Natural ester insulating oil has higher ignition point and flash point, excellent natural degradation ability and good insulation performance, and it is a green substitute for mineral insulating oil. Due to the physical and chemical characteristics of natural ester insulating oil, it is still facing many challenges to widely used in transformer equipment. In this paper, the preparation process of natural ester insulating oil was reviewed combining with its shortcomings. The research results of natural ester insulating oil were expounded from the aspects of additive modification, mixed modification, nano modification, and chemical modification. Finally, the future research direction was prospected.

According to the actual situation of outgoing section at Changping station of Zhangnan-Changping 500 kV round Ⅲ line (Beijing section), a trampled composite insulation cross arm narrow base linear tower for 500 kV line was designed and developed. The cross arm structure used the form of “two pull and two press”, and suspension insulator string was not required, which can reduce the fault caused by the monsoon discharge at tower head. Alicyclic epoxy umbrella skirts were arranged at intervals in the middle of compression bar, which makes the structure can be trampled. According to the original line form, considering the tower structure, air clearance requirements, and deflection deformation calculated by simulation under the conditions of reserved safety factor, we confirmed the structure height and mandrel diameter of the product. The mechanical instability conditions of the structure were analyzed under three severe working conditions, including break line, vertical strong wind, and ice covering. The results show that the overall first-order buckling coefficient is greater than 2.5, the axial stress of each unit is far less than the mechanical strength of metal and composite materials, and the structure has high safety. The mechanical properties of 1∶1 true type test tower and the electrical properties of upper phase composite insulation cross arm can meet the test standard and design requirements.

The thermal conductive and insulating composite material with polymer matrix and high thermal conductive filler is an ideal solution to settle insulation protection of live working equipment and heat dissipation problem of electrical and electronic equipment. In this study, micron alumina (Al₂O₃), surface modified by silane coupling agent KH550, mixed with high thermal conductive carbon nanotubes (CNT) as thermal conductive filler, silicone rubber (SR) with wide temperature range resistant and corrosion resistant was selected as polymer matrix, an SR composite material was prepared, and its performance was tested. The results show that when the total content of Al₂O₃/CNT mixed filler is 10%, the proportion of carbon nanotubes is 0.2%, the thermal conductivity of the SR composite is as high as 0.268 W/(m·K), which is improved by 103.1% compared with SR, the resistivity is 10.5×10¹² Ω·cm, the relative dielectric constant is almost unchanged, and the Shore hardness A and Young’s modulus increase slightly .

Combining with the relationships among the DC submarine selection, voltage level, and transmission capacity of HVDC submarine cable projects in China, taking the raw material input of DC submarine cable required per unit transmission capacity as a typical economic index, we find that with the increase of voltage level and current capacity of the single loop DC submarine cable, the economy of DC submarine cable increases. By comparing the materials consumable variation of different routes after improving the voltage level and transmission capacity, we find that it is more effective to improve the economy of DC submarine cable by increasing the electrical level of DC submarine cable insulation and the maximum operating temperature of conductor to improve the voltage level and transmission capacity, respectively. In addition, a ±525 kV DC submarine cable with the highest conductor operating temperature of 70℃ was selected as the typical specification, and its insulation field strength distribution variations were analyzed through finite element simulation when the applied voltage and conductor operating temperature increased to 972 kV and 90℃, respectively. The results show that the increase of voltage level and conductor operating temperature will both cause a further increase of field strength close to insulation shield after the insulation field strength reversal, and there is a synergism effect between them. On this basis, a semicircular protuberance with 0.125 mm of depth was set on the interface of insulation shield into insulation, and the finite element simulation results show that the field strength at the protuberance increased by 35.4% compared with the case without protuberance.

