A study of various inhibitor mixtures in natural ester oil

The difficulty with the insulation oils is well known and it has been dealt with several years by now. Using the mineral oils as a transformer insulating fluid represents a thread to the environment. With a considerable amount of research, the biodegradable insulating fluids based on natural esters were developed and used as a replacement for mineral oil. Their main features are biodegradability, a good thermal and insulating features, a high fire resistance etc. Compared to the mineral oil, the fluids based on natural esters still have a problematic development of a trend of dissipation factor in the long-term period due to lower oxidative stability. This paper primarily presents a comparison of key dielectric parameters of clean natural ester insulation fluid (rapeseed oil) and several mixtures of this rapeseed oil with 1 wt% of different antioxidants. The aim of this paper is to choose the antioxidant which contributes to achieving the best dielectric parameters of oil based on the measurement of breakdown voltage, dissipation factor and real part of relative permittivity. The oil samples were also exposed to the elevated temperature to evaluate the oxidation stability based on values of dissipation factor. The results of these measurements under high voltage are also supplemented with a detailed analysis by broadband dielectric spectroscopy.