POSITIVE STREAMER PROPAGATION IN LARGE OIL GAPS - EXPERIMENTAL CHARACTERIZATION OF PROPAGATION MODES

This paper presents an experimental study of positive streamer propaga tion in mineral oil, in large point-plane gaps under impulse voltage. A systematic investigation was done concerning the influence of gap di stance (less than or equal to 35 cm), and voltage from streamer incept ion up to large overvoltages. The measurements presented concern break down voltage measurements, time to breakdown, visualization of streame rs (streak and still photographs), electrical measurements (transient current, charge), light emission intensity. Streamers are characterize d and then classified into different modes (2nd, 3rd and 4th modes) ac cording to their propagation velocities up to more than 100 km/s. Acco rding to the applied voltage, transitions between modes are observed, and correlations are established between charge, shape, and streamer v elocity. The physical mechanisms as well as practical consequences are then discussed.