Breakdown behaviour of oil-barrier insulation at lightning impulse voltage

Abstract

The breakdown behavior of oil-barrier insulation is of special interest for design, manufacturing and quality testing for relevant insulations in electrical apparatus. Besides the commonly used mineral oil with different chemical structure (inhibitors) also other insulating fluids as natural and synthetic esters become more and more applied as an alternative ecofriendly fluid. The application in electrical power apparatus needs, at least, the same withstand ability against electrical breakdown as given by the well-known mineral oil properties. In practical design e.g. of power transformers very often interfaces of pressboard material are used mainly in form of insulating barriers. Therefore, the knowledge of breakdown properties of such type of insulation and, in particular, the interaction between the pressboard barrier and the insulating fluid seems to be obligatory for an optimal design of relevant apparatus. The paper deals with some results of breakdown behavior of oil-barrier insulation at inhomogeneous field under lightning impulse voltage stress conditions. The investigation was performed with various types of mineral oil as well as natural and synthetic esters. To provide inhomogeneous field conditions a test arrangement tip-to-semi-sphere was used. For having some reference values also the same test set without barrier (pure oil) was investigated. Additionally, some tests were carried out at standardized conditions (homogeneous field). For generalizing of measured voltage values the electrical breakdown field strength was evaluated. At pure oil test set (reference values) the known polarity effect was measured, what means, that at positive polarity (tip) the breakdown values are significantly lower than at negative one.