THEORY EXAMINATION (SEM–VI) 2016-17 HIGH VOLTAGE ENGINEERING

HIGH VOLTAGE ENGINEERING (EEE021)

SECTION – A

(Short Answer Questions | 2 Marks Each)

a) Gas Law

Gas law in high voltage engineering relates pressure, temperature, and density of a gas. According to Paschen’s law, breakdown voltage in gases depends on the product of gas pressure and electrode spacing.

b) Intrinsic Strength of Solid Dielectric

Intrinsic strength is the maximum electric field a solid dielectric can withstand without breakdown, under ideal conditions and free from defects.

c) Time Lag

Time lag is the delay between the application of voltage and actual breakdown. It consists of statistical time lag and formative time lag.

d) Ripple Voltage

Ripple voltage is the AC component present in the output of a DC voltage source, especially in rectifier circuits.

e) Flashover and 50% Flashover Voltage

Flashover is the discharge over the surface of an insulator.
50% flashover voltage is the voltage at which 50% of the applied impulses cause flashover.

f) Deltatron Circuit

A Deltatron circuit is a high-voltage rectifier circuit used to generate high DC voltages using delta-connected capacitors and diodes.

g) Collision Cross Section

Collision cross section represents the effective area of a particle for collision with other particles and determines the probability of ionizing collisions.

h) Advantages of CVT

Capacitive Voltage Transformer (CVT) is economical for high voltages, provides insulation, and is suitable for both voltage measurement and carrier communication.

i) Mean Free Path

Mean free path is the average distance traveled by an electron between successive collisions with gas molecules.

j) Limitations of Series Resistance Micro-Ammeter Method

This method has low accuracy at very high voltages, is affected by leakage currents, and is not suitable for impulse voltage measurement.

SECTION – B

(Descriptive Questions | 10 Marks Each)

a) Electron Attachment and Importance of Attaching Gases

Electron attachment occurs when free electrons are captured by gas molecules to form negative ions. Attaching gases like SF₆ reduce free electrons, suppress ionization, and thus provide better insulation 

compared to non-attaching gases such as air.

b) Factors Affecting Breakdown of Gases

Breakdown of gases depends on pressure, electrode spacing, gas composition, electrode shape, temperature, humidity, and applied voltage waveform.

c) Marx Circuit Arrangement for Impulse Generator

A Marx circuit consists of capacitors charged in parallel and discharged in series through spark gaps to generate high impulse voltages. During charging, spark gaps remain open; during discharge, they break down simultaneously, producing high voltage.

d) Van de Graaff Generator

A Van de Graaff generator produces very high DC voltages using electrostatic charge accumulation on a hollow metallic sphere. It consists of a moving belt that transfers charges from a lower potential to a high-potential terminal.

e) Sphere Gap Measurements

Sphere gap is used for measuring high voltages by observing sparkover between two metallic spheres.
Advantages: simple, reliable, and absolute measurement.
Limitations: affected by atmospheric conditions and not suitable for continuous measurement.

f) Electrostatic Voltmeter

Electrostatic voltmeter works on electrostatic force between charged plates.
Advantages: no power loss, suitable for high voltages.
Limitations: bulky, limited sensitivity, and affected by stray fields.

g) Stressed Oil Volume Theory

This theory states that breakdown in liquid dielectrics depends on the volume of oil under high electric stress. Larger stressed volumes have a higher probability of breakdown due to impurities and bubbles.

h) Tests on Bushings, Isolators, and Circuit Breakers

Bushings are tested for power frequency, impulse voltage, and partial discharge.
Isolators and circuit breakers undergo dielectric, mechanical, temperature rise, and short-circuit tests.

 SECTION – C

(Long Answer Questions | 15 Marks Each)

3) Impulse Testing of High Voltage Transformers

Impulse testing checks the ability of a transformer to withstand lightning surges. A standard impulse voltage (1.2/50 µs) is applied using an impulse generator. The test sequence includes reduced voltage shots, full voltage shots, and chopped wave tests.

Failure Location:
Failure is identified by observing oscillograms, neutral current, voltage collapse, and changes in wave shape. Comparison of waveforms helps locate insulation failure.

4) Voltage Multiplier Circuits and Cascade Transformers

Voltage multiplier circuits use diodes and capacitors to multiply AC voltage into high DC voltage. Common types include Cockcroft–Walton circuits.

Cascade connection of transformers is used to generate very high AC voltages by connecting secondary windings in series while primaries are supplied in parallel.

5) Measurement of High Impulse Currents

Impulse currents are measured using Rogowski coils, shunt resistors, and current transformers.
A typical impulse current generator uses capacitors, inductors, and switches to produce standard impulse current waves.

Applications:
Used in lightning arresters, circuit breakers, and surge testing.