A 0.6/1 kV cable core with double-layer insulation structure was prepared through the double-layer co-extrusion process, and the effect of extrusion temperatures on its surface quality, eccentricity, microstructure, and mechanical properties was investigated. The prepared cable core with double-layer insulation structure was conducted multiple properties tests and heat-resistance life evaluation. The results show that when the extrusion temperature increases from temperature I to temperature II, the prepared double-layer insulation core has flat surface, good brightness, and lower eccentricity between the inner and outer insulation layers (≤15%), and the elongation at break of the insulation core increases from 109% to 221%. When the extrusion temperature increases from temperature II to temperature III, the surface quality of insulation core is deteriorated with distortion and shrinkage on it. Meanwhile, many different sizes of pores can be observed in the longitudinal section, and the tensile strength and elongation at break decrease to 10.7 MPa and 140%, respectively. Compared with the values of JG/T 442—2014 standard, the insulation resistance constant at 20℃, tensile strength and elongation at break before and after ageing, and thermal extensibility of insulation core after electron irradiation cross-linking are significantly improved, indicating that the mechanical properties, ageing properties, and thermal elongation properties of the insulation core with double-layer structure are relatively excellent. Furthermore, after ageing at 165℃/168 h, the elongation at break of insulation core with double-layer strucure has 98% of retention rate, indicating that the insulation core with double-layer structure possesses the outstanding heat-resistance life.

Air gap defect is one of the common internal defects in GIS, which can cause electric field distortion. The introduction of dielectric functional graded materials can optimize the local electric field of basin insulators. However, under the combined effect of air gap defect and dielectric functional graded materials, its electric field characteristics are not yet known. Therefore, an electric field simulation model of GIS basin insulator with uniform medium and dielectric functional graded materials (ε-FGM) was built in this paper, and the influence of the existence of air gap and air gap position on the electric field distribution of insulators with uniform medium and ε-FGM was discussed. The results show that the introduced dielectric functional graded materials have a relatively ideal regulatory effect. When air gap defects are doped in a uniform medium, the field strength of air gap near the high-voltage conductor, insulator middle, and grounding shell will significantly increase, and the increase rate is 85.32%, 78.10%, and 68.69%, respectively. The influence trend of air gap defects on the electric field characteristics of insulators with ε-FGM is basically consistent with the influence trend on the uniform media, but from a specific numerical perspective, there is still a certain difference. Air gap defects have a greater impact on the electric field of insulators with ε-FGM. When designing insulators with ε-FGM, special attention should be paid to suppress the air gap defects in medium.

To investigate the impact of interface air gap and moisture defects on the multilayer composite dielectric insulation interface of ±320 kV DC cable intermediate joints on their electric field distribution mechanism, a simulation model of ±320 kV DC cable intermediate joint was built, and the electric field distribution of the cable joint with air gap and moisture defects on the multilayer composite dielectric interface region was calculated using the finite element method. The effect of these defects on the electric field distribution of the cable joint were analyzed at no load and load conditions, and their differences under AC/DC electric fields were discussed. The results show that there is significant electric field distortion at the composite dielectric interface of the DC cable joint due to air gap and moisture defects under the action of DC electric field. The multiple of electric field distortion of air gap defects is 9.6 times at no load and 1.7 times at load condition. The electric field strength in the water film defects decrease over 99.9% at both operating conditions. Interestingly, the electric field distortion induced by air gap defects in cable joints at no load is larger than that at load condition, and the temperature differentials across the joint play a role in field homogenization. However, the temperature differentials across the joint induced by moisture at load promote the accumulation of space charge in the joint, which leading to the multiple of electric field distortion at moisture defect bigger than than at no load. Notably, the multiple of electric field distortion resulting from defects in DC cable intermediate joint is bigger than that of AC cables.

Composite insulators are widely used in transmission lines, but their ageing and fracture problems cannot be ignored. In order to study the cause of fracture of composite insulators and the mechanism of mechanical-electro-thermal ageing, a mechanical-electro-thermal ageing experiment platform was designed to carry out multi-field coupling ageing tests on short rod samples of composite insulators. The results show that the ageing insulator rod sample has fracture phenomenon. During the process of rod fracture, the macroscopic characteristics are the expansion of crack and fracture surface, and there are discharge marks near the high-voltage electrode. The microscopic characteristics are epoxy decomposition, glass fiber microcrack and fracture, and fiber-epoxy interface failure. At the same time, a multi-field fracture failure evaluation model of mechanical-electro-thermal coupling is established based on the unit break rate, and the inverse relationship between the unit break rate and the energy barrier is revealed. Under the given electrical, thermal, and mechanical stress conditions, the model can evaluate the ageing degree of composite insulator rod according to the ratio of fracture area, and can provide a reference for the ageing failure analysis of composite insulator rod under the synergistic action of multiple stresses.

To explore the thermal mechanical stress distribution of motor insulation structure at high temperatures, we take the insulation structure of organic silicone system as the research object, test its thermal expansion coefficient and elastic modulus at different temperatures, and analyze the influence of thermal stress parameters at different temperatures on the thermal mechanical stress of organic silicone insulation system. The results show that within -50-200℃, the thermal expansion coefficient of insulation structure show a change trend of increases, decreases, and increases again with the increase of temperature, and its value is 0.8×10^-5-1.7×10^-5 K^-1. The elastic modulus increases at first and then decrease with the increase of temperature, and its value is 0.2×10³-3×10³ MPa. The thermal mechanical stress borne by insulation structures increases linearly with the increase of thermal expansion coefficient and elastic modulus. The thermal mechanical stress of insulation structures firstly increases and then decreases with the increase of temperature, depending on the size of thermal expansion coefficient and elastic modulus at different temperatures. The feasibility of testing parameters and simulation analysis was verified through stress testing of the wire rod and motor, and the deviation between simulation value and testing value is within 15%. The simulation analysis results show that at 120℃, the maximum thermal mechanical stress point of motor insulation is located at the slot insulation, and the maximum stress value is 11.37 MPa. After sealing, the maximum thermal stress value of the slot insulation can be reduced by about 45%.

