Modelling the dielectric response measurements of transformer oil

Abstract

Dielectric polarization based measurements have been used as one of the useful techniques to characterize the quality of transformer oil. Further, understanding the variation in dielectric response of oil is important to accurately interpret the condition of a transformer composite insulation system. In this paper, the behaviour of insulating oil during frequency domain and time domain dielectric response measurements is investigated. A commercially available natural ester and mineral oil are used for this study. Observed dielectric responses have revealed that existing models which have been in use to describe polarization phenomenon in insulating liquids are insufficient to characterize the exact response of oil obtained from frequency and time domain measurements. This paper proposes two new models for characterizing frequency and time domain response of oil based on the charge transport phenomenon at the electrode interface. The phenomenon in frequency and time domain response of oil has been characterized with an equivalent circuit model and mathematical model respectively, which have been deduced by considering electrode polarization effect and diversity of ionic mobility. The proposed models are tested against measured results and then the applicability verified. Correlation between AC conductivity and low frequency dispersion of real permittivity has also been identified in the presented work. This relationship has been explained with a new mathematical model with nearly S- shaped sigmoid growth. Finally, the impact of parasitic phenomenon called Electro Hydro Dynamic (EHD) motion on dielectric response measurement has been investigated. Two mineral oil samples obtained from operational transformers are used to study the influence of EHD motion.