The core content of 500 kV DC cable joint design is the performance and geometric structure of reinforced insulation material. In this paper, the electric field distribution characteristics of the main insulation and reinforced insulation double-layer dielectric of the DC cable joint were calculated and simulated. The development mechanism of DC cable joint breakdown caused by interface discharge was analyzed. The breakdown characteristics of the dielectric interface between cross-linked polyethylene (XLPE) and silicone rubber (SR) in DC cable joint were analyzed. The results show that the conductivity of the reinforced insulation and the main insulation of the cable joint and the interface tangential electric field strength are the key parameters of the reinforced insulation design. Within the changing allowance of temperature and electric field, the conductivity of the reinforced insulating material should always be less than that of XLPE. The tangential electric field strength of the interface between the main insulation and the reinforced insulation is the key control parameter affecting the operation reliability of the DC cable joint, and its threshold is 2.5 kV/mm under the most unfavorable conditions. The research results provide a new method to solve the design problems of DC cable joints, especially for the reinforced insulation.

In order to explore the change law of Shore A hardness and surface roughness of silicone rubber and fluorosilicone rubber under continuous low temperature environment, we put the test pieces made of the two raw materials into the low temperature test box, and carried out low temperature ageing test at -50℃ for 0, 48, 96, 144 h, respectively. The change law of Shore A hardness, surface roughness, and surface morphology with low temperature ageing time were studied, and then its principle was analyzed. The results show that at the constant low temperature environment of -50℃, when the ageing time increase from 0 h to 144 h, the Shore A hardness of the two materials increases slowly at first, then increases sharply, and finally decreases slowly. The surface roughness of the two material decreases at first, then increases sharply, and finally increases slowly. Throughout the ageing process, both the Shore A hardness and surface roughness of fluorosilicone rubber are smaller than that of silicone rubber. It shows that under low temperature environment, fluorosilicone rubber is more stable than silicone rubber.

Ceramizable silicone rubber composites were prepared by using kilchoanite as ceramic filler, glass frit as flux, and nano silica as reinforcing agent. The effect of the content and size of kilchoanite on the tensile properties of the composites was studied. The ceramic specimens were obtained by sintering the composites at 1 000℃, and then the microstructure of its fracture surface was observed. The phase change of ceramic specimens was analyzed by X-ray diffraction, and its linear contraction and flexural strength were measured. The results show that the increase of the kilchoanite with the same particle size can reduce the tensile strength of the composites, while increase the elongation at break. Under the same content, the kilchoanite with a smaller size can improve the tensile strength of the composites. When the content of kilchoanite is too high or too low, the flexural strength and density of the ceramic specimens decrease. With the decrease of the size of kilchoanite, the flexural strength of the ceramic specimens increases, while the linear contraction also increases. The maximum flexural strength of the ceramic specimens can reach to 58.57 MPa by changing the content and size of kilchoanite.

During the operation process, property deterioration of ethylene propylene diene monomer (EPDM) cable is often caused by multiple stresses. In order to explain the variation law and mechanism of dielectric properties of EPDM cable under the extrusion stress and thermal stress, we took EPDM for cable insulation of coal cutter as an object to study. The dielectric spectroscope of EPDM under the combined action of extrusion pressure and thermal stress was measured. The characteristic parameters of dielectric spectroscopy were obtained by simulating the dielectric spectroscopy according to the expression of complex permittivity from the single relaxation Cole-Cole model modified by DC conductivity. The change rule of the DC conductance and relaxation time constant was analyzed. The results show that temperature and extrusion pressure have obvious effects on the complex dielectric constant of EPDM. At a higher temperature, the increase of the real part of the complex permittivity at the lower frequency is obvious. Under the effect of DC conductivity and polarization, with the increase of extrusion pressure and temperature, the increase trend of real part of the complex permittivity at low frequencies slows down, while the real part decreases gradually at higher frequencies. Therefore, the change of DC conductance caused by extrusion stress and temperature is the key to the change of the complex dielectric constant of EPDM.

Polypropylene laminated paper (PPLP) used in DC superconducting cable is in a wrap-around structure. Due to the existence of rectifier, PPLP withstands AC-DC composite voltage, and it is easy to generate partial discharge at the butt gap. Under long-term working voltage, the insulating property of PPLP changes and affects partial discharge. In this paper, the DC electrical ageing experiment was conducted on PPLP in liquid nitrogen, and the butt gap was simulated by internal discharge model. The electric field distribution of internal discharge model was obtained by simulation calculation, and the electrical ageing mechanism and partial discharge results were discussed and analyzed. The results show that with the increase of ageing time, the electrical conductivity and relative permittivity of PPLP show an increasing overall trend; the DC electric strength of PPLP decreases gradually, but still maintained good insulation properties; the intensity of partial discharge increases the partial discharge phase mainly concentrates in the rising stage of composite voltage, and the ratio between AC and DC of composite voltage is the main factor affecting partial discharge.

