Question

Question (06+06+06+06 ) -

a) Examine and Identify the cause of the followings:

i) The air blast breakers are very sensitive to the rate of rise of re-striking voltage.

ii) Circuit interruption easier in an AC circuit than in a DC circuit.

iii) Surge diverters located very close to the equipment to be protected.

b) Assuming the same peak value, compare the following two surges: 1/50 usec surge and 3/50 usec surge.

Answer 1

Answer a:

(i) In air blast CB it is observed that tge rate at which dielectric strength of the gap increases , it lowers than the other circuit breakers. Since air has a much lower dielectric strength than the gases at same temperatures and the pressure in the oil circuit breaker. The dielectric strength of gas increase with pressure. Thus the air blast Circuit breaker are more sensitive to the restriking voltage.

(ii) Circuit breaker is easier in Ac because an Ac sine wave changes its amplitude every time ( 0 , +ve max, -ve max, 0). When it reaches zero therfore the disconnecting are can be successful quenched using a simple arc quenching mechanisms. But Dc is constant and its peak value is at maximum tendency to form a sustain arc causes fire and explosion. Therefore dc arc quenching mechanisms is more complex . Dc breaker use magnetic arc chutes to quench the arc.

(iii) A surge diverter is a piece of equipment that diverts excess voltages ( caused by spikes in the electrical supply) to the earth thus protecting sensitive electrical equipment. It provides low impedance path to ground for the current from a lightning strikes or transients voltages and then restores to anormal operating condition. When a high voltage ( greater than normal line voltage) exist on the line , the arrestor immediately furnishes a path to ground and thus limits and drains off the excess voltages. High voltage surges occurs due to lightning creates insulation failure in the equipment such as in transmission. Thats why surge diverter are placed close to the equipment.