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.

The composite insulation cross arm has the characteristics of light weight and high strength, which can play an important role in reducing the area of the line corridor and strengthening the insulation capacity of the power grid. The electrical insulation performance of the internal insulation interface is related to the safe and stable operation of the power system. In this study, based on IEC 62217:2012, a 144-hour water diffusion test was carried out on the interface specimens of the inner-filled composite crossarm, and the leakage current and breakdown at different stages were measured. Combined with the test results, the insulation crossarm samples were analyzed. Interface ageing characteristics under water diffusion test conditions. The results show that when the water diffusion time is 144 h, the leakage current of the sample is as high as 20 mA, which is 1000 times that of the sample without the water diffusion test, and the interface breakdown time is faster than that of the sample without the water diffusion test. 65%, water molecules have a destructive effect on the interface under the action of high temperature diffusion, and the longer the water diffusion time, the greater the damage to the polyurethane/glass fiber reinforced plastic interface.

The conventional properties, viscosity characteristics, gel characteristics, storage stability of YD319G3 epoxy modified unsaturated polyester impregnating varnish and its application in the partial model product of direct drive permanent magnet synchronous wind turbine generator were studied. The results show that the YD319G3 impregnating varnish has low toxicity and low volatility characteristics, which can meet the requirements of VPI insulation treatment process for large wind turbines, and the impregnated model and product have good insulating properties. It is a suitable impregnating resin for the VPI treatment of wind turbines.

When installing the cable accessories, we usually coat silicone grease on the interface between silicone rubber (SR) insulation of cable accessories and XLPE insulation of cable body, but the silicone rubber will affect the interface pressure after absorbing silicone grease. The interface pressure between SR and XLPE was studied by combining experiment and simulation methods. The elastic modulus variation of the silicone rubber after absorbing silicone grease was measured by experiment, and then the elastic modulus was distributed to three-dimensional model to conduct simulation. The results show that under the silicone grease environment, the greater the expansion degree of silicone rubber, the greater the mass change rate, and the smaller the elastic modulus. The interface pressure between SR and XLPE decreases with the increase of silicone grease absorption time. The larger the expansion degree of cable joint, the greater the interface pressure, and the faster the interface pressure decline speed. The interface pressure decreases slightly with the increase of friction coefficient.

The insulating properties of the doubly-fed motor stator coil after different cycles of cold and heat circulation was compared and its service life was assessed under different high frequency voltage to study the cold and hot shock resistance and high-frequency pulse electrical ageing performance of the doubly-fed generator stator coil. The results show that the cold and heat circulation has little effect on the insulating properties of doubly-fed generator stator coil. With the increase of high frequency ageing time, both the dielectric loss factor and its increment increase, and the insulation resistance increases at first and then decreases. The partial discharge inception voltage of the coil after high frequency pulse is smaller than that of the unaged coil, and the high-frequency pulse electrical ageing life of the stator coil is calculated to be more than 30 years.

The space charge accumulation and dissipation characteristics of the oil-paper insulation systems composed of different types of transformer oils were studied under the conditions of applying and removing different intensity of DC electric field. The results show that compared with paraffin-based transformer oil, the naphthenic transformer oils have higher naphthenic hydrocarbon content, more suitable aromatic hydrocarbon content, and higher charge transfer rate, and the oil-paper insulation systems composed by them has less charge accumulation and the charges dissipates more easily after applying and removing the high voltage DC electric field, which is beneficial to the dissipation of charge in oil-paper insulation for converter transformer.

A ±400 kV HVDC model cable was conducted DC withstand voltage test and impulse withstand voltage test, and the DC breakdown voltage and impulse breakdown voltage of the model cable under the highest operating temperature were obtained. The electric field distribution under DC breakdown voltage and impulse breakdown voltage were calculated. On the basis of average field intensity method and maximum field intensity method, the insulation thickness of ±400 kV HVDC cable was designed, and the electric field distribution of insulation layer was calculated under DC voltage and impulse voltage. At last, the insulation thickness of ±400 kV DC cable was obtained by comparing the electric field distribution of ±400 kV HVDC cable and model cable. The results show that when the insulation thickness of HVDC cables is designed by the average field strength method, the insulation thickness depends on the impulse voltage. When the insulation thickness is designed by the maximum field strength method, the insulation thickness depends on the DC voltage. By comparing the electric field distribution of ±400 kV DC cable and the electric field strength of the model cable when breakdown, it is obtained that the insulation thickness of 400 kV DC cable is 26 mm.

