Characterisation of a corona-stabilised switch in alternative gas mixtures

This paper informs on the characterisation of a corona-stabilised switch in environmentally-friendly alternative gases to SF6. A DC high voltage is applied to an electrode with a sharp edge within the sealed switching system, upon which, the non-uniform electrical field establishes a corona discharge, creating an ionization region. The electrical field strength at this region is low enough that breakdown will not occur, until triggered by an impulsive trigger voltage. It is common that SF6 (Sulphur hexafluoride) is used within these switches, although, due to its high global warming potential, other gases are being tested in order to find suitable, environmentally-friendly, alternatives. The gas tested was the refrigerant 1,3,3,3-tetrafluoropropene, also known as HFO-1234ze, in mixtures with nitrogen (N2). Characterisation of the triggering range, delay times and jitter in the single-shot regime reveals that, with the use of HFO-1234ze, the breakdown voltage can be increased to ~280%, ~290% and ~306% of that using N2 alone, over the pressure range 0–3 bar gauge; these results were achieved with 5%, 10% and 20% of HFO-1234ze by pressure, respectively. The calculated jitter was found to vary between 0–3.3 μs with 100% N2 (at 98% self-breakdown voltage); 0.2–6.4 μs for 5% HFO-1234ze; 1.7–5.9 μs for 10% HFO-1234ze; and 1.8–3.2 μs for 20% HFO-1234ze (all taken at 75% self-breakdown voltage). The maximum triggering range in N2 was 1 kV, compared to a maximum of 10 kV with 5% HFO-1234ze; 13.6 kV with 10% HFO-1234ze; and 12.2 kV with 20% HFO-1234ze.