The natural ester insulating oil transformer has advantages such as green environmental protection and strong overload capacity, which is an important development direction for the high-voltage and large capacity power transformer. The compatibility between insulating oil and insulating paint has a significant impact on the transformer design and operation maintenance. In this paper, the vegetable insulating oil and insulating paint materials were conducted thermal ageing experiments, and the compatibility between insulating oil and six commonly used insulating paint materials for power transformers was studied. The characteristic changes in the kinematic viscosity, moisture content, acid value, dielectric loss, breakdown voltage, and gas chromatography of natural ester insulating oil before and after thermal ageing were observed and analyzed. The results show that the structural integrity and functional stability of the insulating paint sheet remain good. Except for the kinematic viscosity, the physicochemical and electrical properties of the natural ester insulating oil have changed after ageing, but there is no significant deterioration. The dissolved gas content increases significantly after thermal ageing, and the gas chromatography results show good compatibility between insulating oil and insulating paint.

The difference of laying methods in different sections of marine transmission line has a great impact on the ampacity of submarine cable, so it is of great significance to study the ampacity under typical laying methods in transmission section. In this paper, an electric-heat-current coupling model of submarine cable in landing section and submarine section of submarine cable line in ±160 kV DC transmission project was established. On the basis of finite element method, the influence of air domain size, position of angle steel support, seawater and soil factor on the steady ampacity of submarine cable were studied under four typical laying methods, which is the submarine cable laid at the bottom of cable trench and laid on the angle steel support in landing section and the submarine cable laid and directly buried in submarine section. The results show that the submarine cable laid at the bottom of cable trench is greatly affected by the air domain size, and increasing the convection area can effectively improve the steady ampacity of submarine cable. The semi-closed area between the support and the inner wall of trench should be taken into account in calculating the ampacity of cable laid on the angle steel support. The temperature of submarine cable laid at the shallow depth of support is lower and its ampacity is larger. In the submarine section, the submarine cable temperature changes in the same direction with the rise and fall of sea water temperature, which is opposite to the increase and decrease trend of sea water velocity. The increase of soil temperature results in the decrease of buried submarine cable ampacity. The landing section is the bottleneck section of ampacity calculation, and the ampacity can be improved effectively by using the water filling cable trench when the submarine cable is laid at the bottom of cable trench. When the submarine cable is laid on the support, the ampacity can meet the engineering requirements by installing cooling water pipe.

Epoxy composite insulating materials doped with 0%, 1%, and 10% of modified nano Al₂O₃ were prepared and conducted thermal ageing tests at 150℃, and their thermal ageing characteristics and mechanism were studied through dynamic thermal mechanical testing, scanning electron microscopy testing, and linear thermal expansion testing. The results show that an inhomogeneous ageing structure is formed in epoxy composite insulating material during thermal ageing, and the thickness of surface ageing layer is related to the content of nano Al₂O₃. When the doping mass fraction of nano Al₂O₃ is 1%, the thickness of surface ageing layer is the smallest, and the nanoparticles are well dispersed and closely embedded in the epoxy matrix. According to the free volume theory of glass transition, it is found that the free volume fraction of sample is the smallest when the doping mass fraction is 1%, the corresponding α main relaxation time of the epoxy matrix is the longest, and the molecular chain activity is the lowest. Adding an appropriate amount of modified nano Al₂O₃ into the epoxy resin can improve the cross-linking degree of molecular chain and reduce the free volume of system. Thereby the diffusion of oxygen in the material is weakened, and finally the inhomogeneous thermal ageing degree of the epoxy composite insulating material is reduced.

