Hot Plasma Decay and Electrical Breakdown in a Miniature Relay During Hybrid Switching Operation

Small relays carrying high currents of tens to hundreds of amps can become part of novel AC and DC hybrid circuit breakers. In our work, we use numerical simulation of an electrical arc in an ultrafast gas-filled sealed miniaturized relay to predict properties of the hot gas (plasma) during the opening phase of the relay. We assume a hybrid switching operation during which an initial spark occurs between opening contacts of the relay and then a semiconductor circuit bypasses the current. When recovery voltage appears, the relay is fully open to a gap of the order of 0.1 mm. In our simulations, plasma generation is driven by electric current rising to the prospective switching current. When the initial spark vanishes, cooling of the plasma generated inside the relay is simulated. Subsequently, after a given semiconductor bypass time (of the order of 0.1 ms) we investigate the dielectric strength of the gap filled with the decayed plasma. Results for different gases are compared, including pressurized air, N2, SF6, and mixtures of C4F7N with CO2 and with N2. We show that SF6 or the mixtures of C4F7N are not much better than pure nitrogen, as result of faster cooling of lighter N2 molecules. Therefore, pure nitrogen can be recommended for usage in the miniaturized sealed relays.