Firstly, the physico-chemical characteristics of the decay-like core rod were analyzed, and the effect of decay-like on the mechanical properties of the core rod was studied. And then, the vibration modes of composite insulator core rods were analyzed by simulation, and the relationship between characteristic frequency and the decay-like degree was studied. Finally, a vibration excitation detection method was proposed for decay-like composite insulator, and the characteristic frequencies of the 110 kV composite insulators with different degrees of decay-like defects were measured and analyzed. The results show that the complete degradation of epoxy resin and the hydrolysis and pulverization of glass fibers in the core rod make the decay-like region loss mechanical properties, leading to the decrease in the overall stiffness, tensile resistance, and deformation resistance of the core rod. Compared with the intact core rod, the characteristic frequencies of 3-5 order vibration mode of decay-like core rod decrease by 2.8%-14.3%, the decreasing range of characteristic frequencies of higher order vibration modes decrease more obvious, and the decreasing range increases with the increase of decay-like degree. The decay-like detection method based on vibration mode can diagnose the decay-like degree of composite insulator, and also can reflect the changes of its mechanical properties, which is suitable for non-destructive detection of decay-like in composite insulators.

To resolve the issue that the traditional frequency domain reflection (FDR) method is easy to misjudge when diagnosing the moisture defects of cable joints, the influence law of thermal excitation on the characteristic impedance of moistened cable joints was firstly analyzed in this paper, and a diagnostic method for moisture in cable joints was proposed based on the frequency domain reflection characteristics under thermal excitation. Then a moistened joint was set up on a 10 kV XLPE cable with a length of 58 m, the temperature of the moistened joint was changed using a heating belt, and the moisture defects were located and diagnosed. The results show that when applying the thermal excitation, the unit capacitance and characteristic impedance of the cable body and normal joint hardly change with the temperature, the unit capacitance of the moistened cable joint decreases with the increase of temperature, and the characteristic impedance increases with the increase of temperature. The simulation results show that the amplitude of the frequency domain reflection spectrum at the moistened joint increases with the increase of temperature, while the amplitude of the frequency domain reflection spectrum at the normal joint is basically unchanged. Based on the above research results, the amplitude changes of the true cable reflection coefficient spectrum at the joint under different temperatures were obtained through thermal excitation, achieving the localization diagnosis of moisture defects.

A clean air insulation defect decomposition experimental device was constructed based on gas component analysis method, then the main characteristic decomposition components of clean air under different discharge form (spark discharge and corona discharge) and different gas pressure were detected and analyzed by Fourier transform infrared spectroscopy (FTIR). The basic reaction principles and pathways of the main decomposition components were explored, and the effects of different pressures on the partial main characteristic decomposition components of clean air were analyzed quantitatively or qualitatively. The results show that the main characteristic decomposition products of clean air under spark discharge are NO₂, NO, and N₂O₄, and the main characteristic decomposition products under corona discharge are O₃, N₂O₅, and N₂O. The increase of gas pressure can affect the generation ratio of NO and NO₂ under the spark discharge, the proportion of NO is lower in high gas pressure range, and the NO selectivity increases with the decrease of gas pressure. The concentrations of O₃, N₂O₅, and N₂O generated under corona discharge show an overall trend of increase at first and then decrease with the increase of gas pressure under a same voltage level, and the generation of N₂O₅ has a strong correlation with the concentration of O₃.

In order to obtain the escape beam characteristics of environmentally friendly insulating gas under the action of nanosecond pulse voltage, the generation laws of the escape electrons in clean air, SF₆, and C₄F₇N/CO₂ gas mixture were studied. An escape electron collection platform was built, and the charge amount of the generated escape electrons was measured when different amplitudes of nanosecond pulse voltages were applied to clean air, SF₆, and C₄F₇N/CO₂ gas mixture under different gas pressures. The results show that the low-energy electrons will accelerate and transform to escape electrons under the action of nanosecond pulse. The higher the nanosecond pulse voltage amplitude, the lower the gas pressure, and the larger the charge amount of escape electrons generated by insulating gas. The clean air has relatively weak inhibition on the generation of escape electrons, and the SF₆ and C₄F₇N/CO₂ gas mixture have obvious inhibition effect on the generation of escape electrons. As the pressure increases, the inhibition effect on the generation of escape electrons of C₄F₇N/CO₂ gas mixture is more obvious than that of SF₆.

The adsorption of CF₃SO₂F gas on the surface of Al(111) and its decomposition mechanism were studied based on density functional theory. The adsorption energy, charge transfer, differential charge density, electron localization function (ELF), and state density of CF₃SO₂F gas on the surface of Al(111) were calculated, and the compatibility between CF₃SO₂F gas and Al(111) surface was analyzed theoretically. Further, a theoretical calculation model of the decomposition path of CF₃SO₂F gas was established to calculate the free energy of CF₃SO₂F gas under different decomposition paths, and the main decomposition products of CF₃SO₂F gas were obtained through transition state analysis. The results show that CF₃SO₂F gas has good compatibility with Al(111) surface under normal operating conditions, and their interaction is physical adsorption. The typical decomposition gas products of CF₃SO₂F gas mainly include CF₄ and SO₂. The research results can provide theoretical reference for evaluating the gas-solid compatibility and stability of the new environmentally friendly insulating gas CF₃SO₂F.

