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2003 Course High Voltage Eng. (Elective II)

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Total No. of Questions : 12] P1344 [Total No. of Pages : 3 [3764]-214 B.E. (Electrical) HIGH VOLTAGE ENGINEERING (403150) (2003 Course) (Elective - II) Time : 3 Hours] [Max. Marks :100 Instructions to the candidates : 1) Answer any THREE questions from each section. 2) Answers to the two sections should be written in separate books. 3) Neat diagrams must be drawn wherever necessary. 4) Figures to the right indicate full marks. 5) Use of logarithmic tables, slide rule, Mollier charts, electronic pocket calculator and steam tables is allowed. SECTION - I Q1) a) Explain the term Ionisation . With reference to breakdown in gases, discuss the following ionization processes. [9] i) Ionisation by Collision. ii) Photo - ionisation and iii) Secondary ionisation. b) Define Townsend s first and second ionisation coefficients. How is the condition for breakdown obtained in a Townsend discharge? [7] OR Q2) a) What is Paschen s law? How do you account for the minimum voltage for breakdown under a given p d condition? [8] b) What will the breakdown strength of air be for small gaps (0.2 cm) and large gaps (10 cm) under uniform field conditions and standard atmospheric conditions? [8] Q3) a) With reference to conduction and breakdown in commercial liquids explain: [9] i) Suspended Particle Mechanism. ii) Cavitation and Bubble Mechanism. iii) Stressed oil volume Mechanism. P.T.O. b) Draw a neat sketch of Liquid purification system with test cell and discuss function of i) Distilation column. ii) Cooling Tower. iii) Filter. iv) Reservoir. v) Vacuum pump and vi) Test cell with electrodes. [9] OR Q4) a) What is a composite dielectric and what are its properties? Describe the mechanism of short term breakdown of composite insulation. [9] b) What do you understand by intrinsic strength of a solid dielectric? How does breakdown occur due to electrons in a solid dielectric? [9] Q5) a) What are the causes for switching and power frequency overvoltages? How are they controlled in power systems? [8] b) What are the mechanisms by which lightning strokes develop and induce overvoltages on overhead power lines? [8] OR Q6) a) Explain the different aspects of insulation design and insulation co-ordination adopted for EHV systems. [10] b) How are the protective devices chosen for optimal insulation level in a power system? [6] SECTION - II Q7) a) Describe, with a neat sketch, the working of a Vande Graaff generator. What are the factors that limit the maximum voltage obtained? [8] b) Describe, with a neat sketch, the working of a 3-stage cascade transformer for producing very high a.c. voltages. [8] OR [3764]-214 2 Q8) a) b) An impulse generator has eight stages with each condenser rated for 0.16 Micro Farad and 125 kV. The load capacitor available is 1000 pico Farad. Find the series resistance and damping resistance needed to produce 1.2 / 50 Microseconds impulse wave. [10] Describe, with a neat sketch, the working of a trigatron gap? [6] Q9) a) Describe, with a neat sketch, the principle and construction of an electrostatic voltmeter for very high voltages. What are its merits and demerits for high voltage a.c. measurements? [8] b) Describe, with a neat sketch, the basic circuit for measuring the peak voltages of impulse voltage. [8] OR Q10)a) Describe, with a neat sketch, the principle of operation of Hall generator for measuring high d.c. currents. [8] b) Explain how a sphere gap can be used to measure the peak value of voltages. What are the parameters and factors that influence such voltage measurements? [8] Q11)a) What are the different power frequency tests done on insulators? Mention the procedure for testing. [9] b) Explain the method of impulse testing of high voltage transformers. What is the procedure adopted for locating the failure? [9] OR Q12)a) Explain the partial discharge tests on high voltage cables. How is a fault in the insulation located in this test? [9] b) What is an operating duty cycle test on surge diverter? Why is it more significant than other tests? [9] Y [3764]-214 3

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