Dielectric Behaviour of Natural and Synthetic Esters at Inhomogeneous Field Conditions

Abstract

Natural and synthetic esters will be more and more applied as an alternative fluid to the commonly used mineral oil. The application in electrical apparatus needs, at least, the same withstand ability against electrical breakdown or partial discharge (PD) activities as given by the well-known mineral oil properties. Besides the introduced standardized procedure, the behaviour at inhomogeneous field condition is of special interest for design, manufacturing and quality testing of relevant insulations. The paper deals with some results of breakdown property at lightning impulse voltage as well as PD behaviour at AC stress conditions. To provide inhomogeneous field condition a basic arrangement tip-to-semi-sphere was investigated. At impulse testing the highest breakdown values were obtained for synthetic ester at a weak polarity effect. In addition to that an impact of time between successive test impulses (waiting time) could be observed. Especially for shorter values of waiting time (\({<}\)300 s) a statistical evaluation of the obtained values was impossible because the measured data were not statistically independent. That was probably given by the evidence of space charges generated by each impulse and should be further investigated. As described in the paper the PD behaviour shows a certain polarity effect for different oil types. If the test voltage will be increased above the PD inception voltage up to the breakdown value also a different PD behaviour was obtained. It should be noticed, that the final discharges which initiate the breakdown were always in the range of 5 ... 15 nC at both types of esters. The obtained results may contribute to a deeper understanding of the dielectric behaviour of esters at inhomogeneous field condition. Especially the impact of space charges to the breakdown ability should be further investigated.