Flashover phenomena in positive rod‐to‐plane air gaps under impulse and DC voltages

Discharge phenomena in positive rod‐to‐plane air gaps under lightning impulse and dc voltages were investigated. Under impulse voltages, the 50 percent flashover voltages agree with the 50 percent corona inception voltages only in a gap shorter than a certain value d c which increases with the rod diameter φ. The 50 percent flashover voltage at d c rises at a rate of about 14 kV/cm with d c in the range of d c < 2.8 cm (φ ⩽ 1.5 cm), and then it rises at a rate of about 5 kV/cm. Experimental results show that the flashover process in the former region (termed the Gp process) is different from that in the latter region (the Lp process). Under dc voltages, the critical gap length, at which the first corona may lead to flashover without any succeeding streamer corona, corresponds to d c under impulse voltage. In a gap shorter than d c flashover occurs through the Gp process (φ ⩽ 1 cm) or Lp process (φ ≥ 2 cm), but in gap longer than d c flashover may occur through another process which cannot be seen under impulse voltages. In this paper, those flashover characteristics and processes are described on the basis of experimental results and photographic observations with an image converter camera.