Dielectric analysis of vegetable and mineral oils

This paper deals with analysis and comparison of the specific dielectric properties and chemical structure of sunflower, rapeseed and commonly used transformer mineral oil. The measured sunflower and rapeseed oils are natural ester molecules with a triglyceride structure and have mainly excellent fire resistance and biodegradability. The measured common mineral oil is produced from mixture of hydrocarbons from crude oil and belongs to the group of naphthenic oils consisting mainly of cycloalkanes and is relatively flammable and dangerous for living environment. The measurement was performed by means of the dielectric spectroscopy method. The real and imaginary parts of complex permittivity of measured samples were analyzed in a frequency range from 50 mHz to 100 kHz and in temperature range from -50 °C to +90 °C. The measured spectrums displayed relaxation processes (α and β) as well as the conductive component (σ). The results showed that the measured dielectric properties of the investigated vegetable oils are very similar to each other in the evaluated frequency range, but differs from the common mineral oil properties. The common mineral oil had these properties much better than the sunflower and rapeseed oils. Adapted mixtures of vegetable and mineral oils with suitable inhibitors can be considered to using as an alternative insulation liquid into power transformers instead of mineral oils.