Saturday, March 26, 2011

Principle of Arc Quenching in Air Blast Circuit Breaker

5. Description of different Circuit Breaker
5. 2. Air-Blast Circuit-Breaker
5.2.3 Principle of Arc Quenching in Air Blast Circuit Breaker
       The air blast circuit breaker needs an auxiliary compressed air system which supplies air to the air receiver of the breaker. For opening operation, the air is admitted in the arc extinction chamber. It pushes away the moving contacts against spring pressure. In doing so, the contacts are separated and the air blast takes away the ionized gases along with it and assist s arc extinction. After few cycles the arc is extinguished by the air blast and the arc extinction chamber is filled with high pressure air (30kgf/cm2 ). The high pressure air has higher dielectric strength than that of atmospheric pressure. Hence a small contact gap of a few centimeter is enough.
       The flow of air around contacts is guided by the nozzle shaped contacts. It may be axial, across or a suitable combination { Fig. 10(a), (b)}.

Figure 10  Flow of air around contacts in air blast circuit breaker

       In the axial blast type air flow Fig. 10 (a) the flow air is longitudinal, along the arc. In axial blast type air flow, the air flows from high pressure reservoir to the atmospheric pressure through a convergent divergent  nozzle. The difference in pressure and the design of nozzle is such that as the air expands into the low pressure zone, it attains almost supersonic velocity. The mass flow of air through the nozzle is governed by the parameters like pressure ratio, area of throat, nozzle throat diameter and is influenced by the diameter of the arc itself.
       The air flowing at high speed axially along the arc causes removal of heat from the periphery of the arc and the diameter of the arc reduces to a low value at current zero. At this instant the arc is interrupted and the contact space is flushed with fresh air flowing through the nozzle.
       The flow of fresh air through the contact space ensures removal of hot gases and rapid building up of the dielectric strength.
       The principle of cross blast illustrated in Fig. 10 (b) is used only in the circuit breaker of relatively low rating such as 12 KV, 500 MVA.
       The experience has shown that in the cross blast flow, the air flows around the arc and the diameter of arc is likely to remain stable for higher values of current.
       During the period of arc extinction, the air continues to flow through the nozzle to the atmosphere. The mass flow rate can be increased by increasing the pressure system. the increase in the mass flow results in increased breaking capacity.
      After the brief duration of air flow, the interrupter is filled with high pressure air. The dielectric strength of air increases with pressure. Hence the fresh high pressure air in the contact space is capable of withstanding the transient recovery voltage.
      After the arc extinction the interrupter chamber is filled with high pressure air. For closing operation, the air from this chamber is let out to the atmosphere. Thereby  the pressure on the moving contacts from one side is reduced and the moving contacts close rapidly by the spring pressure (Fig. 11).
       The air blast circuit breakers come under the class external extinguishing energy type. The energy supplied for arc extinction is obtained from high pressure air and is independent of current to be interrupted.

Figure 11 Principle of Operation in ABCB

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