Design and Experimental Investigation on Small Currents Magnetic Blow-out Equipment of Air DC Circuit Breaker

A type of magnetic blow-out equipment is designed for air DC circuit breaker in this paper since it is difficult to break small currents. This equipment is composed of field coil and some magnetic plates which is made of iron to supply vertical and approximately uniform magnetic field for arc running zone in the interrupter without influencing performance of the main current circuit. The arc can be driven to the arc chute by the magnetic field and the arc stream can be cut and stretched which leads to the persistent rise of arc voltage. Finally the current of arc drops to zero and air DC circuit breaker breaks small current successfully. The magnitude and distribution of magnetic field result from the blow-out equipment is calculated in this paper by means of finite element analysis and the results has been proved by experimental investigation. To achieve the goal of breaking bidirectional small currents the equipment adopts system power supply of the contact fracture of air DC circuit breaker. There is a capacitor in parallel of the field coil and AC contactors in series. The design of control loop is beneficial to break out excitation circuit after the breaking of arc between the moving and fixed contacts and restrain transient overvoltage of high frequency to avoid burning of field coil. Lastly experimental investigations are done to prove the performance of the equipment. It can be concluded that magnetic equipment for arc driving is helpful for pulling out of the arc and there exists a critical current which leads to a longest time of arcing. Beyond that, arcing time increases and breaking performance decreases when residual magnetism of magnetic material is considered when breaking bidirectional currents. This paper is meaningful for air DC circuit breaker to pass the testing item of "Search for critical currents and low current test duty" described in IEC standards.