The paper proposes a evaluation method for the micro properties of impurity content for cable insulation, which can effectively avoid the effect of subjective factors of manual microscopic observation in the purity evaluation process. On the basis of broadband dielectric impedance spectroscopy method, the change law of dielectric loss factor of 35 kV and 110 kV XLPE cable insulation samples with the test frequency during different thermocycling processes were investigated, and the stress characteristics at the interface between XLPE insulation and impurity during the thermocycling process were simulated. The correlation between the macrophysical parameter broadband dielectric loss and the microscopic property cleanliness for XLPE cable insulation were analyzed. The results show that after 10 cycles of thermocycling between -196℃ and 50℃, the low-frequency dielectric loss factor of 35 kV XLPE cable insulation at 0.1-1.0 Hz is more than doubled, while the dielectric loss factor of 110 kV XLPE sample does not change obviously. The simulation analysis shows that during the thermocycling process, there is interface stress between the polyethylene insulation and the micro-impurities, the stress fatigue enlarges the microscopic defects in the interface area and then accelerates the increase of low-frequency dielectric loss factor.

In order to guide the application of composite cross arms in heavy ice areas, we investigated the icing characteristics of glass fiber reinforced composite material and composite insulated crossarms in this paper. The results show that the icing amount and ice flashover voltage of blank sample under vertical suspension are better than that of horizontal suspension, and its icing amount is less than that of pure aluminum rod and Q345 steel rod with the corresponding hydrophilic. The icing amount of the composite material coated with PRTV coating is less than that of the blank sample, its flashover voltage increases sharply; the ice surface adhesion value is the least, which is 19% of ice surface adhesion value of pure aluminum rod, while the ice surface adhesion value of the blank sample is 78% of pure aluminum rod. The ice coating characteristics of the silicone rubber composite insulated crossarm are just the opposite of that of the composite material sample. Compared with vertical suspending the same composite insulator, when the insulators are horizontal suspending, the icing amount of composite insulated crossarm suspension reduces by 30%, and the flashover voltage increases by 51%.

To improve the UV resistance of room temperature vulcanized silicone rubber (RTV), we used titanate coupling agent (PN) to modify nano zinc oxide (ZnO), and then nano ZnO-RTV composites were prepared by doping different mass fraction of modified nano ZnO into RTV. The ultraviolet accelerated ageing experiments were conducted on pure RTV and nano ZnO-RTV composites, and the effect of nano ZnO on the ultraviolet resistance of RTV was compared and analyzed. The results show that nano ZnO can effectively improve the UV shielding performance of RTV, and the UV shielding rate of nano ZnO-RTV composite with 1.0% of ZnO can increase by 72.63%; after adding nano ZnO, the electrical strength of RTV is slightly improved. After UV ageing for 2 500 hours, the decreasing amplitude of static con-tact angle and the ageing degree which reflected by surface micro-morphology of pure RTV are far greater than that of nano ZnO-RTV composite, it is verified by the changes of Si-(CH₃)₂, Si-CH₃, and C=O of RTV molecular chains before and after ageing. At the same time, after UV ageing for 2500 hours, the thermal stability of nano ZnO-RTV composite is significantly better than that of pure RTV.

The physical process of initiation and development of surface flashover in oil-paper insulation of transformer is complicated, and the discharge mechanism remains unclear. In this paper, through experimental measurement and numerical simulation of the needle-plate electrode model for oil gap discharge and the needle-plate electrode model for oil-paper insulation surface flashover, the discharge characteristics and discharge mechanism of oil gap discharge and oil-paper insulation surface flashover under different thickness of paperboard and different surface distance were obtained and analyzed. The results show that under DC voltage, oil-paper insulation surface flashover voltage is lower than the breakdown voltage of oil gap, this is because the paperboard not only changes the distribution of electric field, increases the parallel electric field component at the needle tip, and decreases the initial voltage of the streamer discharge, but also hinders the spread of space charge, enhances the distortion of electric field, and increases the development speed of streamer. In addition, due to the presence of paperboard, the initiation process of streamer in oil-paper insulation surface flashover discharge is completely different from that of oil gap discharge, that the streamer gradually propagates from the tip to the oil-paper interface. Increasing the thickness of paperboard can increase the parallel component of the electric field at the tip, decrease the initial voltage of the discharge, decrease the development speed of the streamer and reduce the surface flashover voltage. The surface flashover voltage of oil-paper insulation increases with the increase of the surface distance, but its nonlinear variation is due to the effect of applied voltage weakens gradually and the effect of space charge distortion electric field is dominant in the process of the streamer developing to the self-supporting discharge stage.

High-frequency copper-clad laminate is the core foundation to the development of 5G related industries, it has high technical threshold and broad market prospects. In order to clarify the development trend of high-frequency copper-clad laminate field, we comprehensively excavated and deeply studied the patent big data of high-frequency copper-clad laminate and its key technologies, including resin, fiber cloth, filler, and copper foil. The results show that the field of high-frequency copper-clad laminate is still in a period of rapid development, and the technology evolution of resin, fiber cloth, filler, and copper foil are aimed at achieving stable dielectric constant and low dielectric loss. Companies from the USA and Japan occupy the dominant position, the application amount of patent form Hitachi, Sumitomo, Panasonic, Mitsubishi and other Japanese companies are in the front rank. The research on high frequency copper-clad laminate in China starts late but develops rapidly, with two major research and development highlands formed in Guangdong and Jiangsu, and advantageous enterprises emerged such as Shengyi Technology and WAZAM New Materials. However, on the whole, patent technologies of China lack core competitiveness and face strong patent barriers. Accordingly, the countermeasures and suggestions for the high-quality development of high-frequency copper-clad laminate industry in China are put forward.