Breakdown Behaviour of Oil-Barrier Insulation at Lightning Impulse Voltage

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. That was valid for all tested oil types excluding the synthetic ester (FR 3), at which an opposite result was obtained. With pressboard insulation the charging effect of the barrier leads to significant higher electrical breakdown field strength, dependent on used fluid. When the barrier insulation was tested with mineral oil NYTRO BIO, the highest breakdown field strength of ~ 300 kV/mm was obtained (negative polarity). The measured polarity effect for the tested fluids was not so unambiguously as for reference values e.g. so for NYNAS TAURUS, LYRA and FR 3 lower breakdown values have been measured at negative polarity. Also the impact of surface charge on the barrier to the breakdown process was studied by simulation and measurement. Generally, the surface charge on barrier increase the resulting electrical field strength and leads in the insulating gap between barrier and semi- sphere to a (quasi) homogeneous field. If the value of that field strength is high enough, the final breakdown will be initiated. Because of various discharge conditions for “uploading” the barrier under different polarities the mentioned polarity effect might be understandable. That effect should be further investigated. Nevertheless, any evidence of barrier in the insulation gap may increase the electrical breakdown field strength significantly. The obtained results may contribute to a deeper understanding of the breakdown behavior of oil-barrier insulation at different oil / fluid types and inhomogeneous field condition. Especially the impact of surface charges on barrier to the breakdown ability should be further investigated.