7466 2022 05 10 Q2

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Hong Kong Institute of Vocational Education (Tsing Yi) *

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2469

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Electrical Engineering

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Nov 24, 2024

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Q2. If a =9, b=9, c=9 An electric power system has 5 identical generators, each has a rating of 70MW and a force outage rate of (0.02+ (a/1000)). The table of its load vs the duration of the load is given as below: Load Demand (MW) Duration of the Load (in hours within a year) 73 2500 100 1950 240+b*5 956 180+c*5 1330 155 1550 300 480 Total number of hours = 8766 Hours (1 year = 365.25 day=365.25%24=8766 hours) (a) Determine the probability of expected loss of load of the power system in the unit of “number of days in 10 years™. (15 marks) (b) What should be the force outage rate of the system, if the probability of expected loss of load of the power system is required to be very close to (but less than) 0.1 days in 10 years? (5 marks)

Qs. If a =9, b=9 Check, whether it is possible to provide a CDT of 0.4 second in the IDMT overcurrent protection systems of the following diagram. If possible, suggest the values of TMS for all the three relays (show the detailed steps of calculations). If not possible, why not possible, and suggest any changes in the protection system design to make it possible (in this case, no detailed re-calculations on your suggested changes, just to give the area and direction of changes). Note: a) All relay is of the same type, and of 5A rating, b) The tripping time equation for the relays is: TMS * 0.14/(PSM°-1). c) For all IDMT relays, the TMS can be any value between 0.05 to 1.0 d) System voltage is 3-phase 380kV, base 3-phase power rating is 300MVA. (15 marks) 3 A - B » C Q AL Ay \AJ o L JVV LUV L_vv CcT CcT cr IDMT Relay | Plug setting | CT ratio Between the 2 points | line reactance per phase (in p.u.) is: A 125% 500/5 Source to A j(0.08+ (a/2000)) B 100% 400/5 Ato B 1(0.04+ (/2000)) C 75% 300/5 B to C 1(0.04+ (b/2000))

Q4. If a =9, b=9, c=9 For the power system shown in Figure Q4, the generator’s rating is 30 MVA and its inertia constant is 2MJoule.sec/elect radian. (a) What is generator’s synchronizing coefficient and frequency of oscillations under small disturbance when its output power is (0.7+(a/100))pu? (7 marks) (b) Find the critical clearing angle to maintain transient stability for the power system, if there is a solid short-circuit 3-phase symmetrical fault at exactly the middle point of the lower transmission line. The generator and the lines are assumed to be lossless. The electric power from generator at stable steady state is (0.7+(a/100))pu just before the fault. Show the detailed steps of calculations. (13 marks) j0.17 §0.17 |E|=1-15 jO.11 j(0.4+(6/100)) H s 10.08 — 1 — —{ — —O infinite O 3¢ et e S X ¢ j(0.3+(c/100)) e 1— —3 L 11— V=1.05/ 2-pole generator : . g j0.17 j0.17 Figure Q4: Power System for Q4 (all values are in pu, and all components are lossless)

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