Epoxy impregnated paper is one of the main insulation materials of valve-side bushing, and its performance is affected by curing process. In order to explore the effect of curing temperature on the electrical properties of epoxy impregnated paper, we prepared epoxy impregnated papers at 120℃, 130℃, 140℃ and 145℃ of post-curing temperature, respectively, and their glass transition temperature, dielectric properties, and DC breakdown strength were tested. The results show that with the increase of post-curing temperature, the glass transition temperature increases, the DC breakdown strength increases at first and then decreases, and the dielectric loss factor decreases at first and then increases. The analysis shows that the addition of hydroxyl-rich wrinkle paper will increase the curing rate in interface area of epoxy impregnated paper, leading to the frequency of interface relaxation peak decrease at first and increase, which affects the insulation properties of epoxy impregnated paper.

In this paper, the heterogeneous non-naphthenic transformer oil based on natural gas-to-liquid technology and traditional naphthenic transformer oil were used as research object. The streamer branching patterns in oil samples in the 25 mm point-to-plate electrode system was observed under positive lightning impulse voltage. The streamer discharge images were conducted multifractal analysis to obtain the multifractal parameters as the classification and evaluation index of streamer branching morphology. The results show that when the applied voltage is lower than the accelerated voltage, the streamer branching process in the two oil samples can be classified as a two-stage model dominated by the steady growth of side branch and main branch. When the applied voltage is higher than the accelerated voltage, the streamer branching process in naphthenic oil is a three-stage model, each stage dominated by steady growth of side branch, steady growth of main branch, and rapid growth of main branch, respectively, and the streamer branching process in gas-to-liquid transformer oil only includes two stages, which are steady growth stage of main branch and rapid growth stage of main branch. Further theoretical analysis shows that the competition between space charge and primary electric field is distorted due to the higher ionization energy of gas-to-liquid transformer oil components, which is the main reason for the abrupt change of streamer branching model in gas-to-liquid transformer oil under accelerating voltage.

In high-voltage insulated gate bipolar transistor (IGBT) devices, traditional Si-based semiconductor chips are gradually replaced by emerging wide-bandgap semiconductor chips. High-power-density wide-bandgap semiconductors lead to a sharp rise of operating temperature in power electronic modules. Silicone gel is used as an insulating material for the packaging of high-voltage IGBT devices, and its insulating performance faces the challenges of high temperature and thermal ageing. In order to study the effects of high temperature and thermal ageing on the thermal stability and insulating properties of silicone gel, we studied the effects of high temperature and long-term thermal stress on the dielectric properties, volume conductivity, and breakdown strength of silicone gel by high temperature experiments and thermal ageing experiments of silicone gel materials, and analyzed the high temperature characteristics and thermal ageing characteristics of the silicone gel. The results show that with the increase of temperature, the real part of complex permittivity and breakdown strength of silicone gel decreases, and the dielectric loss and DC conductivity increases. With the increase of thermal ageing time, the real part of complex permittivity of silicone gel increases, the dielectric loss decreases at first and then increases, the DC conductivity increases at first and then decreases, and the breakdown strength decreases.

The transformer fault diagnosis technique based on a single detection method is difficult to identify the same type of defects of oil-paper insulation in detail, which cannot meet the requirements of power system on equipment operation reliability under the background of rapid development of deep offshore wind power. Therefore, an oil-paper insulation defect identification method based on information fusion of phase-resolved partial discharge (PRPD) spectrum and dissolved gas analysis (DGA) data was proposed. Six kinds of electrode models were designed and made to simulate the typical defects of surface discharge in transformers with different electric field inhomogeneity coefficients, and PRPD and DGA data were collected. Then convolutional neural network (CNN) and back propagation neural network (BPNN) were adopted to recognize the patterns of PRPD spectrum and DGA feature vector of six kinds of defects, respectively. Finally, the CNN-BPNN information fusion model based on D-S evidence theory was proposed to realize joint diagnosis based on PRPD spectrum and DGA data. The results show that the CNN-BPNN model based on the D-S evidence theory can effectively correct the wrong output of the single criterion model and reduce the uncertainty of the classification results. When the input dimensions of PRPD spectrum are 8×8, 16×16, and 32×32, the recognition accuracy of the model integrated with the DGA feature vector is 93.21%, 97.53%, and 99.17%, respectively, which is 4.81%, 2.78%, and 0.84% higher than that of PRPD single criterion model. The CNN-BPNN model can effectively integrate the electrical physical information and chemical product information of partial discharge, which not only improves the accuracy of defect identification, but also enhances the confidence of the output results, and reduces the data storage requirements, providing accurate, reliable, and lightweight defect identification methods for intelligent operation and maintenance of transformers.

