Streamer initiation in mineral oil Part I: Electrode surface effect under impulse voltage

This paper presents a study of streamer inception in mineral transformer oil, in point-plane and rod-plane geometry under impulse voltage. The measurements performed with points and rods over a wide range of tip radii show a marked decrease of initiation fields when the electrode radius is increased. The initiation field is divided by 30 over the investigated range (from 1 mum points up to 2 cm rods). This effect compares fairly well with the "surface effect" known for breakdown voltages under uniform field with large electrodes. Plotting these results together shows the decrease of streamer initiation fields over a very wide electrode surface range (12 decades), proportional to S-0.17 (S: electrode surface area, cm(2)). These results suggest the influence of electrode surface defects on streamer initiation under impulse voltage.