Defects will occur during production and operation of insulator, which will cause the accumulation of surface charge, and affect the distribution of electric field of insulators. In serious cases, surface flashover will occur, and then affect the operation safety of electrical equipment. Therefore, it is of great significance to study the nondestructive testing and evaluation methods of defects in insulators. Firstly, this paper summarized the influence of insulator defect types such as bubble, crack, and metallic foreign matter on insulator insulation performance, and analyzed the generation and development mechanism of different defects. Then, the testing principle and research status of new nondestructive testing methods based on acoustic, optical, and thermal characteristics were summarized and introduced. Finally, the detection methods were comprehensively evaluated from the perspectives of speed and accuracy, the detection effects of different detection methods on insulator defects were evaluated, and the research direction of insulator defect detection in the future were prospected.

It is found that ablative white spots are often attached to the buffer layer by disassembling high-voltage XLPE fault cables. To study the ablation characteristics of buffer layer, the chemical elements of white spot at different positions of buffer layer were analyzed by using energy dispersive spectrometer (EDS), and the conductivity of the white spots buffer layer were measured. Then on the basis of the measurement results of conductivity, an electrothermal coupling simulation model of high voltage XLPE cable was established, and the heat production characteristics of white spot buffer layer under the action of high frequency overvoltage were calculated. The results show that the white spots in the ablative area of buffer layer are the water-blocking powder precipitated by the buffer layer, and the conductivity of the buffer layer decreases obviously after the white spots are precipitated. Under the action of high frequency overvoltage, the higher the overvoltage frequency, the greater the resistance heat generated in the local periphery of buffer layer white spot, and the higher the temperature rise, which may eventually cause overheating and erosion of the buffer layer.

The existing test method standards of the compatibility between electrical insulating fluids and structural materials at home and abroad were introduced, and the test method standards of the compatibility between electrical insulating fluids and structural materials in transformers, power capacitors, cable terminals, and oil cooling motors were discussed respectively. The similarities and differences of test conditions and evaluation parameters in each standard were summarized, and the shortcomings of current standards were analyzed. At last, the standard proposal of IEC 63177 Test methods for compatibility of construction materials with electrical insulating liquids proposed by Guangzhou Power Supply Bureau of Guangdong Power Grid Co., Ltd. was introduced compendiously, which will address the shortcomings of existing compatibility test methods.

An accelerated thermal ageing test was designed for the main insulation sample of a generator VPI stator bar, and the ultrasonic velocity, ultrasonic spectrum, and ultrasonic defect waveform of samples were analyzed at various ageing temperatures on the basis of the principle of ultrasonic testing. At the same time, artificial hole defects and impurity defects were introduced into the insulation samples to explore the effect of defect introduction on the ultrasonic waveform of the samples. The results show that as the thermal ageing test progresses, the overall ultrasonic velocity in the main insulation of stator bar shows a decreasing trend. The reason is that during the thermal ageing process of the stator bar main insulation sample, the layering of internal structure can cause refraction, reflection, and dissipation of the sound wave during propagation process, resulting in the propagation speed of the sound wave in the insulation sample decrease. The defects can be detected and accurately located through ultrasonic echoes, which indicates that the acoustic performance testing method has good practicality in the diagnosis of stator bar main insulation.

To evaluate the environmental adaptability of insulation system for offshore wind turbine, multi-factor cycling tests including high and low temperature test, humid heat test, salt spray test, and vibration test, were conducted on insulating resin and simulation winding, and their electrical performance changes before and after tests were characterized and compared. The result show that after ageing tests, the performance of the epoxy modified unsaturated polyester impregnating resin does not deteriorate significantly, the normal insulating performance of simulation winding just deteriorate slightly, its residual breakdown voltage is almost no attenuation, and both the key materials and simulation winding exhibit good resistance to marine environments. The insulation system of offshore direct-drive wind turbine still retains stable and excellent dielectric performance after long-term offshore multi-factor cyclic ageing tests, which can meet the requirements of marine environment.

On the basis of surface discharge tests and finite element electric field simulation methods, the surface discharge and electric field characteristics of GIS basin insulators with metal ball or wire particle defects were studied. The results show that under different metal particle defects, both the pulse voltage and ultraviolet photon number increase with the increase of voltage. The surface discharge of basin insulators with metal wire defects is more severe, compared to the situation where there are metal ball defects on the high-voltage side, the discharge is more intense when there are metal ball defects on the low-voltage side, and the change trends of simulation results are consistent with the experimental results. When there are metal particle defects on the surface of GIS basin insulators, the surface discharge intensity is positively correlated with the distortion degree of electric field, and the correlation between the distortion degree of electric field and the average pulse amplitude is significant.

In order to investigate the effect of alkaline corrosion on the insulating strength of superhydrophobic coatings, superhydrophobic coating specimens with insulating properties were prepared and subjected to artificial alkaline corrosion tests. The insulating strength of the superhydrophobic coating specimens with different corrosion degrees was tested, and the effects of pollution degree, applied voltage amplitude, and wetting degree of surface contaminants on their insulating strength under different temperatures were investigated. The results show that the alkaline environment will destroy the substance and structure of superhydrophobic coating surface, leading to the decrease of its hydrophobicity and self-cleaning capability, which in turn leads to the decrease of its insulating strength. The more serious the surface pollution degree of the alkaline-corroded superhydrophobic coating under wet and polluting environment, the higher the applied voltage, and the shorter the wetting time, the lower the insulating strength of the coating surface. The surface pollution degree of the alkaline-corroded superhydrophobic coatings gradually decreases to a stable value with the increase of wetting time, at this time the surface leakage current of the coating does not change significantly, and the lower the temperature of alkaline corrosion environment, the shorter the wetting time required to reach the above stable state.

Light weight and high insulation composite cross-arm has excellent properties of light weight and excellent insulating properties, which has been widely used in power lines in recent years. In this paper, a composite cross-arm with light weight and high insulation of foam core was prepared, and the cross-arm was conducted water absorption, dye penetration, water diffusion leakage current, electric strength, thermal induction, and vibration ageing tests. The results show that the volume water absorption of the composite cross-arm with light weight and high insulation of core is 0.036%, the water diffusion leakage current is less than 75 μA, the AC electric strength is greater than 71 kV/cm, the vibration ageing resistance is good, and the comprehensive performance is better than that of the cross-arm with traditional solid core, which can meet the need of practical engineering application.

In this paper, the main ageing forms and influencing factors of oil-paper insulation for traction transformer were analyzed. The ageing mechanism of oil-paper insulation was studied, the current characteristic parameters characterizing the ageing degree of oil-paper insulation and the main residual life prediction methods were summarized, and the advantages and disadvantages of each characteristic parameter and residual life prediction method and the applicable conditions were analyzed. On this basis, the existing main problems in the ageing state assessment and residual life prediction of oil-paper insulation, as well as the future research directions were discussed.

In order to improve the bonding strength of epoxy resin potting materials to metals, the effects of molecular structure, amine curing agent type, and inorganic fillers on the tensile shear strength of epoxy resin potting materials were studied. The results show that the epoxy materials prepared by polyurethane modified epoxy resin and dimer-acid modified epoxy resin have higher tensile shear strength. The tensile shear strength of epoxy materials cured by polyamide, polyether amine, and aromatic amine decrease in turn. With the increase of inorganic filler content, the tensile shear strength of epoxy materials increases at first and then decreases. For the nano gas-phase SiO₂ powder, when its filling mass fraction is 4%, the epoxy material has higher adhesive strength, while for the micro powder, its filling mass fraction needs to reach 40%, the epoxy material can have higher adhesive strength.