Abstract
This article presents the results of the studies on negative streamer propagation in a point-to-sphere electrode system with a pressboard barrier placed between them. The proposed electrode system gave the opportunity to assess the influence of the insulating barrier on streamer development in the conditions close to the actual transformer insulating system where the liquid gap is typically divided in parts by using pressboard barriers. The studies were performed for five commercial dielectric liquids. Among them two were biodegradable synthetic esters and two were biodegradable natural esters. Mineral oil, as the fifth liquid, was used for comparison. The measurements were based on electrical and optical experimental techniques. From the results obtained it may be concluded that, independently of the liquids tested, the electrical strength of the insulating system considered was increased by about 50%. In the case of streamer development assessed using photomultiplier-based light registration it is not possible to indicate clearly which of the liquids tested is better under the conditions of the experiment. In all cases streamers always developed slowly (2nd mode) at all voltage levels applied during the studies. In turn, the intensity of the discharge processes, comparing the same voltage levels, was mostly higher when streamers developed in ester liquids, however, the differences noticed were minimal.
Highlights
Ester-based insulating liquids, as eco-friendly products, have been developed providing the transformer market with an interesting alternative to mineral oils
They are characterized by quite good dielectric properties, among which special attention is directed to the higher electrical permittivity of the esters than mineral oils, which is an advantage for electrical field distribution in paper-dielectric liquid insulating systems
There is a lack of change in the alternating current (AC) breakdown voltage up to a moisture content of 600 ppm for synthetic esters, and up to a moisture content of 300 ppm for most of the natural esters, respectively
Summary
Ester-based insulating liquids, as eco-friendly products, have been developed providing the transformer market with an interesting alternative to mineral oils For both natural and synthetic esters, several experimental studies have been conducted providing knowledge on different aspects of ester properties [1,2,3,4,5,6,7]. Without a doubt the synthetic and natural esters are friendly for the environment, being biodegradable and having high flash points They are characterized by quite good dielectric properties, among which special attention is directed to the higher electrical permittivity of the esters than mineral oils, which is an advantage for electrical field distribution in paper-dielectric liquid insulating systems. Considering the interaction with the solid insulation in the case of ester liquids, the moisture from thermal aging of the paper is absorbed by the esters, which lengthens the paper lifetime [2,7,8,9,10]
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