The influence of sampling period, voltage rising speed, and overcurrent breaking time on the breakdown voltage of insulating materials in the process of breakdown voltage test was analyzed. Proper improving technology and method was select to eliminate or reduce the error. The results show that the dispersion of test results is reduced by digital filtering, weighted average, and other methods, and the accuracy of dynamic measurement is improved at the same time.

With the development of advanced electronics and high frequency communication technology, polyimide (PI) film faces more and more high thermal conductivity requirements as an important polymer insulating material. The intrinsic thermal conductivity of traditional PI film is smaller, which cannot meet the rapid cooling requirement of electronic components. In recent years, researchers had carried out a lot of researches on thermally conductive PI films and a series of polyimide-based composite films were prepared by adding inorganic thermally conductive fillers. In this paper, we summarized the latest research progress in thermally conductive PI-based insulating films and discussed the relevant thermal conduction behavior. The key factors influencing the thermal conductivity of films, which include fillers types, particle sizes, addition amount, and the interface interaction between fillers and polyimide matrix, were described systematically. In addition, the technical challenges of high performance polyimide-based thermally conductive insulating film materials were summarized and proposed.

The most important component of high thermal conductivity main insulation material for large generators is high thermal conductive epoxy adhesive. As an important component of high thermal conductive epoxy adhesive, BN powder can improve the thermal conductivity of epoxy resin, but it would affect other properties. In this paper, the researches of high thermal conductivity main insulation materials were analyzed and summarized. It was found that BN powder can inhibit the generation and aggregation of space charge in BN/epoxy composites effectively, and reduce the threshold electric field of space charge in BN/epoxy composites. The addition of BN powder could reduce the electric strength of BN/epoxy composites, under the same BN mass fraction, the electric strength of micron BN/epoxy composites was lower, and the conductivity of the composites could be reduced more effectively by nano BN than by micro BN. The surface flashover voltage of the composite decreased after charging on its surface. Then the research direction of high thermal conductivity adhesives in the main insulation materials for large generators in China was proposed.

The interface of thermal conductive polymer composite matrix and filler can affect the overall thermal conductivity of the composite. However, limited by traditional testing technologies, it is difficult to study the interface thermal conduction mechanism from microscopic view. In this paper, the interface thermal conduction mechanism of boron nitride (BN)/low density polyethylene (LDPE) composites was studied by scanning thermal microscope (SThM), and the interface thermal properties of the BN/LDPE composites were analyzed quantitatively. The test process of SThM was simulated by finite element analysis, and the interface thermal conduction process at the inorganic-organic interface was revealed. The results show that with the increase of BN content, the thermal conductivity of the composite increases. When the mass fraction of the BN reaches 20%, the thermal conductivity of the composite increases by 22%. The sample morphology in micro-nano scale and the voltage distribution image reflecting the thermal properties were obtained by SThM, it is found that the thermal conductivity interface width of the BN/LDPE composite is 150–200 nm. At the area that two BN particles are in contact with each other, the high thermal conductivity area increases, and the thermal conductivity interface width changes little. The fitting curve between thermal conductivity and the square of output voltage is obtained by testing the standard samples, and the interface thermal conductivity of the BN/LDPE composites is calculated to be 0.33–39.81 W/(m·K). The simulation results show that the probe tip can distinguish the filler, interface, and matrix, and the thermal conductivity of the composite increases with the increase of the interface width and thermal conductivity.

The effects of thermal conductive fillers on the tensile strength and thermal conductivity of EPDM composites were studied by filling EPDM rubber with multi-scale alumina, nano-zinc oxide alone, and their mixer. An EPDM thermal conductive composite with good mechanical properties was prepared, and its insulating properties were tested. The results show that when the addition amounts of multi-scale alumina and nano zinc oxide are 200 phr, respectively, the thermal conductivity of the composites is 1.12 W/(m·K), the tensile strength is 5.01 MPa, and the tear strength is 21.12 N/mm.

In order to improve the dielectric properties, insulating properties, and thermal conductivity of polymer composites comprehensively, on the basis of boron nitride nanoplates (BNNS) modified polyvinylidene fluoride hexafluoropropylene(P(VDF-HFP)), we prepared a three-layer (P(VDF-HFP)/P(VDF-HFP)-BNNS/P(VDF-HFP)) composite by layer by layer solution casting method, the bottom and top layer were the P(VDF-HFP) film, and the middle layer was the modified P(VDF-HFP) film with 0, 1%, 3%, 5% volume fraction of BNNS, respectively. The properties of the composites were analyzed. The results show that compared with the pure P(VDF-HFP) and the single layer composite films modified with 1% BNNS, the dielectric properties of the three-layer polymer material modified with 1% BNNS is improved obviously, its dielectric constant and dielectric loss factor is 8.52 and 0.022 at 1 000 Hz, respectively. Its electrical breakdown property and thermal conductivity are more excellent, the electric strength is 452.6 MV/m, which is 17.3% and 24.9% higher than that of the pure P(VDF-HFP) and the single layer composite films modified with 1% BNNS, respectively, and its thermal diffusion rate is 0.04 mm²/s, which is 25% and 5.3% higher than that of the pure P(VDF-HFP) and the single layer composite films modified with 1% BNNS, respectively.