First of all, hexagonal boron nitride (h-BN) micro-powder and sericite micro-powder (Mica) were used as raw materials, boron nitride nanosheets (BNNSs) and mica nanosheets (MNS) were exfoliated by freeze-thaw cycle combined with ultrasonic technology. Then the BNNSs and MNS were used as insulating thermal conductive fillers, mica/boron nitride nanohybrid polyimide films (MNS/BNNS nanohybrid PI films) were prepared by in-situ polymerization and two-step water-based polyimide (PI) process. The influence of different MNS/BNNS filling amounts on the performance of PI composite films was studied. The morphology and structure of BN, BNNS, Mica, and MNS were characterized by XRD, TEM, and AFM, and the thermal conductivity, dielectric constant and electric strength of the MNS/BNNS nanohybrid PI films were measured. The results show that when the ratio between MNS and BNNSs is 1∶2, the MNS/BNNS nanohybrid PI films have better comprehensive properties, and the thermal conductivity is significantly improved compared with pure PI. The thermal conductivity is 0.743 W/(m·K), the electric strength is 246 MV/m, and the dielectric constant is 5.28.

Within tens of meters of offshore area in Fujian, a large number of insulation layer is ablated and even on fire for 10 kV overhead insulation line, which causes great security risks to line operation and social security. The ablation location and regional distribution characteristics of the insulation line were investigated, and the main cause was that the offshore lines occurred pollution flashover discharge. Then according to the ablated line samples obtained in the field, the insulating property variation of the line before and after ablation was analyzed. Taking a 10 kV overhead insulation line model as an example, the phenomena and mechanisms of pollution flashover discharge and insulation layer ablation were simulated by finite element analysis and artificial pollution test. At last, according to the mechanism of ablation phenomenon, some specific preventive measures were proposed.

Taking the UHV DC transmission line insulators under four typical climatic conditions such as extremely arid, semi-arid, semi humid, and humid as research objects, we conducted natural pollution accumulation tests on insulators. The pollution accumulation characteristics of UHV DC insulators under different environmental conditions were obtained, and the pollution accumulation characteristics of insulators with different umbrella types and hydrophobic surface were described. The results show that from the extremely arid region to humid region, both the ratio of ESDD to NSDD and uneven pollution coefficient of the insulators decrease. The ESDD ratio of bell-type insulator to outer umbrella insulator in humid area is 0.82, and the ESDD ratio of composite surface insulator to hydrophilic surface insulator of tension string in extremely arid and semi-arid areas is 1.64 and 1.85, respectively. The research results can provide a reference for the differential design of external insulation for UHV DC transmission lines at different environmental areas.

PTC resistance device, which made by positive temperature coefficient (PTC) material, can be used as a passive component to prevent overcurrent fault. In this paper, four kinds of polymer-based PTC materials were prepared by melting blending method, and the effects of conductive filler concentration, coupling agent, and supplementary conductive filler on their properties were studied. According to the temperature-resistance characteristics and heat transfer equation of polymer PTC composite, its current limiting process was simulated. The experimental results show that increasing the concentration of conductive filler will lead to the decrease of the resistivity of composite at room temperature. The PTC properties of the composites can be improved by using silane coupling agent to modify CB and adjusted by the interaction of various fillers. The simulation results show that under certain assumptions, the PTC resis-tor, as a passive component to prevent overcurrent fault, can limit the short-circuit current of power system in a short time.

In order to study the radial temperature field distribution of overhead conductor, firstly, on the basis of heat transfer theory, a 3D model of finite element simulation was established according to the physical structure of conductor, and a simulation experiment was designed. Then the simulation and experimental results were compared. The results show that under natural convection conditions, the maximum radial temperature difference of the conductor can reach 10.4℃, and the maximum surface temperature difference of different position on conductor is 3.4℃. The simulation results coincide with the measured values under natural convection conditions basically, and the relative error between each layer temperature of the conductor is within ±5%, which verifies the reliability of the simulation model. Under forced convection conditions, there are obvious difference between simulation results and measured values, which indicates that the axial heat transfer of the conductor caused by different heat dissipation conditions has significant effect on the radial temperature field distribution of the conductor. For conservative consideration, in practical applications, the core temperature of conductor can be calculated by multiplying the maximum surface temperature of conductor measured from different position of conductor with correction coefficient of 1.05‒1.10.

In this paper, the effect of DC-temperature composite field on the electrical tree growth characteristics was introduced. The effect mechanism of charge transport on the electrical tree deterioration under temperature and electric field coupling conditions and the research progress in electrical properties of epoxy nanocomposite dielectrics were reviewed.