Electrical Power and Machine Drives

Electrical Power and Machin" rel="nofollow">ine Drives Date: May 2016 Time allowed: 2 hours Instructions to Candidates: This is an unseen closed book examin" rel="nofollow">ination. This exam paper is made up of three sections: Candidates should answer all questions in" rel="nofollow">in Section A. All questions in" rel="nofollow">in Section A are marked out of 5. Candidates should answer one question only in" rel="nofollow">in Section B. Each question in" rel="nofollow">in Section B is marked out of 30. Candidates should answer one question only in" rel="nofollow">in Section C. Each question in" rel="nofollow">in Section C is marked out of 30. Materials provided: Graph Paper Materials allowed: A scientific calculator may be used in" rel="nofollow">in this exam. Unannotated paper versions of general bi-lin" rel="nofollow">ingual dictionaries only may be used by overseas students whose first language is not English. Subject-specific bi-lin" rel="nofollow">ingual dictionaries are not permitted. Electronic dictionaries may not be used. Access to any other materials is not permitted. Turn Over Section A Answer all questions in" rel="nofollow">in this section. Question A1 What environmental and economic benefits accrue from the deployment of power factor correction technology in" rel="nofollow">in electrical power transmission and distribution systems? (5 marks) Question A2 Discuss briefly, the purposes of symmetrical short circuit calculations and state the assumptions usually made in" rel="nofollow">in these calculations. (5 marks) Question A3 Discuss briefly the benefits of usin" rel="nofollow">ing the Per Unit (p.u.) system in" rel="nofollow">in electrical power system calculations. (5 marks) Question A4 Explain" rel="nofollow">in the advantages of IDMT protection schemes over time or current graded schemes. (5 Marks) Question A5 In the context of sin" rel="nofollow">ingle-phase power transformers, explain" rel="nofollow">in, usin" rel="nofollow">ing diagrams, the process of short circuit and open circuit testin" rel="nofollow">ing. Discuss what in" rel="nofollow">information is gain" rel="nofollow">ined and explain" rel="nofollow">in how transformer efficiency may be estimated from these tests. (5 marks) Question A6 Explain" rel="nofollow">in briefly how a Variable Voltage Variable Frequency (VVVF) in" rel="nofollow">inverter may be used to vary the speed of an in" rel="nofollow">induction motor above and below its base speed. Illustrate your answer with diagrams of typical output voltage waveforms and motor torque-speed characteristics. (5 marks) Question A7 Explain" rel="nofollow">in how a 3-phase, fully-controlled, 6-pulse converter, may be used as a 2-quadrant drive to control the speed and direction of a separately excited dc motor attached to a hoist. Use circuit diagrams and sketches to illustrate your answer and discuss briefly the flow of active and reactive power durin" rel="nofollow">ing both liftin" rel="nofollow">ing and lowerin" rel="nofollow">ing operations. (5 marks) Question A8 Describe, with the aid of relevant circuit and waveform diagrams, how a separately excited motor’s speed and direction may be controlled usin" rel="nofollow">ing an H-bridge ‘chopper’ circuit. (5 marks) [40 MARKS] End of Section A Turn Over Section B Answer one question only from this section. Question B1 A balanced delta-connected load havin" rel="nofollow">ing an impedance of 15 /25.44 Ω/ph is connected to a 220 V, 3-phase 50 Hz supply. a) Calculate the phase and lin" rel="nofollow">ine currents. (5 marks) b) Calculate the associated complex power. (4 marks) Three identical coils are connected in" rel="nofollow">in star to a 3-phase 440 V, 50 Hz supply. The power consumed is 15 kW at a power factor of 0.85 laggin" rel="nofollow">ing. c) Calculate the lin" rel="nofollow">ine and phase currents. (6 marks) d) Calculate the resistance and in" rel="nofollow">inductance of each coil. (9 marks) e) Sketch a complete phasor diagram for part (a) above. (6 marks) [30 MARKS] Turn Over Question B2 A three-phase, 45 MVA, 33/11 kV transformer has an equivalent impedance referred to the primary of: Zeq = 12.10 / 70o  per phase. a) Express this impedance in" rel="nofollow">in p.u. to both the primary and secondary ratin" rel="nofollow">ings and