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.

The oil‐impregnated transformer is an important part of the power system. The ageing degree of the internal oil‐paper insulation determines the service life of the transformer. The low frequency of the frequency domain dielectric spectroscopy can effectively characterise the ageing degree of transformer oil‐paper insulation. For accurately and quantitatively evaluate the aging state of oil‐paper insulation of transformer, the physical process of low‐frequency electrode polarisation is theoretically analysed in this work. Considering the polarisation of the low‐frequency electrode, the theoretical expression of the dielectric response of the equivalent XY model of the transformer is obtained. Meanwhile, to verify the rationality of the theoretical expression, the accelerated thermal ageing test of the oil‐paper insulation samples were carried out under laboratory conditions. The DC conductivity, ion mobility and FDS tests of oil‐paper insulation samples were tested on ageing 5, 10 and 15 days, respectively. The test results were combined with the theoretical expression of the low‐frequency dielectric response. It is found that the difference between the calculated result and the measured result is small. Therefore, the theoretical expression of the transformer equivalent XY model can be applied to calculate the dielectric parameters of the oil‐paper insulation at low‐frequency.

Over a transformer's operational lifetime, the moisture content of its insulation pressboard increases because of the ageing of its internal insulation. Thus, this moisture content becomes an important index for judging the ageing of this insulation. In this study, to analyse the moisture content of a transformer's insulation pressboard using the dielectric response insulation technique, the XY ‐equivalent model analysis method for classical transformers is combined with a dielectric‐constant algorithm for mixed media. In addition, a new method for calculating the moisture content of a pressboard is obtained via cyclic iterative comparison calculations based on the dielectric response translation formula, the temperature, and the material activation energy. The XY ‐equivalent models of transformers with various proportions are derived under laboratory conditions. The moisture contents of the transformers’ insulating pressboards are obtained under the corresponding dielectric loss factor curves by using the proposed moisture content calculation method, which provides a result that is within 4.83% of that provided by the IDAX‐300 moisture content analysis algorithm, thus demonstrating its validity.

In order to reduce resistance loss due to skin effect and proximity effect in high-frequency inductive wireless transmission coils, an impedance calculation model is built in this paper for analyzing the electromagnetic field and resistance loss of the double-layer composite conductor in the frequency range of less than 100 kHz. The analytical formulas on calculating high-frequency resistance and power loss of the composite conductor are obtained. It is found that there is a critical frequency above which the resistance of the double-layer composite conductor is significantly lower than that of a pure copper conductor. The loss density within the aluminum layer is lower than that of a copper conductor, and will increase sharply at the interface between copper and aluminum layer. The wiring mode, coil shape factor, and the magnetic field angle determine the distribution of the magnetic field intensity around the composite conductor. When the angle of magnetic field vector varies from 0° to 360°, the magnetic field intensity will vary in a spiraling way. Finally, the high-frequency resistances of the two double-layer composite conductor coils are measured and are consistent with the analytical results, which is helpful to the high-frequency coil design in inductive wireless power transmission.