In order to assess the feasibility of using C₅F₁₀O/dry air gas mixture to replace existing dry air or nitrogen in insulation equipment under low partial pressure conditions, we analyzed the insulation performance of C₅F₁₀O/dry air gas mixture systematically. The results show that in a quasi-uniform electric field, when the partial pressure of C₅F₁₀O is 5-10 kPa, the insulation strength of the C₅F₁₀O/dry air gas mixture is 1.1-1.9 times and 1.4-2.3 times bigger than that of dry air and nitrogen under the same condition, respectively, which indicates that even at low partial pressures, the C₅F₁₀O/dry air still demonstrates strong insulation performance and has technical advantages compare to dry air or nitrogen. Furthermore, the C₅F₁₀O/dry air shows good stability in multiple breakdown tests, and the slope of fitting curves between breakdown voltage and breakdown times approaches zero, indicating that its insulation performance has not significantly deteriorated.

In order to realize the accurate assessment of the ageing state of transformer insulating paper, an ageing state assessing method of insulating paper based on combined assignment and improved TOPSIS was proposed. Firstly, in order to overcome the shortcomings of single feature quantity evaluation, polymerization degree, tensile strength, elongation at break, breakdown voltage, and dielectric loss factor were selected as ageing feature quantities to establish an evaluation system for the ageing state of insulating paper. Secondly, the combined weight value of feature quantities was calculated using the combined assignment model. Finally, an improved TOPSIS model was proposed to construct the Euclidean-gray correlation distance measure. The closeness degree was calculated using the improved TOPSIS model, and the closeness degree interval was set to grade and evaluate the ageing states of insulating paper. The results show that the ageing state assessing method of insulating paper based on combined assignment and improved TOPSIS not only considers the subjective experience weight of experts, but also uses the inherent characteristics of feature quantity data for weight correction, making the weight more scientific and reasonable. At the same time, it compensates for the distance criterion deficiency of the traditional TOPSIS model, making the calculated closeness degree more reasonable. Through the electricity-heat-force combined ageing test, the evaluation method has been verified to determine the ageing states of insulating paper accurately and effectively, and the grading evaluation of ageing states can be achieved by setting a closeness degree interval.

In order to explore the generation mechanism of perfluoroisobutyronitrile (C₄F₇N) gas decomposition products under the presence of micro-water, the decomposition process of C₄F₇N gas and the recombination process after decomposition were investigated when micro-water participated in the reaction. On the basis of density functional theory and transition state theory, the thermodynamic parameters of C₄F₇N gas under micro-water participation reaction conditions, as well as the generation paths and generation rates of some characteristic products, were calculated. The results show that when micro-water participates in the reaction, the activation energy required for the decomposition of C₄F₇N gas is lower than that required for the decomposition of dry C₄F₇N gas. The presence of micro-water makes the C₄F₇N decompose easier. The reaction rate constant of CHF₃ gas is relatively high in the decomposition products of C₄F₇N gas, and its chemical properties are stable and detectable, which can be used as a characteristic product of C₄F₇N gas decomposition under the presence of micro-water.

The influence mechanism of thermal-cooling cycling times and temperature change rate on the electrical properties of epoxy resin were studied using a linear rising and cooling method. The results show that as the cycle times increases, the DC electric strength and volume resistivity of epoxy resin increase at first and then decrease, and this trend slightly increases in early cycles with the increase of temperature change rate, but decreases significantly in later cycles. The analysis indicates that thermal and cold alternating environment will promote the dynamic adjustment of chain segments in epoxy resin, making its structure more compact and regular. Although thermal expansion and cold contraction may cause micro-cracks appear on the surface of epoxy resin during early cycles, dynamic adjustment plays a dominant role, which leads to the improvment of electrical properties of epoxy resin. However, with the increase of cycle times, long-term thermal expansion and cold contraction intensifies the fracture of molecular chain segments and the expansion of micro-crack, resulting in the decrease of electrical properties of epoxy resin. With the increase of temperature change rates, the internal chain segment in epoxy resin cannot be fully adjusted, and the impact of temperature stress increases, which lead the epoxy resin more prone to deterioration.

C₆F₁₂O has good environmental protection characteristics and insulation properties, and has good application prospects in medium and low voltage gas-insulated equipment. In order to explore the influence of micro-water inside the gas-insulated equipment on the insulation performance and decomposition characteristics of C₆F₁₂O/CO₂ gas mixture, a gas power frequency breakdown test platform was set up, and the breakdown voltage of 4%C₆F₁₂O/96%CO₂ gas mixture under 0.14 MPa pressure within the range of 0-1 000 μL/L micro-water concentration was studied.The influence of different micro-water concentration on the power frequency breakdown characteristics of gas mixture was analyzed. The types and concentrations of the decomposition products of C₆F₁₂O/CO₂ gas mixture were analyzed quantitatively and qualitatively by gas chromatography-mass spectrometry (GC-MS) after breakdown, and the influence laws of micro-water concentration on the concentration of breakdown decomposition products and effective gas production rate were obtained. The results show that micro-water will reduce the power frequency breakdown voltage of C₆F₁₂O/CO₂ gas mixture. The main breakdown decomposition products of C₆F₁₂O/CO₂ gas mixture are CF₄, C₂F₆, C₃F₆, C₃F₈, C₃F₇H, CF₂O, C₄F₁₀, C₅F₁₂, and CF₃H, among them, the concentration and effective gas production rate of CF₄, C₂F₆, and C₃F₇H are positively correlated with the micro-water concentration, and the concentration of CF₄, C₂F₆, C₃F₆, C₃F₈, and C₃F₇H are positively correlated with the breakdown times. Considering the insulation performance and decomposition characteristics, it is recommended to strictly control the concentration of micro-water inside the C₆F₁₂O/CO₂ gas mixtures equipment during engineering application, and increase the test frequency of micro-water in the equipment.