SECTION A – Short Answers (2 Marks Each)
(a) Characteristics of liquid dielectrics
Liquid dielectrics have high dielectric strength, good heat dissipation capability, self-healing property, and uniform electric field distribution.
(b) Time lags for breakdown
Time lag is the delay between application of voltage and actual breakdown. It consists of statistical time lag and formative time lag.
(c) Need for generating high AC and DC voltages
High voltages are required to test insulation strength, study breakdown characteristics, and ensure reliability of power system equipment.
(d) Resonant transformer and its use
A resonant transformer operates on resonance between inductance and capacitance. It is used for generating high AC voltages efficiently with low power loss.
(e) Front and tail times of an impulse wave
Front time is the time taken for the impulse to rise from 10% to 90% of peak value. Tail time is the time taken for voltage to fall to 50% of peak value.
(f) Trigatron gap
A trigatron gap is a triggered spark gap used for controlled switching in impulse generators.
(g) Factors affecting spark over voltage of sphere gap
Gap spacing, sphere diameter, atmospheric conditions, surface condition, and polarity affect spark over voltage.
(h) Protective devices
Protective devices protect electrical equipment from overvoltages. Examples include lightning arresters, surge diverters, and rod gaps.
(i) Withstand voltage
Withstand voltage is the maximum voltage an insulation system can tolerate without breakdown for a specified duration.
(j) Complex permittivity
Complex permittivity represents dielectric behavior and is given by ε* = ε′ − jε″, accounting for energy storage and losses.
SECTION B – Long Answers (10 Marks Each)
(a) Solid dielectric materials and their properties
Solid dielectrics include paper, mica, glass, ceramics, plastics, and polymers. They have high mechanical strength, good insulation resistance, and varying dielectric constants. Properties depend on moisture content, temperature, and frequency.
(b) Cockcroft-Walton voltage multiplier
Cockcroft-Walton circuit is preferred due to simplicity, high voltage capability, and low ripple for DC applications. It uses cascaded diode-capacitor stages to multiply voltage.
(c) Capacitance Voltage Transformer (CVT)
CVT uses series capacitors to step down high voltage. A tuned CVT improves accuracy by resonance, minimizing phase error. It is widely used for high-voltage measurements and protection.
(d) Retardation of wave front (Numerical – Theory explanation)
When a rectangular wave strikes a capacitive termination, the wave front slows due to charging of capacitance. Retardation depends on surge impedance and capacitance.
(e) Three-electrode arrangement in dielectric measurement
Three-electrode system eliminates surface leakage errors. Different electrode arrangements are used for solid and liquid specimens to ensure uniform field distribution.
SECTION C – Long Answers (10 Marks Each)
(3a) Breakdown strength of air
For small gaps (1 mm), breakdown strength is about 30 kV/cm.
For large gaps (20 cm), breakdown strength reduces due to non-uniform fields and atmospheric effects.
(3b) Breakdown in pure liquid vs commercial liquid
Pure liquids show higher breakdown strength due to absence of impurities. Commercial liquids have lower strength due to moisture, gas bubbles, and contaminants.
(4a) Front and tail time calculation of impulse wave
Front and tail times depend on wave front resistance, wave tail resistance, generator capacitance, and load capacitance. Standard impulse is 1.2/50 µs.
(4b) Tolerances for impulse wave
Front time tolerance is ±30% and tail time tolerance is ±20% as per standards.
(5a) Methods of measuring high DC voltages
Methods include resistance dividers, generating voltmeters, electrostatic voltmeters, and sphere gaps. Limitations include accuracy, sensitivity, and environmental effects.
(5b) Generating voltmeter design
Generating voltmeter measures DC voltage without direct contact. Capacitance is chosen based on required current range and voltage range.
(6a) Switching and power frequency overvoltages
Switching overvoltages arise from breaker operations. Power frequency overvoltages occur due to faults and load rejection. Control methods include surge arresters and controlled switching.
(6b) Short notes
Rod gaps act as protective devices by providing controlled spark over.
Ground wires protect overhead lines by shielding conductors from lightning.
(7a) Dielectric constant and complex permittivity
Dielectric constant is calculated from measured capacitance. Complex permittivity is determined using dielectric constant and loss tangent.
(7b) Balanced detection method for partial discharge
Balanced detection uses bridge circuits to eliminate external noise and accurately locate partial discharges in high-voltage equipment.
