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.

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 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.

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.

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.

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.

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.

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 contamination of DC silicone rubber insulator has an important influence on its insulation performance and service life, and the adverse effects of swelling effect under the insulation cleaning agent is the key issue to be solved. The volatilization rate of two kinds of electrified cleaning agent, the swelling and hydrophobicity of silicone rubber before and after cleaning by different cleaning agents were studied, and the corresponding mechanism was discussed. The results show that the volatilization rates of No.1 (fast volatilization) and No.2 (slow volatilization) electrified cleaning agents at 25℃ are 1 095.78 g/(h·m²) and 108.01 g/(h·m²), respectively. The swelling index of silicone rubber increases with the increase of time, it reaches saturation after 10 h, and the swelling index of the saturated silicone rubber are 55% and 130%, respectively. The hydrophobicity of silicone rubber is weakened by No.1 cleaning agent, and the contact angle decreases by 14% after soaking for 10 min. The hydrophobicity of silicone rubber is enhanced by No.2 cleaning agent, and the contact angle increases by 6% after soaking for the same time. Therefore, the cleaning agent with excellent volatility can reduce the adverse effect of swelling on the silicone rubber insulators, and can provide basis and reference for the electrified cleaning of silicone rubber insulation equipment.