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# Bengal Engineering and Science University 2007 B.E Electrical Engineering High Voltage Engineering - Question Paper

Thursday, 17 January 2013 07:40Web

Ex/BESUS/ EE-803/1/07 B.E. (EE) Part-IV 8th Semester Examination, 2007

High Voltage Engineering (Elective-II) (EE-803/1)

Time : 3 hours    Full Marks : 100

Use separate answerscript for each half.

Answer SIX questions, taking THREE from each half.

Two marks are reserved for neatness in each half.

FIRST HALF

}. a) What are the various mechanisms by which ionization can occur in a gas?

b) What is secondary emission? Explain the conditions to be satisfied for secondary emission to take place from a metal surface.    |8+8|

2.    a) Define Townsends first and second ionization coefficients.

b) In a laboratory experiment with certain gas, it was observed that the steady state current between two parallel plate electrodes reaches a value of 600 |aA when the applied voltage is 10 kV, distance between electrodes is 0.5 cm, while the cathode is irradiated by U.V. ray. Reducing the gap distance to

0.1 cm while keeping the field constant, reduces the current to 60 [oA. Detennine the Townsends first ionization coefficient for the gas.    [8+8|

3.    a) What is Paschens Law? How the condition for minimum breakdown voltage

in a gas can be derived from Paschens Law?

b) Find the (P x d)mjn and Vj, mjn values for air. Given that for air the constants A = 12, B = 365 and y = 0.02.    [8+81

4.    a) Explain with neat sketches the operation of a Cockroft Walton Voltage

multiplier circuit.

b) Derive the expression for voltage drop under load in a Cockroft Walton circuit and suggest how this voltage drop can be minimized.    |8+8|

5. a) Describe with neat diagram a three-stage cascaded transformer and label the power rating of the various stage transformers.

b) Prove that the utilization of the total installed kVA of the stage transformers in a cascaded assembly is poor.    (8+8|

SECOND HALF

6.    a) Why is a uniform field gap in high voltage measurement introduced? Describe

the construction of uniform field spark gap arrangement. State its advantages and disadvantages.

b) Explain with neat diagram how rod gaps can be used for measurement of high voltages. Compare its performance with a sphere gap.    |8+8|

7.    a) Why are capacitance dividers preferred for measurement of a.c. high voltage?

What is C.V.T.? Explain with Phasor diagram how a tuned C.V.T. can be used for voltage measurement in power system.

b) State the principle of operation and construction of E.S.V. for a.c. high voltage measurement. What are its merits and demerits for a.c. high voltage measurement?    |8+8|

8.    a) Explain the principle of operation and construction of Resistance divider.

State its limitations. Why the high voltage resistors are helixed and composed of large number of smaller resistances connected in series?

b) Why is compensation needed for high voltage impulse measurement? Justify the use of capacitors across high and low voltage resistors.    [8+8|

9.    a) Explain different theories of charge formation in the cloud.

b) What is HOT and CLOD lightening? What is Isokeraumic level? State and explain A-Stroke and B-Stroke.    |8+8]

10.    a) What is meant by insulation co-ordination? What is BIL? On what factor BIL

depends? State the condition for which BIL for any particular system voltage is minimum.

b) Establish insulation co-ordination amongst substation equipments. 18+8| 