Question 1:
1.1 The following observations were made in an experiment for determining of dielectric strength of transformer oil. Determine the power law equation:
|
Gap spacing:
|
4
|
6
|
8
|
10
|
|
Breakdown Voltage (kV)
|
93
|
139
|
176
|
222
|
1.2 How is the breakdown voltage of an air gap affected by air density, air pressure and air temperature?
1.3 Determine the following:
(a) The breakdown voltage of a uniform sphere gap in air with a uniform gap of 10mm and a 100mm under standard atmospheric conditions?
(b) The correction factor to be applied to the breakdown voltage determined under standard atmospheric conditions if the tests are carried out at a Laboratory 1300m above sea level and 28ºC ambient temperature. (Assume the atmospheric pressure decreases by 1.32% per 100m altitude above sea level.)
1.4 A steady current of 0.9 kA flows through a plane electrode separated by a distance of 6 mm when a voltage of 22 kV is applied. If a current of 75 A flows when the distance of separation is reduced to 1 mm and the field is kept constant at 22 kV, determine the following
(a) Townsend's first ionization's coefficient
(b) The saturation current.
1.5 Calculate at what speed a one meter width belt of a 22.5MV Van de Graaf generator must be driven to generate a maximum power output of 20kW if the medium surroundings the high tension terminal is air with an electric field strength of 3MV/m.
Question 2:
2.1 Derive an expression for maximum thermal voltage and show that the voltage is independent of thickness of specimen. Clearly state the assumptions made.
2.2 Derive an expression for critical electric field and show that the field is independent of the critical temperature of the dielectric. Clearly state the assumptions made.
2.3 When a dielectric material is subject to an electric field, charges of opposite nature is induced on the two opposite surfaces of the material and hence a force of attraction is developed and the specimen is subjected to electrostatic compressed forces and when these forces exceed the mechanical withstand strength of the material, the material collapses. If the initial thickness of the material is d0 and is compressed to a thickness d under the applied voltage V, then the compressed stress F developed due to electric flied is:
F = 0.5s0 sr (V2/d2)
Where sr is the relative permittivity of the specimen. If ç is the Young's modulus, the mechanical compressive strength is:
ç ln (d0 /d)
Derive a formula for Ea, the highest apparent field strength under equilibrium conditions in terms of s0, sr and Young's modulus.
2.4 An eight-stage Cockcroft-Walton circuit has all capacitors of 0.4µF. The secondary maximum voltage of the supply transformer is 88kV at a frequency of 310Hz. If the load current is 0.008A, determine:
(a) The voltage regulation
(b) The ripple
(c) The optimum number of stages
(d) The maximum output voltage
(e) The no-load output voltage
(f) Vomax under load conditions.
2.5 A generating voltmeter is required to measure voltage between 15kV to 200kV. If the indicating meter reads a minimum current of 2µA and maximum 30µA, determine the capacitance of the generating voltmeter and the current indicated at 200kV. Assume that the speed of driving synchronous motor is 1500rpm.
Question 3:
3.1 Describe the construction of various components used in the development of an impulse generator.
3.2 Explain with a neat diagram triggering and synchronization of the impulse generator.
3.3 Resolve the following two problem regarding an absolute electrostatic voltmeter:
(a) The voltmeter has a movable circular plate of 100mm in diameter if the distance between the plates during a measurement is 10mm, determine the potential difference when the force of attraction is 7.85mN
(b) The voltmeter has two parallel plates. The movable plate is 100mm in diameter. With 22kV between the plates, the pull is 5mN. Determine the charge in capacitance for a movement of 1mm of movable plate.
3.4 Discuss various resistance potential dividers and compare their performance of measurement of impulse voltages.
3.5 Discuss various capacitance potential dividers and compare their performance of measurement of impulse voltages.