In order to obtain carbon-ceramic linear resistor with stable performance, the preparation of high resistance layer glaze slurry used in the manufacturing of carbon-ceramic resistor was studied systematically in this paper. Firstly, the glazing technology of glaze slurry prepared by different binders was compared, then the mixing time and water material ratio of slurry were systematically studied to obtain the influence of mixing time and water material ratio on the viscosity of glaze slurry, and finally the optimal mixing process parameters were determined. The results show that the glaze slurry has good suspension and coating properties when the Zuoyun clay is used as binder. When the mass fraction of Zuoyun clay is 15%, the glaze slurry has good processability and adhesion. The optimum mixing process parameters of high resistance glaze and binder is as below: Zuoyun clay is used as binder, the mass fraction of clay is 15%, the ratio of water and material is 0.6∶1, and the mixing time of ball glaze is 1 h. The breakdown strength of carbon-ceramic resistors coated with high resistance layer is 78% higher than that of the uncoated resistors, and the dispersion is smaller.

Ethylene-vinyl acetate copolymer/ethylene-α-olefin copolymer (EVA/POE) was used as the base material, and phosphorus-nitrogen halogen-free flame retardant (HF-601AE) compounded with surface modified aluminum hydroxide (ATH) was used as the flame retardant system, a halogen-free flame retardant polyolefin material for heat shrinkable tube was prepared by melt mixing method. The variation of limiting oxygen index, electrical properties, and mechanical properties of the composites under different ratios were studied, and the flame retardant mechanism of composites was investigated by thermo-gravimetric analysis (TGA) and the morphology characterization of combustion residue carbon. The results show that and the 2^# sample prepared with 50 phr polyolefin materials, 35 phr flame retardant HF-601AE, and 20 phr ATH has the best mechanical properties, and the tensile strength is 7.1 MPa, the elongation at break is 502.86%. The limiting oxygen index is 37.2%, which meets the requirements of EN45545-2 R22 HL3 flame retardant grade. The thermogravimetric rate at 371.5℃ reaches -18.11%/min, the carbon residue is low after combustion, and there are no holes and pores in the carbon layer, which indicate that the comprehensive properties are the best.

In order to improve the upper limit temperature and high temperature stability of BaTiO₃ for MLCC insulating dielectric material, a Ba_1-xCa_xTiO₃ nano-powder was prepared by using BaCl₂·H₂O, CaCl₂, and H₂TiO₃ as raw materials and KOH as mineralizer under the condition of 210℃/22 h hydrothermal reaction. The phase structure, microstructure, and dielectric properties of the samples were characterized and analyzed by XRD, SEM, LCR detection methods. The results show that the obtained powders are uniformly dispersed and the particle size is (110±20) nm. The doping of Ca²⁺ increases the Curie temperature of BaTiO₃, and a Ba_0.91Ca_0.09TiO₃ ceramics with 136.2℃of Curie temperature, 1 882 of dielectric constant, and 0.02 of dielectric loss factor are obtained at 1 250℃/2 h. The preparation of MLCC insulating dielectric materials with small particle size, narrow distribution, low agglomeration, and high performance is realized.

To deeply understand the self-recovery mechanism of electrical tree in silicone rubber (SiR), the self-recovery characteristics of SiR electrical tree were studied in this paper. The morphology variation during the self-recover process of electrical tree in SiR were observed, the elastic modulus and crosslinking density of SiR during the self-recovery process of electrical tree were tested, and the self-recover mechanism of SiR electrical tree was analyzed. The results show that the electrical trees in SiR exhibit self-recovery characteristics without external interventions including external electrical field and healing fillers, wherein some branches of electrical tree gradually degrade and eventually disappear, meanwhile the fractal dimension gradually decreases, and the rate of self-recovery exhibits the stage characteristics of fast at first and then slow. During the self-recovery process, the elastic modulus of SiR sample increases slightly from 0.964 MPa to 0.977 MPa, and the crosslinking density decreases slightly from 1.886×10^-4 mol/g to 1.883×10^-4 mol/g. After recovery, the physical crosslinking density of electrical tree deteriorated area increases slightly from 0.55×10^-4 mol/g to 0.58×10^-4 mol/g. Combined with the growing mechanism of SiR electrical trees, it is analyzed that the self-recovery process of SiR electrical tree is essentially the elastic contraction of tree channel, which is controlled by the gas flow in the channel and accompanied by the reconstruction of hydrogen bond.

The dissociation of impurities such as crosslinking by-products can easily lead to the accumulation of heteropolar space charges, resulting in local field strength distortion. At the same time, after the cross-linked polyethylene (XLPE) DC cables is degassing, the crosslinking by-product impurity in the insulating layer will form a concentration gradient distribution of "high inside and low outside", and the effect of impurity concentration gradient on the space charge is still unclear. Therefore, based on the bipolar charge transport model, the effect of cross-linked by-products impurity with uniform distribution and gradient distribution on the XLPE space charge and electric field distribution were simulated and calculated, and various factors affecting the gradient effect of impurity were analyzed. The results show that under the impurity gradient distribution, the space charge and electric field distribution of XLPE show obvious impurity gradient effect, that is, there are more opposite polarity charges accumulated on the low concentration side, which leads to the enhancement of the nearby electric field. While the impurity dissociation barrier and the impurity distribution concentration are the main reasons of affecting the impurity gradient effect. When the impurity concentration gradient is constant, the lower the activation energy, the higher the temperature, the lower the dissociation barrier, and the lower the carrier mobility, which results in the impurity gradient effect more obvious. The larger the insulation layer thickness, the higher the concentration of impurity distribution, and the more obvious the impurity gradient effect.

