blast circuit breaker
Fast operations, suitability for repeated operation, auto reclosure, unit type multi break constructions, simple assembly, modest maintenance are some of the main features of air blast circuit breakers. A compressors plant necessary to maintain high air pressure in the air receiver. The air blast circuit breakers are especially suitable for railways and arc furnaces, where the breaker operates repeatedly. Air blast circuit breakers is used for interconnected lines and important lines where rapid operation is desired.
High pressure air at a pressure between 20 to 30 kg/ cm2 stored in the air reservoir. Air is taken from the compressed air system. Three hollow insulator columns are mounted on the reservoir with valves at their basis. The double arc extinguished chambers are mounted on the top of the hollow insulator chambers. The current carrying parts connect the three arc extinction chambers to each other in series and the pole to the neighbouring equipment. Since there exists a very high voltage between the conductor and the air reservoir, the entire arc extinction chambers assembly is mounted on insulators.
Since there are three double arc extinction poles in series, there are six breaks per pole. Each arc extinction chamber consists of one twin fixed contact. There are two moving contacts. The moving can move axially so as to open or close. Its position open or close depends on air pressure and spring pressure.
The operating mechanism operates the rod when it gets a pneumatic or electrical signal. The valves open so as to send the high pressure air in the hollow of the insulator. The high pressure air rapidly enters the double arc extinction chamber. As the air enters into the arc extinction the pressure on the moving contacts becomes more than spring pressure and contacts open.
The contacts travel through a short distance against the spring pressure. At the end of the contact travel the port for outgoing air is closed by the moving and the entire arc extinction chamber is filled with high pressure air as the air is not allowed to go out. However, during the arcing period the air goes out through the openings and take away the ionized air of the arc.
While closing, the valve is turned so as to close connection between the hollow of the insulator the reservoir. The valve lets the air from the hollow insulator to the atmosphere. As a result of the pressure of air in the arc extinction chamber is dropped down to the atmospheric pressure and the moving contacts close over the fixed contacts by virtue of the spring pressure.
The opening is fast because the air takes a negligible time to travel from the reservoir to the moving contact. The arc is extinguished within a cycle.
Therefore, air blast circuit breaker is very fast in breaking the current.
Closing is also fast because the pressure in the arc extinction chamber drops immediately as the valve operates and the contacts close by virtue of the spring pressure.
Principle of Arc quenching in Air Blast Circuit Breaker (ABCB)
It 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. In doing so, the contacts are separated and the air blast takes away the ionized gases along with it and assists in extinction. After a few cycles the arc is extinguished by the air blast and the arc extinction chamber is filled with high pressure air (30 kg/cm2). The high pressure air has higher dielectric strength than that of atmospheric pressure. Hence a small contact gap of few centimeters is enough.
In axial blast type air flow, the air flows from high pressure reservoir to the atmospheric through a convergent divergent nozzle. The difference is pressure and the design of the nozzle is such that 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 a high speed axially along the arc causes the 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 dielectric strength.
The principle of operation of cross blast flow is, 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 of high pressure system. The increase in the mass flow results in the increased breaking capacity.
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.
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 form 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.
The post zero resistance of contact space is high in air blast circuit breakers. This is because the contact clearance space is filled with high pressure air after final current zero and high pressure air has high dielectric strength. The high restriking voltage appears across the contacts does not damp out through the gap because of the high post zero resistance.
Further, voltages of the order of several times the normal voltage appear across the contacts because of current chopping. If these voltages are not allowed to discharge, they may cause break down of insulation of the circuit breaker or the neighboring equipment. The overcome this difficulty resistance switching is adopted. The usual procedure is to connect a resistance in shunt with the arc.
During the opening operation, air is admitted in the arc extinguishing chamber. It separates the main contacts and pushes the auxiliary contacts. The auxiliary contacts close, thereby the resistors are connected across the arc for a short time of arcing. The auxiliary contacts are located in the inclined V shaped insulators while the resistors are located in the vertical insulators. Immediately after the arc extinction the pressure on either side of the piston of auxiliary contacts gets so adjusted that the auxiliary contacts open and resistor circuit is interrupted. Ceramic resistances of non linear characteristics, similar to those used in the lightning arresters are used for resistance switching.
These consist of silicone carbide, bound by inorganic binders subjected to heat treatment. During high current, non liner resistor offers low resistance. Thus the main arc current is partly diverted through resistor unit. As current reduces, the resistance offered by non linear resistors increases causing a greater drop across the resistor units. Thereby the voltage available for arc between auxiliary contacts is no more sufficient and arc between auxiliary contacts is automatically extinguished.