Temporal and frequency‐domain assessment of thermal and electrical ageing of mineral and vegetable oils

Abstract

This study focuses on the effect of accelerated thermal and electrical ageing on the dielectric properties of two vegetable oils (olive and soya oils) and a mineral oil which are thermally aged in sealed bottles at 110°C for durations up to 720 h and electrically submitted up to 2500 low-energy discharges under DC voltage. The investigated properties are the real part of the complex relative permittivity, dissipation factor and conductivity in a large frequency range (20 Hz–10 MHz) using frequency domain spectroscopy. This study also discusses the discharge currents generated in these oils and their corresponding fast Fourier transforms followed by power spectral density analysis. The electrical discharges are applied to the oil samples in a plane–plane configuration electrodes and 2.5 mm gap under DC voltage. The frequency-domain investigated descriptors are: maximum peak, mean value, rms value, peak factor, variance and standard deviation of magnitude spectrum, in addition to the resonance and bandwidth frequency of discharge currents. Furthermore, the spectrograms of these discharge currents are presented, illustrating how the frequency composition and the intensity of each frequency change with time.