In order to study the change rule of microstructure and dielectric properties of polyethylene (PE) materials under the action of thermal stress, PE samples were conducted accelerated thermal ageing tests at 90℃, and the microstructure and charge transport properties of the thermal aged PE samples were characterized by Fourier transform infrared spectrometer (FTIR), differential scanning calorimetry (DSC), X-Ray diffraction (XRD), high field conductivity (HFC) test, and isothermal surface potential decay (ISPD) tests. The results show that the thermal ageing process of PE samples can be divided into recrystallization stage and thermo-oxidative degradation stage. In the recrystallization stage, the crystallinity of the samples increases, the crystal structure tends to be perfect, the deep-trap density and deep-trap energy level increase, the carrier migration is restricted, and the threshold field strength of space charge injection increase. In the thermo-oxidative degradation stage, the crystallinity of the samples decreases, the crystal structure appears deterioration, the deep-trap density and deep-trap energy level decrease, the carrier mobility increases, and the threshold field strength of space charge injection decreases. Through the characterization of conductivity properties under different temperatures and field strengths, the variation rules of three parameters A, B, and φ in Steven Boggs’ conductivity function were obtained. With the increase of ageing time, the φ and A increase at first and then decrease, and the B decrease at first and then remains unchanged, among which the material coefficient A is the most sensitive to thermal ageing.

To reduce the partial discharge (PD) of the basin-type insulator in gas-insulated switchgear (GIS), reduce the occurrence of insulation faults, and improve the safety and stability of GIS operation, we took the 252 kV basin-type insulator as the simulation analysis object. The basin-type insulators with and without grounding shield ring were conducted simulation calculation by Comsol software, and basin-type insulators with different sizes of grounding shield ring were conducted simulation comparison analysis. The insulating and mechanical properties of the basin-type insulator were further verified through experiments. The results show that adding a ground shield ring in the basin-type insulator can effectively improve the electric field in the wedge air gap region and significantly reduce the partial discharge value of the basin-type insulator.

Crepe paper is an essential insulating material for transformers and high-voltage cables. Currently, all high-performance crepe paper is imported. In order to realize the localization of crepe paper for high-end power equipment and improve the long-term reliability of crepe paper, it is urgent to carry out research on the physicochemical properties and performance improvement technology of crepe paper. The physicochemical and macroscopic properties of crepe papers from different manufacturers were tested and analyzed in this paper, and the key physicochemical parameters that affect the performance of crepe papers were extracted. Three functional groups were introduced into the cellulose systems through grafting modification, the properties of the cellulose systems before and after grafting modification were analyzed using Materials Studio software, and the effects of different functional groups on the various properties of the cellulose systems were investigated. The results show that the benzene ring has the most obvious improvement on the thermal properties of the cellulose system, the methylene long chain can improve the insulating properties of the cellulose system, and the benzene ring can improve the mechanical properties of the cellulose system.

Partial discharge often occurs due to condensation on the surface of insulator inside the switchgear of box-type substation at the bottom of the wind turbine tower, which seriously threatens the operational safety of equipment. In order to further understand the electric field distribution and discharge process on the surface of epoxy resin with condensation, we used artificial spray condensation was to simulate the natural condensation process in this study. The influence of condensation on the electric field distribution of the insulation surface was simulated and analyzed when the electric field non-uniform coefficient was changing. The spectrogram of partial discharge on the epoxy resin surface under different condensation amount was measured, and the change law of the insulation surface discharge was further discussed. The results show that the artificial spray condensation can quantitatively simulate the condensation amount on the insulation surface under natural condensation. Condensation at different locations can cause the distortion of partial electric field, leading to the accumulation and increase of surface charges on the discharge path, which increases the possibility of partial discharge. When the non-uniformity coefficient of surface electric field is larger, the partial discharge initial voltage is lower, and partial discharge is easier to occur. The increase in the adhesion amount of condensation on the insulation surface promotes the partial discharge. With the increase of condensation amount, the degree of surface electric field distortion and the probability of water droplet fusion increase, while the initial discharge voltage decreases, the number and amplitude of surface discharge increase.

The electric field distribution of cable joints under DC operation is influenced by the conductivity of insulating materials, which exhibits non-linear characteristics under the effects of temperature and electric field, so the electric field distribution within the joints is exceptionally complex. In order to select or determine suitable insulating materials for joint in engineering applications and improve the electric field distribution inside the joint, we constructed a simulation model for the prefabricated joint structure of 10 kV voltage level XLPE-insulated DC cables. By changing the non-linear characteristic parameters of the conductance of silicone rubber, the influence of the conductance activation energy and conductance field strength dependence of silicone rubber on the electric field distribution inside the joint under full load operation was explored. The results show that under high load conditions, the DC steady-state field strength at the insulation interface of the joint is influenced by the conductance activation energy and the conductance field strength dependence of the insulating material, the conductance activation energy has a greater impact. From an economic perspective, the ratio of the conductance activation energy and field strength dependence coefficient of conductance between silicone rubber and XLPE insulating material should not be higher than 0.94 and not be lower than 0.50, respectively.