In order to obtain the temperature rise characteristics of a new environmentally friendly gas HFO-1336mzz(E) applied in gas insulated transmission line (GIL), a multi-physics coupled model of magnetic-heat-fluid field was established based on the finite element method. The internal temperature distribution of GIL under rated operating conditions was simulated and analyzed, and the effects of the type of buffer gas, filling gas pressure, mixing ratio, and operating current on the temperature rise of GIL were studied. The results show that the temperature field inside the GIL shows a temperature gradient distribution of upper high and lower low under rated working conditions. The temperature rise of the A-phase conductor is only 0.70℃ greater than that of the B-phase conductor, and the temperature rise of grounded enclosure is the smallest. Under the same conditions, the temperature rise of conductor in HFO-1336mzz(E)/CO₂ mixture is only 0.38℃ lower than that in HFO-1336mzz(E)/N₂ mixture. Considering the impact of solid deposition from the decomposition of HFO-1336mzz(E), CO₂ is more suitable as a buffer gas than N₂. Increasing the filling gas pressure and mixing ratio can reduce the temperature rise of conductor, and the temperature rise of conductor in 10%HFO-1336mzz(E)/90%CO₂ mixture at 0.70 MPa is only 5.02℃ higher than that in pure SF₆ at 0.50 MPa. The temperature rise of GIL conductor and grounded enclosure increases with the operating current, and the effect of filling gas pressure on the current-carrying capacity of GIL is greater than the mixing ratio. When the operating current exceeds 3.0 kA, the temperature rise of GIL would exceed the temperature rise limit.

With the rapid development of offshore wind power, high-voltage XLPE submarine cables are palying critical role in transmitting electricity in the marine transmission system. In order to investigate the effect of thermal oxygen ageing on the frequency domain dielectric properties of high-voltage XLPE submarine cables, XLPE samples were conducted accelerated ageing experiments at 135℃, and the frequency domain dielectric properties of XLPE samples with different ageing degree were tested. When analyzing the dielectric properties of measured samples, we found that a large amount of conductance loss and polarization loss were superimposed in the low and medium frequency domains, which seriously affected the accuracy of applying frequency domain dielectric properties to assess the ageing state of high-voltage XLPE submarine cable insulation. To slove this problem, a method for separating conductance loss and polarization loss in dielectric response data was deduced. Finally, the effect of conductance loss on the accuracy of model parameter identification was explored, and a characteristic parameter to characterize the ageing degree was proposed. The results show that the frequency domain dielectric curves of high-voltage XLPE submarine cables shift towards high frequency as the ageing progresses, and the low-frequency dispersion phenomenon becomes more apparent. The accuracy of H-N (Havriliak-Negami) model parameter identification can be improved by removing the conductance loss in the imaginary part of complex dielectric constant. The relaxation strength ∆ε and high-frequency dielectric constant ε_∞ could characterize the insulation ageing state of high-voltage XLPE submarine cables, which can realize the assessment of insulation ageing for high-voltage XLPE submarine cables.

The performance of insulating paper determines the reliability and life of power transformers. In order to ensure the safe and reliable operation of transformers, we combined with domestic and foreign literature and related research results, described the current research status of physical modification technology to improve the performance of insulating paper from the nanoparticle modification and thermal stabilizer modification, analyzed the advantages and disadvantages of the two modification methods, and explored the economic and feasible modification method of cellulose insulating paper for power transformers to essentially improve the reliability of transformer operation.