Air Blast Circuit Breaker

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The most common method of arc control in air circuit breakers is that of subjecting the arc to high pressure air blast. There are two types of air blast circuit breakers:

(i) Axial blast types

(ii) Cross blast types.

The designations refer to the direction of the air blast in relation to the arc.

Axial Blast Circuit Breaker

The fixed and moving contacts are held in closed position by spring pressure. The chamber when a tripping impulse opens the air valve. The air entering the are chamber exerts pressure on the moving contacts which moves when the air pressure exceeds the spring force. The air moves with sonic velocity near the nozzle and the arc is subjected to high pressure and there is considerable heat loss due forced convection. With this the diameter of the arc is reduced and the core temperature is very high. The temperature gradients set up within the are very steep which results in greater heat losses.

When the current passes through zero, the air blast is more effective because the residual column is very narrow and the high rate of heat loss becomes increasingly effective. It is known that with a given are length and heat loss per unit surface area, the total rate of heat loss is proportional to the arc diameter, whereas the total energy content of the arc is roughly proportional to the square of the diameter The narrower the residual column, the more effective are the heat losses in reducing the temperature and conductivity, Such conditions may allow the column to recover dielectric strength very rapidly at current zeros.

It is important to note here that the air pressure from the reservoir is maximum initially and falls thereafter It is known that for a particular reservoir pressure there is a certain optimum contact gap at which the breaking capacity is a maximum. This gap (s usually small (in mm) and may reach very quickly it the inertia of the moving parts is kept to a minimum. The shorter the gap, relatively smaller amounts of energy are released in the arcing chamber. The arc is kept in the high velocity blast of air converging into the nozzle throat The falling reservoir pressure and short optimum gap result in three important features of the axial blast principle.

1. The interruption must take place at the first current zero after the optimum gap has reached otherwise restrikes may take place at subsequent zeros due to falling air presssures. It is to be noted here that the chances of interruption in case of OC.В. increase if arcing persists beyond the first current zero.

2. The axial blast circuit breaker gives high speed clearance because of the short gap needed for interruption. This is desirable for improving transient stability on high voltage transmission and interconnection net-works.

3. The small contact gap after interruption constitutes inadequate clearance for the normal system voltage; therefore, an auxiliary switch known as an isolating switch is incorporated as part of this C.B. and opens immediately after fault interruption to provide the necessary insulation clearance. The moving contact is allowed to return and engage the fixed contact as the air pressure in the chamber falls below the spring pressure. The air pressure on the moving contact must be maintained until the isolator is fully open.

For low voltages the isolating switch is not required and an adequate travel is provided instead for the moving contact.

The arcing time of arc-controlled circuit breaker varies considerably depending upon the breaking current. The higher the breaking current (within the rating of the breaker), the smaller the arcing time. The arcing time in case of air blast circuit breaker is independent of the breaking current because of the fixed air pressure and the optimum short gap. The arc duration as a function of breaking current is almost flat as can be seen in fig. the short gap along with an isolating switch gives a total break time of 2 to 5 cycles.

The operation of the air blast circuit breaker is very much affected by the circuit natural frequency. When the current is passing through zero value the residual column has relatively high resistance which reduces the likelihood of the restriking voltage transient being damped. Now the effect of rate of rise of restriking voltage during these zero current conditions is more serious especially where the chance of extinction decreases after the optimum gap has reached. It is to be noted that the chance of extinction in case of oil circuit breaker increases form one current zero to the next, The effect of natural frequency on the performance of the air blast circuit breaker is overcome by shunting the arc with resistors of suitable values.

Cross-Blast Air Circuit Breakers

In this case the blast is directed transversely, across the arc and the physical conditions are different from the axial blast. The cross blast lengthens and forces they are into a suitable chute and serves rather the same purpose as electromagnetic force in the low voltage air C.B. discussed earlier. The final interruption gap is good enough to provide normal insulation clearance so that a series isolating switch is unnecessary. Consistent high-speed operation is not usually obtained to the extent possible with the axial blast air C.Bs.

Air blast C.Bs. can also be of (i) live tank type, and (ii) dead tank type Live tank has a metal tank insulated from ground and compressed air is used for insulation between contacts. The tank is supported by a porcelain insulator. In case of dead tank type, the tank is held at ground potential. The breaker constants are insulated from the tank by compressed air in parallel with solid insulation immersed in the air.

Most of the circuit breakers up to 11 kV are either of the air break type or of the oil break type. Between 11 kV and 66 kV mainly oil C.Bs. are in use while between 132 kV and 275 kV the market is shared by oil (both minimum as well as bulk oil) and gas blast breakers. At the highest system voltages i.e. between 400 kV and above the C.Bs. are of the gas blast type.

Also Read- Types of Insulators

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