Research on the variation of dielectric properties of oil-paper insulation for power equipment over a wide temperature range

The main part of oil-immersed power equipment is oil-paper insulation. At lower ambient temperatures, transformer oil will become viscous. As a result, compared to those at room temperature or operating temperature, the frequency domain dielectric properties of oil-paper insulation at low temperatures are drastically different. For evaluating the insulation condition of electrical equipment, it is essential to accurately determine their dielectric properties. This paper described the FDS test, the DC conductivity test, and the transformer oil viscosity test, which was carried out in a laboratory environment for different ageing of oil-immersed pressboard and transformer oil at 233-373 K. The effects of temperature on the dielectric properties at the competing mechanisms of polarisation and conductivity loss were clarified based on variations of the FDS curves. Considering the viscosity change of ageing transformer oil at low temperatures, the Arrhenius and VFT viscosity equations were used to achieve a fitting calculation in the different temperature zones. Based on the molecular/ion transition model in the force field and electric field, the characterisation relationships between the dynamic viscosity, DC conductivity, and test temperature of transformer oil were established. The limitations of conventional transformer oil equivalent dielectric relaxation models over a wide temperature range were finally clarified by combining the test findings of the DC conductivity over a wide temperature range with observed and simulated calculations. This paper provides an essential theoretical reference when using the FDS test to diagnose the insulation performance of oil-immersed power equipment in extremely cold regions.