Frequency-dependence of conductivity of new mineral oil studied by dielectric spectroscopy

Mineral oil has been widely used in power transformers and understanding the mechanism of the conduction process in mineral oil is important as it reflects the status of oil conditions. Dielectric spectroscopy, as a non-destructive measurement, has been employed for analyzing dielectric properties of material for many years. This paper presents findings from the study on new transformer oil by a frequency domain method. It has been found that the conductivity of the oil calculated from dielectric response in the low frequency range (100Hz~0.001Hz) comprises three stages: two steady states joined by a transient process. Both the distance between the two electrodes of the testing cell and the temperature of oil influence the starting frequency of the transient process. The oil model used to analyze the DC conductivity of oil has been applied to simulate the frequency-dependence conductivity curve and a new factor has been introduced into the model to improve its accuracy. According to this model, the mobility of charge carriers increases with temperature significantly, whilst density of initial charge carriers only increases slightly, indicating the crucial factor that influences the conductivity is the viscosity of the oil.