Abstract: In order to study the discharge phenomena of plastic film overlapping on UHV DC transmission lines and explore the cause of flashover fault of a ±800 kV UHV line, an experimental study is conducted on the flashover characteristics of plastic film with three different gap distances of 1.0 m, 2.0 m and 11.5 m, and the relationship between flashover voltage and plastic states, including dry and wet state, spreading and rolling state, is studied through analyzing the equivalent salt density of three typical areas on the surface of plastic film. The results show that, when the gap distance is 1.0 m and 2.0 m, the gap flashover potential gradients of wet plastic film are 1.22 kV/cm, 0.96 kV/cm, 0.82 kV/cm and 0.7 kV/cm respectively for the spreading out and rolling up state; When the gap distance is 11.5 m, the gap flashover potential gradient of the wet plastic film is 0.56 kV/cm. Under the condition of moisture, the plastic film in rolling up state is more prone to flashover than that in spreading out state, and the average flashover potential gradient decreases with the increase of gap distance. Finally, the causes for the failed three times of restart of a ±800 kV UHV line caused by short circuit of plastic film is analyzed.

Key words: UHV, DC transmission, plastic film, gap distance, flashover potential gradient