Time-dependent impedance characteristics of a high-pressure SF6 gap triggered by enhanced plasma injection at extremely low working coefficient

The SF6 triggered gap was a highly potential technique routine, which can quench high-energy fault arcs in large oil-filled power equipment by ultrafast current transfer, thus suppressing detonation. However, the impedance characteristics of SF6 triggered gaps were a vital factor to be focused on. In this paper, the time-dependent impedance of a high-pressure SF6 gap triggered by enhanced plasma injection (EPI) at extremely low working coefficient was investigated and the influence of gap length and conduction delay on gap impedance was studied. The results showed that an 8–12 cm 0.5 MPa SF6 gap could be conducted within 160–500 μs by EPI. The time-dependent impedance of the plasma channel showed three typical modes after gap breakdown. Mode I: the channel resistance rapidly dropped to milliohms and current was injected into the gap continuously. Mode II: the channel resistance rapidly decreased to tens of ohms and thereafter remained basically unchanged at a 50–100 μs transition period and then the channel resistance dropped sharply to milliohms. Mode III: the channel resistance decreased to hundreds of ohms. Subsequently, the discharge channel resistance continued to increase and the gap transferred into a high resistance state. When the gap distance was 8–10 cm, the conduction modes were generally Mode I and Mode II with a 160–300 μs conduction delay and a milliohms-level minimum resistance; when the gap distance was near 12 cm, the conduction mode was only Mode III with a conduction delay of over 300 μs and a kiloohms-level minimum resistance. The results provided useful guidance for the design and application of ultrafast bypass switches.