Comparison of negative streamer development in synthetic ester and mineral oil in a point-sphere gap divided by a pressboard barrier

This article presents the results of the studies on development of negative streamers in synthetic ester and mineral oil with electrode system using a pressboard barrier. The tests were performed at standard lightning impulse voltage in point-sphere electrode configuration as reference system and a 5 mm thick pressboard barrier placed in different locations between the point and the sphere. Introducing pressboard between the electrodes allowed to relate the observations to the real insulating systems where long oil gaps are divided on shorter by insulating barriers. It was done to show whether pressboard barrier changes the streamers behavior in the considered electrode gap distance what is important from the viewpoint of assessment of dielectric properties of alternative liquids versus mineral oils. The results showed that the pressboard barrier for both liquids tested prevents breakdown. In addition, the barrier causes reduction of the propagation velocity of the streamers in synthetic ester, which is benefit in relation to the results concerning free liquid gaps.