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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 2 Examinations 2011 / 2012
Module Title: Power Engineering
Module Code: ELEC
School: Electrical & Electronic Engineering
Programme Title: Bachelor of Engineering in Electrical Engineering
Programme Code: EELEC_7_Y
External Examiner(s): Ms Mary Desmond, Mr Colm Murray Internal Examiner(s): Mr Joe Buckley
Instructions: Answer 3 Questions ONLY. All questions carry equal marks. In marking scripts, the examiner will take account of clarity of expression and communication skills.
Duration: 2 Hours
Sitting: Summer 2012
Requirements for this examination:
Note to Candidates: Please check the Programme Title and the Module Title to ensure that you have received the correct examination. If in doubt please contact an Invigilator.
Q1. Part 1 “When a complex current wave exists in a circuit, each of the components harmonics can be looked upon as acting independently”.
(i) Discuss this in the context of the following circuits: (a) An inductive circuit in which the resistance can be regarded as negligible. (b) A capacitive circuit in which the resistance can be regarded as negligible. (5 marks)
(ii) Derive an expression for the current value of the third, fifth and seventh harmonic for (a) and (b) above. The expression to express the current in terms of the fundamental reactance x 1 and the respective harmonic emf Eȝ, Eƽ and E7. (5 marks)
Part 2 An e.m.f. is represented by e=100 sin ϴ + 30 sin (3ϴ+ 20˚) +10 sin (5ϴ + 10˚). The fundamental frequency is 50 Hz.
i. Determine the r.m.s value of the current in a circuit in which R=5Ω, L = 0.02H. (5 marks) ii. Derive the equation for the current wave. (5 marks)
Q3. (a) List three applications for differential relay protection in Electrical Power Engineering. (5 marks)
(b) A single phase two winding 10 MVA, 80kV/20kV transformer has differential relay protection fitted. (i) Outline a differential relay protection circuit for the above application. (5 marks) (ii) Select suitable CT ratios for the above circuit. (5 marks) (iii) What tests would be carried out on the above circuit prior to commissioning (5 marks)
Q4. (a) Explain why reverse active current protection and reverse reactive current protection are necessary in an industrial installation which is fed in parallel by a Utility Supply Co. and an onsite generator.
Draw a diagram of a typical installation showing the location of reverse active and reverse reactive protection relays. (10 marks)
(b) Describe an experiment you carried out in the laboratory as part of your year’s work where you measured reverse reactive current. What was the function of this experiment and discuss (with graphical support) the results you got. (10 marks)
