Behavior of positive streamers in ester liquids and mineral oil in a non-uniform field with and without insulating pressboard barrier

This article presents the results of the studies on positive streamer behavior in non-uniform field the insulating system, in which the free liquid gap is divided on parts by pressboard barrier. The streamer characteristics were assessed mainly on the basis of registration of light emitted during streamer development and additionally on the basis of propagation velocity evaluated. Three liquids were tested during the studies namely mineral oil, synthetic ester and natural ester. A non-uniform electric field was created by point-to-sphere electrode system, a standard practice in laboratory, although very much unlikely taken place in real transformers. Two liquid gaps 20 mm and 40 mm were considered. The studies included LIBV determination of free liquid gaps, acceleration voltage determination in each case and streamer behavior assessment when 4 mm in thick pressboard barrier was placed 10 mm below HV tip. The results showed the differences in LIBV of the liquids considered in favor of natural ester for 20 mm gap and in favor of mineral oil for 40 mm gap. However when comparing acceleration voltage much better properties indicated mineral oil tested, what was expected based on literature data. When introducing the pressboard barrier the streamers significantly changed their characteristics in relation to the system without barrier. In most cases, at the values of the voltage at which the streamers developed as fast, they have been slowed down. Additionally, the differences in breakdown voltage when barrier introducing were significantly reduced. This fact is a benefit in assessment of ester liquids at lightning impulse stresses and in consideration for their use in power transformers.