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
