To investigate the partial defects positioning method of low voltage cable, frequency domain reflectometry (FDR) was used to test the low voltage cable with defects, and the positioning results were analyzed in this paper. Three partial defects of D1, D2, and D3 were made successively in phases A, B, and C of 14.1 m long low voltage cable. The D1 defect was a 20 mm × 2 mm of rectangular insulation (expose cable core) in phase A made at 8.7 m away from the head-end of cable and was paralleled resistors with different resistance to simulate high and low resistance faults. The D2 defect was a longitudinal knife mark defect with 5 mm of length in phase B made at 4.3 m away from the front end of cable, while D3 defect was a V-shaped knife mark defect in phase C made at 11 m away from the head-end of cable. The A phase samples without defects and A, B, and C three phase samples with defects were tested by FDR instrument. Then, the D3 defect was immersed in saturated salt water and the three phases end of cable was applied 1 kV voltage for accelerated ageing. The ageing time was four weeks. After ageing, the three phases A, B, and C were detected by FDR. The waveform characteristics in frequency domain and time domain of different defects were analyzed according to FDR detection results. The results show that the peak amplitude in frequency domain of the sample without defect decreases, but the peak amplitude at the defect of the sample with defects increases and is larger than the waveform amplitude on both sides, and the peak amplitude increases more significantly when there is a low resistance fault. In addition, the time domain waveform of high resistance fault shows a positive wave, but the peak is not significant. The low resistance fault presents a significant negative wave. FDR waveforms in frequency domain and time domain can be used to locate low voltage cable faults effectively, and have higher sensitivity to locate low resistance fault.

Electrical tree is a common phenomenon in XLPE cable, and there may be different scales of spike defects during the operation of cables. In this paper, the relationship between electrical tree growth morphology and partial discharge characteristics under different tip curvature radius was studied. The results show that the sample with small tip curvature radius will eventually develop into branch electrical tree under the tree initiation voltage, while the sample with large curvature radius will eventually develop into cluster electrical tree, and the discharge capacity of cluster electrical tree is greater than that of branch electrical tree. The positive and negative half cycle of PRPD diagram of the cluster electrical tree is similar to the “wing shape”. The discharge shape of the positive half cycle of the branch electrical tree is “triangle shape”, and the negative half cycle shape is “wing shape”. The growth rate of the two electrical trees shows the characteristics of “fast-slow-fast”. The growth rate of the branch electrical tree is higher than that of the cluster electrical tree, and the branch electrical tree is more harmful in the actual cable.

High performance glass fiber reinforced epoxy resin composites were prepared by vacuum impregnation and autoclave molding using multifuctional AG80 epoxy resin and phenolic epoxy resin as composite epoxy resin matrix, composite diaminodiphenyl sulfone (DDS) as curing agent and 2-ethyl-4-methylimidazole as accelerator. The results show that the prepared high performance epoxy resin composites have excellent high temperature and high humidity resistance. When the content of DDS is 35 phr, the tensile strength, compressive strength, and bending strength of epoxy resin composites reach 385, 549 and 407 MPa at room temperature, respectively, the insulation resistance reaches 1.0×10⁹ Ω, and the bending strength reaches 210 MPa at 200℃. In addition, the epoxy resin composite still maintains good mechanical properties and insulating properties after high temperature and high pressure hygrothermal test at 205℃ and 140 MPa, which can meet the application requirements of petroleum logging tools in high temperature and high humidity environment.

High voltage power module packaging insulation endures unipolar electric stress, but its space charge phenomenon has never been paid enough attention. The space charge behavior and trap energy distribution of silicone material for packaging were investigated at various temperatures in this paper. The space charge and isothermal relaxation current of silicone samples were tested at 40, 60, and 80℃ under the applied electrical field from 3 to 15 kV/mm by simultaneous measurement system of space charge and trap energy states. The results show that the space charge accumulation is getting worse with the increase of temperature and applied field. There are charges with the same polarity accumulated near the electrode, which distorts the local electrical field distribution. Furthermore, a modified isothermal relaxation current analysis method based on non-negative least-squares iterative algorithm was proposed, and the depolarized relaxation currents at various temperatures were analyzed to provide the trap energy distribution from 0.8 to 1.2 eV, which was beneficial for understanding the mechanism of space charge behavior. The accumulation of space charges results in local field distortion, and the distorted field can cause partial discharge and insulation failure of packaging. Therefore, it is suggested that the space charge problem should be taken into account at packaging design of high voltage power modules.

In order to improve the comprehensive performance of bisphenol A epoxy resin (DGEBA), bisphenol A epoxy resin was modified by alicyclic epoxy resin through blending, and the thermal, mechanical and electrical properties of the blending system were comprehensively analyzed. The results show that when the mass fraction of alicyclic epoxy resin 2021P is 10%, the blending system have better thermal, mechanical and electrical properties. Compared with the pure bisphenol A epoxy resin, the glass transition temperature of the blending system increases by 3.18%, and the bending strength and tensile strength increase by 3.95% and 4.65%, respectively. The leakage current decreases by 6.03%, the dielectric loss factor decreases by 24.25%, and the power frequency electric strength increases by 7.16%.