Partial discharges (PDs) in electrical equipment are normally detected with 50 Hz AC sinusoidal voltage stimulus in an online situation, but can also be measured at other non-sinusoidal and non-power frequency stimuli in off-line situations. In this work, three types of non-sinusoidal voltage waveforms, which are symmetric triangular-wave, symmetric trapezoidal-wave, and approximately square-wave voltages, were applied to investigate the effect of ambient temperature on the PD activity produced in an air gap between mica-epoxy surfaces, in combination with the frequency-domain dielectric spectroscopy (FDS) measurement of mica-epoxy dielectric. The results show that the PD inception voltage (PDIV) decreases greatly with the increase of temperature at three types of applied voltages. The PD behavior at symmetric triangular-wave and sinusoidal-wave voltages shows no obvious changing with the increase of temperature. With the increase of ambient temperature, the average PD charge exhibits regular increasing trend and the increment has almost proportional relation with the increasing rate of applied voltage; the maximum PD charge has the largest value at approximately square-wave voltage and also shows the increasing trend. The most obvious effect of temperature on PD activity lies in the constant voltage period of applied voltage. In this period, the average PD number and average PD charge both exhibit almost exponential increasing trend with the increase of temperature.

An oil-paper sample was conducted accelerate thermal ageing treatment, and its ageing process was divided into five ageing stages according to the variation of polymerization degree. Partial discharge tests were conducted on the air gap discharge model, and the PRPD patterns of the oil-paper sample were collected at different ageing stages. The feature quantities were extracted by using statistical operator, the dimension of the original feature data was reduced by factor analysis method, and the clustering characteristics of the feature data before and after dimension reduction were compared. A probabilistic neural network model (PNN) was established to identify the ageing stages of oil-paper insulation, and a back propagation (BP) neural network model and a support vector machine (SVM) model were built as comparison. The three models were trained by the same data, and their recognition results were compared. The results show that ageing will cause pores in the pressboard, which promotes the occurrence of partial discharge. Compared with other models, the FAM-PNN model has obvious advantages in recognition accuracy and operation efficiency. The ageing state of transformer oil-paper insulation can be evaluated accurately and efficiently using the FAM-PNN model.

The ageing characteristics of composite insulators are often obtained by the accelerated ageing test at laboratory, but this method is difficult to simulate the actual complex environment, and the equivalence between accelerated ageing and natural ageing need further verification. In view of this, we selected 153 composite insulators in operation with different operating years and operation environments in Ningxia area to conduct multiple tests, and studied their natural ageing characteristics from three aspects, including the physical and chemical properties of silicone rubber material, the electrical properties and mechanical properties of insulator. The results show that the operating years, pollution degree, and pollution composition are the main factors affecting the ageing of silicone rubber material, and the chemical pollution can accelerate the ageing of materials. Under the same operating years, the ageing degrees of electrical and mechanical properties of insulator are lower than that of the electrical and mechanical properties of silicone rubber material. The mandrel of insulators operating for more than 15 years in heavy pollution areas can still maintain good performance. The junctions of mandrel and sheath, mandrel and hardware are the weak points of the electrical and mechanical properties of insulator, respectively. The manufacturer formulation and process are the important factors affecting the natural ageing, and there will be agglomeration performance degradation in low-quality batches. In the operation and maintenance sampling inspection of composite insulators, it is necessary to pay attention to the insulators with long service life in the heavy pollution area or chemical pollution area, and the composite insulators in operation with performance aggregation degradation from the same manufacturer batch need to conduct supplementary sampling inspection.