JNTU
JNTU B.Tech III Year HIGH VOLTAGE ENGINEERING Previous Question Paper (March - 2021 - R18)
Answer any five questions. All questions carry equal marks.
Main
Explain the difference between photo ionization and photo-electric emission.
Answer
Photo-ionization is a process occurring in the gas phase where an atom or molecule absorbs a photon with energy equal to or greater than its ionization potential, resulting in the ejection of an electron and the formation of a positive ion. This process is a primary mechanism for sustaining ionization in a gas discharge.
Photo-electric emission is a surface phenomenon where photons incident on a metallic cathode surface transfer their energy to electrons within the metal. If the photon energy exceeds the work function of the metal, electrons are emitted from the surface into the gas. While photo-ionization involves the bulk gas, photo-electric emission is critical for secondary electron production at the cathode, contributing to the growth of a discharge.
Define Townsend’s first and second ionization coefficients. How is the condition for breakdown obtained in a Townsend discharge?
Answer
Townsend’s first ionization coefficient (α) is defined as the average number of ionizing collisions made by an electron per unit distance traveled in the direction of the electric field. Townsend’s second ionization coefficient (γ) represents the average number of secondary electrons produced at the cathode per incident positive ion, photon, or excited particle.
The condition for breakdown is derived by considering the total current I at the anode, which includes primary and secondary ionization processes:
As the denominator approaches zero, the current I tends to infinity, signifying a self-sustaining discharge or breakdown. This occurs when:
This is known as the Townsend breakdown criterion, where d is the gap distance between electrodes.
Describe the various theories that explain breakdown in commercial liquid dielectrics.
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A solid specimen of dielectric has a dielectric constant of 4.2, and δ=0.001 at a frequency of 50 Hz. If it is subjected to an alternating field of 50 kV/cm, calculate the heat generated in the specimen due to dielectric loss.
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Discuss one method of controlled tripping of impulse generators. Why is controlled tripping necessary?
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Calculate the peak current and wave shape of the output of the following generator. Total capacitance of the generator is 53µF. The charging voltage is 200 kV. The circuit inductance is 1.47 mH and the dynamic resistance of the test object is 0.51 ohm.
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What are the different types of resistive shunts used for impulsive current measurements? Discuss their characteristics and limitations.
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A resistance divider of 1400kV(impulse) has a high voltage arm of 16 kilo-ohm and a low voltage arm consisting 16 members of 250 ohms, 2-watt resistors in parallel. The divider is connected to a CRO through a cable of surge impedance 75 ohms and is terminated at the other end through a 75-ohm resistor. Calculate the exact divider ratio.
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Describe the importance of switching over voltages in EHV power systems. How is protected against over voltages achieved.
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Give the mathematical models for lightning discharges and explain them.
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What are the various standards for HV Testing of electrical apparatus.
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What are the different power frequency tests done on insulators? Mention the procedure for testing.
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Explain various methods to generate high A. C. voltages.
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Write short notes on the following:
Generation of impulse currents
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Measurement of dielectric constant.
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