Behaviors and Mechanism of Electrical Breakdown in Short Vacuum Gaps Under Pulsed DC Voltage

A topology consisting of a mechanical bypass switch and a power semiconductor switch (IGBT and IEGT) in parallel is widely used in high-voltage flexible dc transmission converter and static var generator (SVG). The mechanical switch will close quickly when the power semiconductor switch fails, which will cut off the failure device to protect the whole converter valve. Therefore, the mechanical contacts of the bypass switch need to close fast (within 3 ms) without contact bounce, which could only be achieved if the gap between the two contacts is extremely short (approximately 1 mm). Vacuum interrupter (VI) is most suitable for this application due to its advantages, such as compact size, high safety, environmental friendliness, long service life, and hardly oxidized contacts. In this article, a vacuum test rig is built, and the electrical breakdown behaviors of the short vacuum gaps under continuous and pulsed dc voltages are studied. It is confirmed that breakdown voltage under pulsed dc voltage is much lower than that under continuous dc voltage owing to the free and weakly bound microparticles on the contact surfaces. Finally, a new model describing the breakdown mechanism is proposed to explain the behaviors revealed in the experimental test.