Question 1

Q.1. (a) Explain electromechanical breakdown in solids. [8]

Accepted Answer

Electromechanical breakdown in solids occurs when internal Maxwell compressive stress exceeds the mechanical (Young's modulus based) restoring stress, causing the solid dielectric to collapse under applied electric field; transformer/insulating oil in power apparatus serves as both an electrical insulant and a cooling medium, additionally used for arc-quenching in circuit breakers.

Question 2

Q.1. (a) What is Townsend's breakdown mechanism? Explain with suitable exampl…

Accepted Answer

Townsend's breakdown mechanism explains gas breakdown as a cumulative avalanche process driven by electron-impact ionization (coefficient alpha) and secondary emission from positive-ion bombardment of the cathode (coefficient gamma), with breakdown occurring when the Townsend criterion gamma(e^(alpha*d)-1)=1 is satisfied; breakdown in electronegative/electromagnetic gases such as SF6 additionally involves electron attachment, which competes with ionization and raises the effective breakdown strength.

Question 3

Q.2. (a) Draw the circuit diagram of basic voltage multiplier circuit. Explai…

Accepted Answer

A basic voltage multiplier (Cockcroft-Walton) circuit uses a ladder network of diodes and capacitors driven by an AC source to progressively charge capacitors in a stacked configuration, producing a DC output voltage that is a multiple of the peak AC input; cascaded transformers connect multiple identical transformer units in series (with the secondary of one stage supplying the primary/tertiary excitation of the next), allowing generation of very high AC test voltages beyond what a single transformer unit could economically provide.

Question 4

Q.2. (a) Explain the working of Marx multistage impulse generator. [8]

Accepted Answer

A Marx multistage impulse generator charges a bank of capacitors in parallel through charging resistors, then simultaneously discharges them in series via spark-gap switching, producing a high-voltage impulse of magnitude n times the individual stage charging voltage; a Klydonograph is a device that records the characteristic Lichtenberg figure produced by a surge/impulse voltage on a photographic film in contact with an electrode, used to determine the surge's polarity and estimate its magnitude.

Question 5

Q.3. (a) What is partial discharge? Draw and explain partial discharge equiva…

Accepted Answer

Partial discharge is a localized electrical discharge that only partially bridges the insulation between two conductors, typically occurring in voids/cavities within solid or liquid insulation, modeled by an equivalent circuit of a void capacitance Cc in series with a capacitance Cb representing the sound insulation in series with the void, both in parallel with the capacitance Ca of the remaining healthy insulation; the High-voltage Schering Bridge is an AC bridge used to precisely measure the capacitance and dielectric loss (tan delta) of high-voltage insulation specimens by balancing against a known standard capacitor and variable resistive/capacitive bridge arms.

Question 6

Q.3. (a) How can the resistivity of a dielectric be measured? Explain. [8]

Accepted Answer

The resistivity of a dielectric is measured by applying a known DC voltage across a guarded electrode arrangement on the specimen and measuring the resulting leakage current with a sensitive galvanometer/electrometer, then computing volume and surface resistivity from the specimen geometry; dielectric constant (relative permittivity) is the ratio of a material's permittivity to that of free space, while the loss factor (tan delta) quantifies the fraction of stored electrical energy dissipated as heat per cycle, both measured together using bridge methods such as the Schering Bridge.

Question 7

Q.4. (a) What are the causes of over voltages? Explain in brief. [8]

Accepted Answer

Over-voltages in a power system arise from internal (switching operations, faults, resonance, load rejection) and external (lightning) causes; lightning during thunderstorms results from charge separation within cumulonimbus clouds building up a large potential difference, which discharges via a stepped leader from the cloud meeting an upward streamer from the ground/structure, producing the visible return-stroke flash and an associated steep-fronted current surge.

Question 8

Q.4. (a) Explain the attenuation phenomenon in travelling waves flowing in ov…

Accepted Answer

Attenuation of travelling waves on an overhead transmission line occurs due to line resistance (I2R losses), conductor/insulator corona losses, and dielectric losses, causing both a reduction in the wave's peak magnitude and a distortion (rounding/flattening) of its wavefront as it propagates; at a T-junction where a line splits into two further lines, the reflected and refracted (transmitted) wave components are determined by the surge impedance mismatch, calculated via reflection and refraction coefficients based on the parallel combination of the branch impedances.

Question 9

Q.5. (a) What is insulation coordination? What are the problems associated wi…

Accepted Answer

Insulation coordination is the systematic process of selecting the dielectric strength of equipment insulation relative to the expected over-voltages and the characteristics of protective devices, so that insulation failures are confined to self-restoring, non-damaging locations while protecting costly, non-self-restoring equipment insulation; volt-time curves plot the breakdown voltage of an insulation/protective gap against the time-to-breakdown for impulses of varying steepness, and are used in insulation coordination to ensure the protective device's characteristic lies safely below the protected equipment's characteristic across the full relevant time range.

Question 10

Q.5. Write short notes (any two) - [2x8=16]

Accepted Answer

An expulsion-type lightning arrester uses an internal arc confined within a fibre tube, generating high-pressure de-ionizing gas that is expelled through a vent to extinguish the follow-on power-frequency current; a metal-oxide gapless arrester uses stacked zinc-oxide varistor discs with highly non-linear voltage-current characteristics to conduct only during over-voltage conditions without any series spark gap; coordination of insulation levels refers to systematically grading the withstand voltages of successive equipment along a protection scheme so each is adequately protected by the arrester's characteristic with sufficient margin.

High Voltage Engineering