Download Electrical Engineering Exam Questions - Summer 2007 and more Exams Electrical Engineering in PDF only on Docsity!
Cork Institute of Technology
Bachelor of Engineering in Electrical Engineering - Award
(NFQ Level 7)
Summer 2007
Electrical Engineering
(Time: 3 Hours)
Instructions Answer FIVE questions.
Examiners: Mr. J. Buckley Mr. M. Hennessy Prof. E. McQuade
Q1. “When a complex current wave exists in a circuit, each of the component harmonics can be looked upon as acting independently”.
(a) Discuss this in the context of the following circuits: (i) An inductive circuit in which the resistance can be regarded as negligible. (ii) A capacitive circuit in which the resistance can be regarded as negligible.
(b) Derive an expression for the current value of the third, fifth and seventh harmonic for (i) and (ii) above. The expression is to express the current in terms of the fundamental reactance x 1 and the respective harmonic e.m.f. E 3 , E 5 , and E 7. (10 marks)
(c) An e.m.f in which the amplitude of the fundamental (frequency 50 Hz) is 500 volts, and which contains a 21st harmonic having an amplitude of 1 per cent of the fundamental, exists in a circuit in which R = 2 ohms, L = 0.115 henry, and C = 0.2 microfarad. Calculate the amplitude of (i) the harmonic current and the harmonic p.d. across the capacitor, (ii) the fundamental current. (10 marks)
Q2. Based on the following information, calculate the potential fault currents at a, b, c, d and e in the circuit below. (20 marks)
Q3. (a) Compare and contrast Bulk Oil, Vacuum and Sulphur Hexafluoride (SF6) Medium Voltage (10 kV / 20 kV) circuit breakers. (b) Your employer wishes to supply power to a new 1600 kVA transformer which he has purchased. This will necessitate extending the existing 10 kV / 20 kV medium voltage switchboard. The extension will consist of a circuit breaker cubicle of the SF6 type and will be equipped with integral earth switch on the cable side. The circuit protection will comprise overcurrent, short circuit and earth fault protection. The cubicle will be equipped with 3 ammeters. Having carried out the relevant calculations prepare a specification and line diagram of the circuit. The line diagram should include the circuit breaker, earth switch, protection and metering devices and all associated equipments. You may assume:
- the short time rating is 250 MVA for 3 seconds
- the breaker will operate at 20 kV 3Ø, 50 Hz.
- BiL 95 kV/45kV
Fig Q
Q7. (a) Electrical Safety is of fundamental importance. Discuss electrical safety under the following headings: (i) Legislation (ii) Non-Statutory regulations (iii) Enforcement (iv) Safety Statement (employer) (8 marks)
(b) The National Rules for Electrical Installations specify the requirements for the design and erection of electrical installations so as to provide safety and proper functioning for the intended use. With this in mind, discuss the importance of insulation resistance testing. Describe an experiment you carried out as part of your years’ work where you measured the insulation resistance of electrical equipments. Discuss the results you got. (10 marks)
