Sparkover characteristics of air-CO2 mixed gas in nonuniform field gap

The atmospheric carbon dioxide concentration has increased since widespread industrialization began. Moreover, there are currently places where the carbon dioxide concentration in the air is high, for example, inside tunnels and forest fire area. Hence, it is important to research the effect of the CO₂ concentration on the insulation characteristics of air. This study examined the sparkover characteristics of air-CO₂ mixed gases. Two electrodes were used: a rod 5 mm in diameter and a plane electrode 400 mm in diameter. The gap length was 62.5 mm. Synthetic air (nitrogen: 79%, oxygen: 21%) was used in the sparkover tests, and the sparkover voltage was measured in several air-CO₂ mixtures. High DC and AC voltages were applied to the electrodes in the sparkover voltage measurement tests. Streamers were examined under several conditions using an image intensifier. The results show that the sparkover voltage depends on the CO₂ concentration. Specifically, when the CO₂ level increased 0.01% from 0.75% to 0.76%, the sparkover voltage fell rapidly by approximately 20 kV (from 54.3 kV to 33.7 kV) under a positive DC voltage. Similarly, the sparkover voltage also depends on the CO₂ concentration under a negative DC voltage and a high AC voltage.