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.

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.

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.

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.

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.

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 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.

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.

Using bisphenol A epoxy resin, phosphorus flame retardant, aluminum hydroxide, and molecular sieve as the main agent, organic anhydride and accelerator as curing agent, we prepared a halogen-free flame retardant potting material for film capacitor. The effects of different flame retardants and filling content on the glass transition temperature and flame retardancy of the potting materials and the durability of the capacitor were studied, and the optimum formula of halogen-free flame retardant potting material for film capacitor was determined. The results show that the glass transition temperature of the potting materials prepared by the optimum formula is 104℃, and the flame retardant is V0 (3 mm), and the electrical insulation performance is good. The film capacitor potted with this potting material can meet the 600 h durability test under the condition of 1.2 times of voltage and 85%RH /85℃, and the capacitance change rate before and after test is 3.93%.