Question

2. (a) (i) Explain clearly various processes which explain electric breakdown in vacuum.

(ii) Discuss about the properties of composite dielectrics.

Or

(b) (i) Explain briefly various theories of breakdown in liquid dielectrics.

(ii) Explain the Townsends criterion for a spark.

Answer 1

2. (b)

(i) 1.SUSPENDED SOLID PARTICLE MECHANISM
Commercial liquids will always contain solid impurities either as fibers or as dispersed solid particles. The permittivity of these solids (1) will always be different from that of the liquid (2).

2. CAVITATION AND BUBBLE MECHANISM:
The dielectric strength of liquid depends upon the hydrostatic pressure above the gap length. The higher the hydrostatic pressure, the higher the electric strength.

3. THERMAL BREAKDOWN; Extremely large currents are produced just before breakdown. The high current pulses originate from the tips of the microscopic projections on the cathode surface with densities of the order of 1 A/cm . These high density current pulses give rise to localized heating of the oil which may lead to the formation of vapour bubbles.

4. STRESSED OIL VOLUME THEORY: In commercial liquids where minute traces of impurities are present, the breakdown strength is determined by the largest possible impurity" or 'weak link". The electrical breakdown strength of oil is defined by the weakest region in oil.

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(ii)