. MVA rating LJkVlo I jt ,_,__------~,,·,,A~~:.}-~~ ( 1 : 1 ~drop· + Nptl VA -. Fig.Q6 Voltage Relay Load Center Three single-phase Type B regulators are connected in wye. The equivalent line impedance between the regulators and the load center node is: Zz= 0.5 + j 1.0985 (a) Determine the appropriate PT and CT ratios. (suppose the secondary side of PT and CT are typically 120V and SA, respectively) (4 marks) (b) Determine the R' and X ' settings in ohms and volts for the compensator circuit. (6 marks) ( c) Determine the regulators taps setting for a load of 24 MV A, 0.9 lagging power factor with the output voltages of the substation transformer balanced three- phase is 13.8 kV. The regulators have a total of 32 steps (16 raise and 16 lower) with each step changing the voltage by 5/8% (10 marks) *** END OF PAPER *** EEE/ELEC3141/ 2018 Dec page 4 of 4
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