Aimed at the problems of low resolution and high data redundancy in traditional defect location functions, we proposed a cable defect location method based on Z transformation of reflection coefficient spectrum. Firstly, a reflection coefficient spectrum model of cable was established by the transmission line theory, and the feasibility of locating cable defect by the reflection coefficient spectrum was analyzed. Then, the principle of Z transformation was introduced. On the basis of reflection coefficient spectrum characteristics, the parameter values in Z transformation algorithm were determined, and a new cable defect localization function was constructed. The simulation and test results of defect location experiment show that compared with the traditional methods, the proposed cable defect location function has higher resolution and lower data redundancy, which can locate cable defect more accurately and reduce the effect of interference peaks, providing a new idea for cable defect location.

Contact method and non-contact method are two commonly used methods to measure relative dielectric constant of polymer films, and the electrode edge effect is one of the unavoidable errors in the measurement process. However, the error caused by the electrode edge effect is difficult to quantify. In order to solve this problem, COMSOL simulation software was used to model and simulate the three-electrode structure, and the errors caused by the electrode edge effect in the contact method and the variable capacitance method of non-contact method were obtained and compared respectively. The results show that the electrode edge effect of the variable capacitance method is much greater than that of the contact method. Taking the edge effect less than 0.50% as the standard, when the film thickness is less than 40 μm, the electrode edge effect of the contact method can be ignored, while for the variable capacitance method, only the film thickness is less than 30 μm, the electrode edge effect can only be ignored.

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.

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 ethylene propylene rubber for wind turbine twisting cable insulation needs to have high electrical properties and mechanical strength simultaneously to meet the demanding operating conditions. Different morphologies of zinc oxide nanoparticles doped ethylene propylene rubber samples were prepared to investigate the influence of zinc oxide morphology on the mechanical, electrical, and thermal conductive properties of ethylene propylene rubber. The results show that the larger the specific surface area of zinc oxide nanoparticles, the bigger the crosslinking density of ethylene propylene rubber, while the crystallinity remaines unchanged basically. In terms of mechanical properties, the excessive crosslinking caused by high specific surface area of zinc oxide would slightly decrease the elongation at break and tensile strength of ethylene propylene rubber. In terms of electrical properties, the specific surface area of zinc oxide has little effect on the resistivity, dielectric loss, and breakdown strength at room temperature, but the dielectric constant and breakdown strength at 90℃ increase slightly with the increase of specific surface area of zinc oxide. There is no apparent variation rule in the regulation of thermal conductivity of ethylene propylene rubber by zinc oxide nanoparticles.

The high power density development and application demand in aerospace field of silicone carbide (SiC) power modules have proposed higher requirements for the high and low temperature resistance of organic silicone encapsulation materials used in modules packaging. In this paper, the temperature resistance of three silicone encapsulants (SEMICOSIL 915HT, Duraseal 1533, R-2188) was compared. The results show that the reaction in the methyl oxidation stage during the thermal degradation process is the same as the early reaction in the thermal oxygen ageing process of silicone encapsulants. The R-2188 encapsulant has 249.5 kJ/mol of highest activation energy for methyl oxidation reaction, 335℃ of initial thermal decomposition temperature, and 5% of lower mass loss during the methyl oxidation stage, which is more suitable for the packaging of SiC power module under high-temperature operating conditions. The SEMICOSIL 915HT encapsulant has -81.1℃ of initial crystallization temperature and 0.006 4 min^-1 of crystallization rate, which is more suitable for the packaging of SiC power module under low-temperature operating conditions.

The flexible low-frequency transmission system is superior in enhancing transmission capacity, reducing line charging reactive power, and improving voltage quality at the end of transmission channels, which can effectively meet the urgent demand for efficient aggregation and transmission of medium- and long-distance offshore wind power. In order to study the operating characteristics of submarine cables under low frequency condition, an electric-magnetic-thermal-flow coupled finite element simulation model of 220 kV cross-linked polyethylene was built considering the influence of external laying environment, and the steady-state ampacity and temperature field distribution of submarine cables operating at 50 Hz and 20 Hz in different laying sections were analyzed. The finite element simulation was verified by building a steady-state thermal path model of submarine cable on the basis of IEC 60287:1995 and previous simulation results of thermal field distributions in submarine cable under low frequency. The results show that under different laying environments of land section, sea section, and submarine section, the relative errors between the ampacity and temperature field distribution calculation results of simulation model and the analytical equation of IEC are within 3%, indicating that the temperature field simulation model of 220 kV cross-linked cable proposed in this paper has good accuracy and high efficiency. Meanwhile, it is found that the frequency reduction can reduce the AC resistance of cable conductor, improve the current distribution in the cable conductor, and reduce the operating loss of each part of cable, thereby the overall operating temperature of cable is reduced eventually and the cable transmission capacity is improved favorably.

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 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.

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.