A micro/nano co-doping epoxy/boron nitride (BN) composite was prepared using micro-BN and nano-BN as fillers, and the variations of thermal conductivity and breakdown characteristics of the epoxy composites with the nano-BN doping content were studied when the total doping content of BN was fixed. The results show that when the total mass fraction of BN is 20%, with the increase of nano-BN doping content, the thermal conductivity of the composite decreases slightly, the power frequency electric strength increases at first and then decreases, and the endurance time of the sample with thickness of 0.2 mm is shortened under the bipolar square wave voltage of 8 kV and 25 kHz. The thermal conductivity of the epoxy composite doped with pure micro-BN is the largest (0.83 W/(m·K)), and its endurance time is the longest (193 s) under high frequency bipolar square wave voltage, which are 277% and 408% higher than that of pure epoxy resin, respectively. When the mass fraction of nano-BN is 1%, the power frequency electric strength of the epoxy composite is the highest (131 kV/mm), which is 27% higher than that of pure epoxy resin. Therefore, for the micro/nano co-doping epoxy composite system, the addition of nanoparticles can improve the power frequency electric strength of composite, but it will reduce the thermal conductivity of the composites and shorten the withstand time under high frequency bipolar square wave voltage.

According to the structure characteristics of HX_D1 locomotive high voltage wall bushing, the electric fields of an imported high voltage wall bushing and the domestic high voltage wall bushing were analyzed by COMSOL finite element method. Then a domestic high voltage wall bushing was produced by the automatic pressure gel (APG) forming technology combining with the application situation of the imported high voltage wall bushing, its electrical properties were compared, and environmental tolerance tests of thermal-cold oil cycling, vibration impact, and high and low temperature alternating were conducted. The results show that the domestic high voltage wall bushing has reasonable structure and good environmental adaptability, which is suitable for the traction transformer field of HX_D1 locomotive.

In order to study the effect of nano Al₂O₃ on the DC electrical tree of epoxy resin, epoxy and epoxy/Al₂O₃ nanocomposites were prepared. The samples were conducted electrical tree test and partial discharge detection by using needle-plane electrode, and the inception, growth, and ageing of DC electrical tree and corresponding partial discharge characteristics were analyzed. The results show that the addition of nano-Al₂O₃ particles can decrease the initiation probability and growth rate of DC electrical tree in epoxy resin. When the mass fraction of Al₂O₃ is less than 3%, the higher the content of Al₂O₃, the stronger the ability to suppress the electrical tree. In addition, the faster the voltage rise rate, the higher the initiation probability of DC tree in epoxy resin. When the voltage applying time is about 1 800 s, the DC electrical tree stops growing, and the electrical trees are all branch trees. Partial discharge is an important reason for DC electrical tree ageing of epoxy resin, and the partial discharge pulse clusters appear in the voltage rise and fall phases in each cycle mainly, which is due to that the electrical trees grow faster at these two phases. By comparing the partial discharge test results of pure epoxy resin and the nanocomposites with 1% Al₂O₃, it is found that nano-Al₂O₃ can suppress the partial discharge in the electrical tree developing process, which leads to the decrease of discharge pulse amplitude and appearing of more sparse partial discharge areas, indicating that the nano particles can suppress the development of electrical tree by suppressing partial discharge.

When we design and optimize the insulation structure of a DC cable, its electric field distribution characteristics is an important reference basis. A simplified model of 320 kV DC cable was established by COMSOL simulation software, and its steady-state and transient electrical characteristics were studied. Then the reliability of the simulation model was verified by experiments. The result shows that the maximum temperature of conductor and the maximum temperature difference between inner and outer surfaces of insulating layer were used as the constraint conditions, when the ambient temperature is lower than 12℃, the decisive factor of DC cable ampacity is the tem-perature difference between inner and outer surface of the insulating layer (20℃). When the ambient temperature is higher than 12℃, the decisive factor of DC cable ampacity is the maximum operating temperature of conductor (70℃). In the process of simulated switching impulse test, lightning impulse test, and load cycle test under 30℃ of insulation temperature difference, the maximum transi-ent and steady-state breakdown field strength are 58 kV/mm and 25 kV/mm, respectively. According to the performance parameters of DC insulating material, the DC cable structure can meet the design requirements. The test results indicate that the COMSOL multi-physical field simulation has important guiding significance for the structure design of DC cable.