In this paper, the aggregation morphology of copper products in transformer oil and its influence on the ageing performance of transformer oil were investigated. The aggregation morphology of copper products in oil were tested by molecular weight distribution test method, and the ageing properties of oil samples were also analyzed periodically. The results show that the distribution of copper products changes with the ageing time obviously. The proportion of ionic copper decreases gradually with the ageing time, while the proportion of colloidal and suspended copper exhibit a tendency of continuous accumulation. The acid value and dielectric loss factor of the transformer oil increase significantly with ageing time, while the breakdown voltage decreases continuously. The concentration of ionic, colloidal, and suspended copper are positively correlated with the acid value and dielectric loss factor, and negatively correlated with the breakdown voltage.

In DC transmission lines, cable line is easy subjected to the impact of operating overvoltage and lightning overvoltage, which leads to cable line superimposed by impulse voltage on the basis of DC voltage, making the space charge accumulation on insulation layer change. In order to study the effect of impulse voltage superposition on the charge accumulation in the operating DC cable lines, a PEA method space charge measurement system under DC superimposed impulse voltage was developed through improvement on the traditional PEA method, combining with equivalent circuit formula deduction and Pspice software simulation, and reasonable selection of RC-component in measurement system. The space charge characteristics of XLPE samples under DC superimposed impulse voltage were measured. The results show that the coupling efficiency of impulse voltage and DC voltage is very high, and the space charge characteristics show that the superimposing the same polarity of impulse voltage can promote the injection and migration of the same polarity charge more compared with superimposing different polarity of impulse voltage.

The application status of polypropylene (PP) cables were introduced in this paper, and the modification principles, existing advantages and disadvantages, and research status of polypropylene insulation materials were summarized. The research and development emphasis of polypropylene semi-conductive shielding materials were described. Finally, the research progress of PP cable at home and abroad was introduced, and the research direction of PP material was pointed out.

In order to compare the thermal ageing properties and application characteristics of polyamide fiber paper commonly used for wire wrapping in the market, the thermal ageing properties in air, metal, and oil, oil absorption properties, and oil pollution properties of 2 mil aramid paper for wire wrapping from home and abroad were studied. The results show that the thermal ageing properties of three kinds of meta aramid paper in air are basically at the same level. When the aramid paper contacts with metal, the thermal ageing properties of the domestic aramid paper (2^#) is significantly lower than that of foreign aramid paper (1^#) and ZhuZhou Times Fiber Pioneer aramid paper (3^#), the gap with the latter two is at least 15%, and the performance retention rates of the latter two are basically the same. After the aramid paper conducts thermal ageing in mineral oil (105℃), synthetic ester oil (130℃), and silicone oil (150℃) for 168, 336, and 504 h, respectively, the properties of three kinds of aramid paper do not decrease significantly. After the aramid paper is immersed in insulating oil, the oil absorption characteristics of three kinds of aramid paper are close, and there is no obvious pollution to oil, which indicates that the aramid paper has good compatibility with insulating oil. The research shows that the comprehensive performance of 3^# aramid paper is better than that of 2^# aramid paper, which indicates that 3^# aramid paper can realize the domestic substitution of high level aramid insulating paper.

Dibenzyl disulfide (DBDS) is a corrosive impurity in transformer oil, which can easily lead to transformer failure. In this paper, a γ-Al₂O₃ was prepared from pseudo-boehmite by calcination method, and a series of Ag/γ-Al₂O₃ were prepared by loading Ag⁺ on γ-Al₂O₃ carrier by equal volume impregnation method. The adsorption phase equilibrium and kinetics curves of dibenzyl disulfide on Ag/γ-Al₂O₃ and the removal rate of dibenzyl disulfide in operating transformer oil by Ag/γ-Al₂O₃ were studied. The results show that the adsorption isotherm of DBDS on Ag/γ-Al₂O₃ belongs to Langmuir model. The equilibrium adsorption capacity of Ag/γ-Al₂O₃ for DBDS is 69.5 mg/g at 298 K, which is 105.9 times higher than that of γ-Al₂O₃, showing excellent adsorption performance. The removal rate of Ag/γ-Al₂O₃ for DBDS in the operating transformer oil is close to 100% under appropriate mass ratio of oil and adsorbent, and the DBDS adsorption capacity of Ag/γ-Al₂O₃ is not affected by temperature changes. The adsorption kinetic process of DBDS on Ag/γ-Al₂O₃ is well fitted by the pseudo-second-order kinetics model, and the activation energy of DBDS desorbing from Ag/γ-Al₂O₃ is 71.673 kJ/mol.