Effect of contact velocity on the behaviour of decaying arcs in air

Hybrid direct current circuit breaker (HDCCB) consists of an ultra-fast electromechanical switch combined with power semiconductors to interrupt fault currents. When the ultra-fast electromechanical switch opens, arc plasma is generated between its contacts that commutate the fault current to the auxiliary circuit consisting of power semiconductors. Understanding of the arc behaviour due to the ultra-fast contact opening is necessary as the current commutation is driven by the arc voltage. This paper presents experimental results of a dynamic voltage-current (V-I) characteristics of a decaying arc plasma in air having contact opening velocities from 5 to 15 m/s. A pair of hemispherically capped copper contacts was used for the experiments. The contacts were covered by a glass tube, open from one end which makes the arc partially wall constricted. The contacts were opened with a dedicated Thomson coil based electromagnetic actuator. A computer controlled test system was used that allowed controlling the shape of the current pulse and the time instant of the contact opening on the current waveform. The conductance of the arc was calculated for different contact opening velocities. It was observed that the conductance decreased with an increase of the contact opening velocity. High speed imaging was performed to observe the physical behaviour of arcs having different contact opening speeds.