Regular Article

Post-breakdown stages in transformer oil

¹ Department of Physics, Faculty of Electrical Engineering, University of Žilina, Univerzitná 1, 010 26 Žilina, Slovakia
² Department of Design and Mechanical Element, Faculty of Mechanical Engineering, University of Žilina, Univerzitá 1, 010 26 Žilina, Slovakia

^a e-mail: kudelcik@fyzika.uniza.sk

Received: 25 August 2016
Revised: 3 March 2017
Accepted: 7 April 2017
Published online: 19 May 2017

Abstract

The external pressure influences significantly on the electric strength of liquid dielectrics. Quantitative explanation of this experimental fact is one of the main evidences for the bubble breakdown theory. The measurements of negative dc breakdown voltage were made in transformer oil ITO 100 for various external pressures and the developments of post-breakdown stages were recorded by high-speed camera. The initiation of breakdown was characterized by the growth of narrow streamers the creation of which was attributed to field injected electrons at local asperities of the cathode surface. Once the streamers reached the anode, large currents were found to flow through the gap leading to formation of a plasma channel. Post-breakdown stage in transformer oil consisted of vapour channel between the electrodes. This channel was created during breakdown and it expanded into space and then contracted. Time development of its length and diameter from records of high-speed camera were determined. The times of expansion and collapse were dependent on the breakdown voltage and the external pressures. These parameters decreased with the increase of the external pressure.