Air blast circuit breakers are mainly used for high-voltage applications for fast circuit breaking and making. These breakers offer advantages like faster operation, suited for repeated operations, auto reclosing, unit-type multi-break construction, simple assembly and maintenance, etc. The air blast circuit breaker requires a compressor plant to maintain the necessary pressure in the air receiver.
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Construction of air blast circuit breakers
These breakers are also called compressed air circuit breakers where high-pressure air is forced through a nozzle on the arc at the instant of contact separation. The ionized particles resulting in the arc between the contacts are blown away by the blast of compressed air. After the extinction of the arc, the chamber is filled with high-pressure air, which makes sure that no restrikes occur after the arc extinction.
In low-capacity circuit breakers, the isolators remain an integral part of the breaker which operates immediately after the circuit breaker providing an additional gap. But, in the EHV circuits, the isolators are independently mounted along with the air blast circuit breaker.
Air at a pressure of 20 to 30 kg/cm2 is stored in the air reservoir (1), which is taken from the compressed air system. The three hollow insulator columns (2) are mounted on the reservoir with a valve (6) at the base. The arc extinguishing chamber (3) is mounted on the top of the hollow insulators because of the existing high voltage between the conductor and the air reservoir. The current carrying part (9) connects the three arc extinguishing chambers in series per phase to the neighboring equipment terminal.
There are three double arc extinguishing poles (3) in series per phase. Therefore, the total number of circuit breaks per phase is six. Each arc extinction chamber (b) consists of a twin fixed contact (7). There are two moving contacts (8), which are shown in open condition. These moving contacts move axially to open or close the circuit. The open and close position is dependent on or controlled by the air pressure and spring force (10).
The operating mechanism (3) operates the rod (5) when it gets a pneumatic or electrical signal. The valve (6) opens up and allows entry of high-pressure air through the hollow insulator, this air with high pressure enters the arc extinction chamber and strikes the moving contacts(8). As the pressure increases beyond the spring pressure, which holds the contacts in closed position, opens up quickly. The moving contact travels through a short distance against the spring pressure and closes the port (15) for the outgoing air and the chamber is filled with high-pressure air. During the arcing period, the air moves out taking the ionized particles with it through the opening (11).
In the closing operation of the breaker, the valve (6) is closed via the electrical signal which ceases the connection of the hollow insulator and air reservoir. The valve lets the hollow insulator to atmospheric air and as a result, the pressure of the arc extinction chamber (3) drops to atmospheric pressure. This drop in pressure allows the moving contact (8) to get into a closed position with the closed contact (7) via the spring action.
The opening is very fast as air takes negligible time to reach the moving contact from the reservoir and the arc is extinguished in one cycle. The closing operation also takes negligible time because the pressure drop on the operation of the valve (6) is also instantaneous making the contacts close with the full spring pressure.
This type of construction opted for air blast circuit breakers of voltage rating 145KV and above. However, for 420KV and more, the construction is modified by adding a number of arc interruption chambers in series. For breakers with voltage ratings 12KV and below, a single break interrupter chamber is normally used. Grading capacitors are connected across the interrupter for an equal distribution of voltage. Closing resistors are connected across the interrupter to limit the overvoltage during closing operation. Opening resistors are connected across to make the circuit breaker restrike-free.
Typical ratings of Air blast circuit breakers are: –
- 12 kV, 40 kA
- 22 kV, 40 kA
- 145 kV, 40 kA, 3 cycle
- 245 kV, 40 kA, 50 kA, 2 ½ cycle
- 420 kV, 40 kA, 50 kA, 63.5 kA, 2 cycle
Principle of Arc Quenching in Air Blast Circuit Breaker
The air blast circuit breakers require an auxiliary compressed air system that will supply the compressed air to the air receiver of the breaker. For breaker opening operation, the pressurized air is admitted into the arc extinction chamber, which pushes the moving contacts and in doing so the air blast takes away the ionized particles along with it aiding in the arc extinction. After a few cycles the arc is extinguished by the air blast and the extinction chamber is filled with pressurized air at 30kg/cm2. This high-pressure air has higher dielectric strength than the atmospheric air.
Hence a small contact gap of a few centimetres is enough to restrict the restriking after the arc extinction. The nozzle-shaped contacts guide the flow of air around the contacts. The flow of air may be axial, cross, or a suitable combination.
Axial blast-type breakers
In axial blast-type breakers, the air flows from the high-pressure reservoir to the atmospheric pressure via a convergent-divergent nozzle. The difference in pressure and the design of the nozzle is such that the air expands into the low-pressure zone, attaining almost supersonic velocity. The mass flow rate through the nozzle is governed by parameters like pressure ratio, area of the throat, nozzle throat diameter as well as the diameter of the arc.
The high-speed air flowing axially along the arc causes the removal of heat from the periphery of the arc, reducing the diameter of the arc 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 this fresh air through the contact space ensures the removal of the hot gases and the rapid building up of dielectric strength.
Cross-flow type breakers
Cross flow is mostly utilized in low-rating air blast circuit breakers such as 12KV, 500 MVA, and below. In cross blasts, the airflow pushes the arc, and the length of the arc is increased. It is because of this increase in the arc length the arc resistance also increases. During the period of arc extinction, the air continues to flow through the nozzle. Increasing the pressure of the compressed air increases the mass flow rate resulting in increased breaking capacity. After a brief duration, the interrupter is filled with the pressurized air. The higher the pressure, the higher the capacity to withstand the transient recovery voltage, as the dielectric strength increases with pressure.
Merits Of Air Blast Circuit Breaker
- It can be used at high pressure.
- Reliable operation due to external source of extinguishing energy.
- Air blast circuit breakers are free from decomposition.
- Clean, non-inflammable.
- Air is freely available everywhere.
- Fresh medium is used every time. Hence the breaker can be repeatedly operated.
- At high pressure, a small contact travel is enough.
- The same air serves the purpose of moving the contact and arc extinction.
Demerits of Air Blast Circuit Breaker
- Air blast circuit breakers have a complex design of arc extinction chambers
- Complex operating mechanism,
- Problems due to switching overvoltages.
- An auxiliary high-pressure air system is necessary. The cost can be justified if there are several breakers in the switching yard.
- Problems arising out of compressed air system.
