Basic Design of Arc Extinction Chamber:
The considerations for design for the Arc Extinction chamber used in oil circuit breakers and air-blast circuit breakers would naturally be different. As far as the oil circuit breakers are concerned, the design of the arc extinguishing chamber would involve:
- Calculation of the arc voltage at various instances of time.
- A clear analysis of the process of gas formation under the influence of the arc.
- Calculation of pressure inside the Arc Extinction chamber due -to liberation of the gases.
- Velocity with which the gases move in the zone around the arc.
- Calculation of recovery voltage in the gap between contacts both for very high currents as well as for very low currents.
- Evaluation of the time of Arc Extinction for different values of current.
- Calculation of the amount of oil expended by burning during one opening operation.
- An analysis of the process by which the Arc Extinction chamber is filled up with oil after the burnt gases escape.
The arc extinguishing chamber in an air-blast circuit breaker has to satisfy the following basic requirements:
- Reliable extinction of the electric arc at the nominal voltage and the maximum rated current which may sometimes by 10,000 amp or more.
- Successful Arc Extinction within an extremely short duration of the order of two cycles or less.
- Reliable and quick extinction without restriking when opening capacitive currents of unloaded transmission lines and highly inductive currents of unloaded transformers.
- Suitable operation without any change in operating characteristics over a long period of time and when operating repeatedly.
- Contact system should be simple and have easy access.
- Quantity of arc extinguishing medium (compressed air in the case of air-blast circuit breakers and special electronegative gases like SF, in the case of some of the modern circuit breakers) should be minimum for the given operation.
Taking the above requirements into account, the design of arc extinguishing chamber in an air-blast circuit breaker would therefore involve:
- Design of the complete pneumatic system like pipes, valves, etc.
- Proper choice of the arc extinguishing medium (at the present time the choice is between compressed air and SF6).
- Calculation of the voltages appearing across the various gaps in a multibreak system.
- Calculation of the air pressure and velocity.