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.

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.

Free metal particles are usually regarded as one of the most hazardous contaminants in the liquid-phase insulation of converter transformers, which can lead to the partial discharge (PD) initial voltage and breakdown voltage of equipment decrease obviously. In this paper, a transformer oil circulation system was constructed, and DC partial discharge and breakdown experiments were conducted on the transformer oil containing metal particles at different oil flow velocity. The PD characteristic parameters were extracted, and the variation of characteristic breakdown voltage with oil flow velocity was obtained. Then the affecting mechanisms of oil flow velocity on the PD and breakdown characteristics were discussed combined with the particle distribution images between electrodes. The results show that when the oil is static, the partial discharge degree of the transformer oil containing metal particles is highest. With the increase of oil flow velocity, the PD initial voltage increases, and the discharge repetition rate and the accumulation discharge magnitude per unit time decrease. When the oil is static, the breakdown voltage of the transformer oil containing metal particles is lowest. With the increase of oil flow velocity, the Weibull distribution curve of breakdown voltage shifts to the right, and the characteristic breakdown voltage increases, that is, the flowing of oil improves the insulating properties of transformer oil and reduces the breakdown possibility remarkably.

The buffer layer of high-voltage cable will generate gas products during the ablation process. However, the correlation between the gas product concentrations and conductive property of buffer layer during the ablation process is still unclear at present. Therefore, we conducted a simulated ablation experiment on cable buffer layer and studied the evolution law of current density and gas product concentrations during the ablation process of buffer layer. The results show that under both the dry and wet conditions, the current density of buffer layer exhibits a bi-exponential function decay law with the increase of ablation time, and the decay rate of current density shows a trend of decrease rapidly at first and then unchanged basically with the increase of ablation time, only the wet sample exhibits peak current density during the initial stage of ablation. The gas chromatographic test results show that under the wet condition, the gas products during the ablation process of buffer layers are CO₂, H₂, CO, CH₄, C₂H₄, C₂H₆, and C₂H₂. Under both the dry and wet conditions, the gas product concentrations increase rapidly at first and then tend to a stable value with the increase of ablation time, and the increase time of gas concentration is consistent with the decrease time of current density, which indicates the change laws of current density and gas concentration with the ablation time is closely related. It is analyzed that there is a correlative relationship between the current density decay rate and the gas products generation rate during the ablation process of buffer layer, and the variation of gas product concentration can reflect the severity of buffer layer ablation.

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.

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.

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.

After the ageing of sealant, the moisture will invade the crimping interface from the sealant interface, leading to insulator ageing. To investigate the affecting mechanism of various ageing factors on the interface samples between sealant and armour clamp, immersion tests were conducted on the sealant samples in deionized water, salt solution, nitric acid solution, sulfuric acid solution at room temperature for 28 days, as well as immersion tests in mixed solutions of sulfuric acid or nitric acid with salt at different temperatures. Then the ageing situation of the samples was analyzed by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) scanning electron microscope (SEM), and energy dispersive spectrometer (EDS). The results show that the sulfuric acid solution has no significant effect on the siloxane matrix of sealant, while the salt solution can cause the breakage of methyl chain of the siloxane side chain, and the nitric acid solution can oxidize the siloxane side chain to generate -OH. The influence of nitric acid and salt mixed solution on the siloxane matrix at room temperature is greater than that at high temperature, and the effect of nitric acid and salt mixed solution on the siloxane matrix is basically equivalent to the superposition of the two solutions acting separately. The influence of sulfuric acid and salt mixed solution on the sealant at high temperature is greater than that at room temperature. The main affecting mechanism of sulfuric acid and salt mixed solution on the siloxane matrix is that the salt solution causes the methyl of silicone side chain to break, making the Si-O-Si main chain expose to sulfuric acid and undergo chain breaking, then the salt solution can continue to affect the deeper methyl groups.

The technical standards of insulating oil at home and abroad were summarized and the technical requirements of insulating oil for high voltage instrument transformer were pointed out. The using properties of five kinds of insulating oils commonly used in high voltage instrument transformer in China were compared and analyzed. The results show that No.1 and No.2 insulating oils are negative gassing insulating oil, which are suitable for high voltage instrument transformers. No.3, No.4, and No.5 insulating oils are positive gassing insulating oil, which are not suitable for high voltage instrument transformers. On the balance of aromatics content, gassing properties, gas production properties, sludge solubility, and oxidation stability, No.1 insulating oil has the best comprehensive performance, the lowest gassing, and the best oxidation stability.

Thermoplastic polypropylene materials have excellent electrical properties and thermal properties, and their production process does not require cross-linking and degassing, with low energy consumption and recyclability. Compared with cross-linked polyethylene, it is environmentally friendly and one of the best materials for the production of power cable. Taking polypropylene materials as the discussion object, the basic structure and physical properties of polypropylene and their relationship with electrical properties were introduced in this paper. The effects of copolymerization modification, blending modification, chemical modification, and nano particle modification on the structure, mechanical, thermal and electrical properties of polypropylene were reviewed. The research progress and application of polypropylene cables at home and abroad were introduced. Finally, the furure development of polypropylene materials were summaried and prospected from the production and preparation technology of polypropylene, relationship between structure and properties, and modification techniques.