Switchgear Riddle No.17 – Low inductive current in current chopping

Due to the short arcing time at the instant of interruption the gap between the contacts is relatively short, and since the voltage is at its peak, then in many cases this small gap may not be sufficient to withstand the full magnitude of the recovery voltage, which begins to appear across the contacts immediately following the interruption of the current. A further complication that may take place is that due to the low magnitude of the currents the currents may be forced to zero prematurely due to arc instabilities, thus creating the condition known as current chopping. This suggests that the overvoltages generated during the switching of reactive loads are the result of current chopping, reignitions or a combination of both. In a successful interruption the energy trapped in the load side will redistribute itself between the load capacitance C_L and the load inductance L_L (refer to figure). If there is a reignition, the energy trapped in the load side inductance and capacitance of the circuit will oscillate between the inductance and the parallel capacitance thus generating an overvoltage. However, because of the randomness of the point at which the reignition takes place and due to the inherent damping of the circuit, it is very unlikely that the upper limit of these overvoltages will exceed a value of 2 per unit. Unless an attempt is made to interrupt the high frequency current oscillation, which is in the range of 50 to 1000 kHz, that is superimposed on the source current which at this instant is essentially zero. Nevertheless, since the elapsed time between the reignition and the new interruption attempt is extremely short it is possible that the change in the contact gap is not enough to withstand the new recovery voltage and therefore another reignition may occur. However, the motion of the contacts during the interval between the two reignitions do increase the gap distance and therefore the breakdown voltage had to be somewhat higher than for the prior reignition. During this interval more magnetic energy is accumulated in the inductance of the load and consequently additional energy is available to trigger a breakdown which would occur at a voltage that is higher than the previous one.