In this paper, class H insulation system of high voltage motor was studied. The basic properties of electromagnetic wire, dry mica tape, and insulating impregnated resin used in class H insulation structure were tested. The coil sample was manufactured, and the electrical properties, electric ageing, and electrothermal ageing tests were conducted on the coil sample. The results show that the insulating coil sample has excellent electrical performance in breakdown voltage and dielectric loss factor, which can meet the technical requirements of class H 10 kV high voltage motor.

In order to study the effect of winding preload on thermal ageing characteristics of insulating pads, four groups of insulating pads were conducted accelerated thermal ageing tests at 135℃ after appling 0, 200, 500, and 1 000 N of preload, respectively, and the mechanical properties of insulating pads were tested. At the same time, in order to simulate the mechanical ageing of insulating pad under the influence of winding vibration in the actual working condition, a sinusoidal cyclic mechanical load with 5 N of amplitude and 2 Hz was applied on the accelerated thermal ageing of insulating pad with 200 N of preloading force. The results show that that thermal ageing causes the stress-strain curve of insulating pad gradually shift to the right under 0 N preload, and its elastic modulus decreases gradually. The elastic modulus of insulating pad decreases to 79.935% of the new insulating pad after accelerated ageing for 336 h at 135℃. Applying preloading force can slow down the thermal ageing of insulating pad, and the greater the preloading force, the lower the ageing rate and ageing degree of insulating pad. But applying cyclic mechanical load will accelerate the ageing of insulating pad. The research results have certain reference and practical application value for the research of insulation pad ageing and transformer vibration.

To investigate influence of high-altitude environment on the electrical ageing performance of the main insulating material of new energy vehicle (NEV) drive motor, NHN insulating paper was selected as the main research object. According to the actual working conditions of NEV, the NHN insulating paper was applied bipolar repeated square waves, and its partial discharge inception voltage (PDIV) under different air pressures was measured. The PDIV value under standard atmospheric pressure was used as standard voltage, and 1.5 times of standard voltage was applied to the samples under different air pressures. The physical and electrical parameters of the samples under the condition of electrical ageing for 30 minutes and ageing to breakdown were measured, and the ozone content generated during the electrical ageing process was detected. The results show that the PDIV value of the main insulation sample decreases significantly under low pressure, and when the applied voltage is greater than the PDIV, the sample surface is corroded more severely compared with the plain area, leading to the sample break down easier. During the electrical ageing process with the same time and voltage, the electrical parameters of the samples under low pressure have bigger change, the ozone production rate of partial discharge is faster, the corrosion of the material is intensified, and the deterioration of insulating properties is more serious. Based on above studies, the influence of altitude factors on the corona resistance performance of insulating materials should be fully considered when designing the insulation system of motor for NEV.

Medium frequency transformers have high power density, small volume, complex winding structure, and strong local electrical stress, so the design of transformer must fully consider the electro-thermal stress margin to ensure reliability. In this paper, the method of adding electrostatic ring and angle ring were proposed to suppress the electric field strength at the end of primary winding, and the electric field strength distribution of multi-winding transformer was calculated by electrostatic field finite element analysis method. Combined with the sensitivity analysis method, the influence of the size parameters of electrostatic ring and angle ring on the electric field strength was studied to determine the decisive factors of the maximum electric field strength. The response surface function related to electric field strength was constructed by response surface method to optimize the insulation structure of transformer. The results show that the optimal sizes of electrostatic ring and angle ring are obtained by the above response surface function through taking a 50 kW, 10 kHz multi-winding medium-frequency transformer as an example, which can reduce the maximum electric field strength by 15.4% and 17.49% under short-term power frequency withstand voltage test and rated operating conditions, respectively. It is proved that the response surface method can be applied to the insulation structure design of multi-winding transformer.

Inter-turn short circuit is the main fault type of dry-type air-core reactors. At present, there is a lack of effective offline detection method to discover potential faults in time before dry-type reactors put into operation. Based on the theory of multiconductor transmission line, a distributed parameter model of dry-type air-core reactor winding was built to simulate the refraction and reflection processes after pulse signal injection, and the influence law of inter-turn short circuit on the response signal at the head and end of winding was discussed. On this basis, a characteristic waveform was constructed by the integral value of response signal absolute value, and a characteristic value was extracted to diagnose the inter-turn insulation state of winding. The results show that the test results of four-eigenvalue method proposed in this paper based on the wave process and multiconductor transmission line theory are consistent with the simulation results. Through the simulation calculation of different structural reactor models, it is found that the eigenvalue change trend of different structural reactor is also consistent when the inter-turn short circuit fault occurs. Therefore, the above method can be applied to diagnoze the inter-turn short circuit fault of reactors with different structures. The research results can provide guidance for the diagnosis of the severity and regional localization of inter-turn short circuit in dry-type air-core reactor.