comment on the results. (8 marks) Figure B2 The transformer described is employed in" rel="nofollow">in the balanced power system represented by the sin" rel="nofollow">ingle lin" rel="nofollow">ine diagram shown in" rel="nofollow">in Figure B2. If the power system is operatin" rel="nofollow">ing at its rated receivin" rel="nofollow">ing end voltage VR = 11 kV when deliverin" rel="nofollow">ing a current of 1500 A at p.f. 0.8 laggin" rel="nofollow">ing to the load, calculate (usin" rel="nofollow">ing Sb = 45 MVA): b) The sendin" rel="nofollow">ing-end voltage (Vs) and the power angle. (12 marks) c) The sendin" rel="nofollow">ing-end real and reactive power. (6 marks) d) Comment on the effect of addin" rel="nofollow">ing load components that ‘generate’ VArs. (4 marks) [30 MARKS] Turn Over Question B3 R1 and R2, shown in" rel="nofollow">in Figure Q4 below, are 5 A over-current IDMT relays havin" rel="nofollow">ing a time/psm characteristic given by: a) Usin" rel="nofollow">ing the tabulated data, calculate appropriate Plug Settin" rel="nofollow">ings and Time Multiplier Settin" rel="nofollow">ings to ensure min" rel="nofollow">inimum relay operate times and a min" rel="nofollow">inimum gradin" rel="nofollow">ing margin" rel="nofollow">in of 0.5 seconds for the fault conditions given. Show all relevant relay operate times in" rel="nofollow">in your workin" rel="nofollow">ing. (20 marks) b) Recalculate the relay operate times and the gradin" rel="nofollow">ing margin" rel="nofollow">in for all relevant faults with only one transformer in" rel="nofollow">in service. Comment on the change in" rel="nofollow">in gradin" rel="nofollow">ing margin" rel="nofollow">in. (10 marks) Relay Feeder Load (Maximum) Fault Level (Both transformers in" rel="nofollow">in service) Fault Level (One transformer in" rel="nofollow">in service) R1 380 A 200 MVA 110 MVA R2 290 A 150 MVA 92 MVA Figure B3 [30 MARKS] End of Section B Turn Over Section C Answer one question only from this section. Question C1 A 400 V, 3–Phase, 4-Pole, 50 Hz, in" rel="nofollow">induction motor has the followin" rel="nofollow">ing per-phase parameters referred to the stator win" rel="nofollow">indin" rel="nofollow">ing: Rotor resistance 1.4 Ω Rotor reactance 2.9 Ω Stator resistance 1.2 Ω Stator reactance 3.4 Ω No load current (1.3 - j2.6) A a) Draw the simplified, per phase, equivalent circuit. (8 marks) b) Calculate the motor in" rel="nofollow">input current and gross torque at a slip of 4% when the motor is supplied at 50 Hz. (12 marks) c) The supply frequency to the motor in" rel="nofollow">in (b) above is now reduced to 30 Hz usin" rel="nofollow">ing a VVVF drive. If the rotor frequency remain" rel="nofollow">ins the same, calculate the new rotor current referred to the stator together with gross torque and slip values. (10 marks) [30 MARKS] Question C2 A 3-phase, fully-controlled, 6-pulse converter is fed from a 400 V, 50 Hz supply. The dc motor is rated at 450 V, 600 A, 1500 rpm and has an armature resistance of 25 mΩ. When the motor is liftin" rel="nofollow">ing at its ratin" rel="nofollow">ing, calculate: a) The active power supplied (P) (3 marks) b) The approximate firin" rel="nofollow">ing angle (α) (6 marks) c) The reactive power supplied (Q) (6 marks) d) If the hoist is now operated in" rel="nofollow">in lowerin" rel="nofollow">ing mode at 500 rpm on two thirds rated load, recalculate the correspondin" rel="nofollow">ing values for (P), (α) and (Q). (15 marks) [30 MARKS] Turn Over Question C3 A variable frequency, chopper circuit with a fixed on time (Ton) of 500 μs, is used to control the torque and speed of a series dc traction motor. The dc supply voltage is 600 V provided via an LC circuit that ensures current drawn from the source is smoothed. The motor is rated at 500 V, 200 A and 1600 rpm. Its combin" rel="nofollow">ined armature and field resistance is 0.12 Ω and the correspondin" rel="nofollow">ing in" rel="nofollow">inductance is sufficient to ensure contin" rel="nofollow">inuous current flow. a) Calculate the chopper frequency and the average lin" rel="nofollow">ine current drawn when the motor is stationary drawin" rel="nofollow">ing full load current. (20 marks) b) Calculate the motor speed when the chopper frequency is 200 Hz and the motor is developin" rel="nofollow">ing half rated Torque