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

Step 1

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a)

Step 2

Part (i):

Definition of the rate of rise of re-striking voltage:

It is defined as the slope on the restriking voltage curve from zero voltage to the first peak of the curve. It is abbreviated by the RRRV. When the rate of rise of re-striking voltage is greater than the rate of rise of the dielectric strength of the medium between the contacts, the arc will re-strike. Otherwise, it will fail to re-strike.

The dielectric strength of the air is much lower than the medium used in other circuit breakers. Thus, the air blast breaker is more sensitive to the rate of rise of re-striking voltage.

Step 3

Part (ii):

Since the energy is the rate of power and the power is proportional to the square of the current. In a DC circuit interruption, the current becomes zero from a certain value in a very short time. Thus, the rate of the change of power becomes very high, and huge energy is produced in the circuit that can damage the whole circuit by its fire.

In the case of the AC circuit, the current flowing in the circuit becomes zero for numerous times in a time period. If the AC circuit interruption is done at the moment when the current becomes zero in the circuit, then, there is no loss of the energy in the circuit and the arc is easily quenched.

Thus, the circuit interruption is easier in the AC circuit than in a DC circuit.

Step 4

Part (iii):

A surge-diverter is a device that provides a path for the current to the ground when a surge voltage is sensed by the surge-diverted in the circuit. One terminal of the surge-diverter is connected to the line close to the electrical equipment and the other terminal of the surge-diverter is connected to the ground.

At the normal temperature, the surge-diverter provides high impedance and the whole current from the line passes through the electrical equipment. But, when a surge voltage appears due to the lighting, the surge-diverter provides a low impedance, and the whole current passes through the surge-diverter to the ground. The electrical equipment becomes safe from the lighting in the transmission line.