Effects of metallic debris and environmental factors on negative DC needle-plate air-gap discharge

To investigate the effects of metallic debris and environmental factors on the discharge characteristics of a negative DC needle-plate air gap, a high-voltage discharge test platform was established. Discharge spectra in the solar-blind ultraviolet band, ultraviolet images, high-speed images, and breakdown voltages were obtained. The results show that the emission spectra in the 240-280 nm band are mainly composed of the N₂ Vegard-Kaplan band system and the NO γ band system, and the spectral profiles remain essentially unchanged under different metallic-debris conditions. Compared with the clean gap, metallic debris significantly reduces the breakdown voltage. At a gap of 2.5 cm, the breakdown voltage decreases from 48.66 kV to 15.50 kV and 18.55 kV after adding 0.6 g of aluminum and copper debris, respectively. At a 1 cm air gap, increasing the temperature from 15 °C to 65 °C reduces the breakdown voltage from 18.44 kV to 10.16 kV, corresponding to a decrease of 44.89%, indicating a reduced insulation margin at high temperature. Increasing pressure raises the breakdown voltage, whereas humidity has a relatively weak effect. These results provide an experimental basis for discharge analysis and condition assessment of air-insulated equipment under particle-contaminated conditions.