Three typical imported PP resins with a large market share for capacitor application were chosen, and their molecule structure, aggregation structure, and electrical properties were systematically measured. The influence law of molecular weight and distribution on the crystallization properties, as well as the action mechanism of crystallization properties on dielectric and breakdown properties was investigated, and in consequence the correlations between microstructure and macroscopic electrical properties was built. The results show that optimizing the molecular weight and distribution avoiding too large and narrow can contribute to regular and compact alinement and stacking of molecules, and then the orientation of dipole and transportation of electrons can be inhibited, resulting in the decrease of dielectric loss and increase of breakdown strength.

Metal particles in gas insulated metal enclosed switchgear (GIS) and gas insulated enclosed transmission line (GIL) can bounce near the insulator under the electric field, which intensifies the charge accumulation and may cause surface flashover in serious cases. In this paper, a 126 kV real size disc insulator was selected as the test sample. A sealed chamber and coaxial structure electrodes were designed, and the effects of bouncing metal particles on the surface charge accumulation behaviors of insulator at different temperatures (23, 40 and 60℃) were studied by using Kelvin electrostatic probe. The results show that the charge accumulation on the nonplanar region is closely related to the motion mode of metal particles at room temperature. And the charge accumulation on the insulator surface is positively correlated with the temperature. As the temperature rise to 60℃, the metal particles moving to the vertical surface of the ground electrode are easy to cause surface flashover, and a large number of heteropolar charges would be accumulated on the non-plane region in the same radial direction as the flashover.

In order to study the effects of different damping paths on cables, we set up a variety of endogenous and exogenous damping paths according to operating experience. Accelerated damping test were conducted on cables for 90 cycles, and the insulation resistance, partial discharge, and depolarization current were measured regularly. Then the moisture state of the cable was comprehensively evaluated by TOPSIS method. The results show that during cable operation, moisture can enter and rapidly spread inside the cable through the broken body, cable joints, and various interface structures of the cable terminals. The degradation effects of moisture through cable joints and endogenous moisture on the insulation is the most prominent, which should be prevented and taken seriously. Furthermore, the comprehensive evaluation of electrical properties based on insulation resistance, partial discharge, and depolarization current can effectively explore the damping process and identify the moisture state, which can be applied to practical engineering to evaluate the damp problem of distribution network cables in warm-damp environment.

The simulation model of 35 kV prefabricated cable terminal was established to simulate and calculate the electric stress, thermal stress, and mechanical stress distribution of the cable terminal under the impulse voltage. The electric field, temperature, and stress of body insulation, semi-conductive layer, and shielding layer were calculated and analyzed, and the influence of harmonic wave with different frequency on the electric field distribution was considered. The results show that the influence of high frequency signal on permittivity does not lead to large distortion of electric field intensity, and the change of electric field intensity is not obvious. At the same time, the temperature rise of cable terminal caused by impulse voltage is small, and its influence can be ignored. The impulse voltage increases the mechanical stress at the intersection of the semi-conductive layer and the stress cone, and the maximum radial strain reaches to 2.54%, which makes the position of the semi-conductive layer prone to produce air gap and is the main cause of cable terminal damage. It is concluded that not only the electric field optimization but also the elastic strain of the material should be considered in the material selection and structural design.

A thermal-mechanical simulation model of cable insulation fused joints was established, and then the effect of the difference in fluid temperature and flow rate between water-cooled and air-cooled cooling methods on the internal stress distribution of fused joints was analyzed. The effect of the material performance difference between the new cross-linked polyethylene and the old cross-linked polyethylene on the stress distribution was also studied. The results show that the change of water-cooling parameters has a great influence on the stress of the insulation layer of the fusion joint, and the stress distribution is concentrated near the interface of the new and old insulation layer, and the difference of the material properties leads to the similar stress distribution.

The relationship between the withstand voltage and the thickness of the sheath was studied when the air gap distance is 0, 5, 10, 15 cm, respectively. The influence of the length of sheath on the development of creeping discharge was studied when the sheath surface is dry, wet, and dirty, respectively. The results show that with the increase of the length and thickness of gap, the breakdown voltage of the insulating sheath-air combined gap increases nonlinearly, and the increase rate of the breakdown voltage also increases. The surface flashover electric field intensity of the insulating sheath decreases obviously in dirty state, the average flashover electric field intensity is 1.2 kV. Based on the above research, the configuration of anti bird droppings flashover insulating sheath was obtained. The thickness of the sheath is determined according to the line voltage level, and then the covering length is determined by the maximum surface flashover distance. It is recommended that the thickness and covering length of sheath in 110 kV transmission line is 6 mm and 60 cm, respectively; while the thickness and covering length of sheath in 220 kV transmission line is 8 mm and 110 cm, respectively.

The accumulation test of metal particles in oil between spherical electrodes under different ratios of AC/DC composite voltage was carried out, the partial discharge (PD) signals at different moments were collected simultaneously. And then the discharge frequency and average discharge quantity were counted, and the correlation between the aggregation behavior of metal particles and PD characteristics was analyzed. The results show that the accumulation area of metal particles under DC voltage is the largest, and the metal particles aggregated the most closely under 3:1 AC/DC composite voltage. The discharge frequency decreases with time, while gradually increases with the increase of AC component in AC/DC composite voltage. It is considered that the collision frequency between metal particles and electrodes and the closeness of particle accumulation are the main factors affecting the frequency of partial discharge and the average discharge quantity.

According to isotope tracing technology, we used stable isotope tracer ¹⁸O₂ to carry out SF₆ partial over-thermal decomposition experiments to simulate the decomposition of SF₆ under sustained high temperature. The influence mechanism of trace O₂ on the overheating decomposition of SF₆ was analyzed by obtaining the changes in the volume fractions of the main oxygen-containing products labeled by ¹⁸O. The results show that under overheating conditions, trace O₂ preferentially reacts with SF₃, SF₂, and SF to generate SOF₃, SOF₂, and SOF, followed by generating a small amount of SO₂F₂. It is said that O₂ is the main source of oxygen for the formation of SOF₂. Besides, under overheating conditions, SF₆ can be completely decomposed to generate elemental S, which reacts with trace O₂ to generate SO₂.

The broadband dielectric response of oil-paper insulation was studied, and broadband dielectric response tests were carried out on laboratory samples of oil-paper insulation with different moisture contents. According to the relaxation polarization part of dielectric spectrum, the spectral parameters characterizing the moisture degree were extracted, and their relationship with the moisture content in the oil-paper insulation was analyzed. Further moisture tests for the oil-immersed bushing scaled-down model were designed, and the application of the spectral parameters in the moisture diagnosis of bushing was discussed. The results show that the low-frequency polarization process in the relaxation polarization spectrum is more sensitive to the moisture content, and its loss peak value can be used as a characteristic parameter to characterize the moisture content; under different moisture types of the bushing, the change patterns of the loss peaks and characteristic frequencies of the low-frequency and high-frequency relaxation polarization are different, which can be used as a preliminary criterion for the moisture type of the bushing.

The typical defects model of heat-shrinkable and cold-shrinkable cable accessories were established, and partial discharge tests were carried out at 0.1 Hz ultra-low frequency and oscillatory wave voltage. The differences in the partial discharge inception voltage (PDIV), the PD quantity, the number of PD and other parameters of the typical defects under the two voltages were measured and compared, and the formation mechanism of these differences was analyzed. The results show that for most of the measured defects, compared with under 0.1 Hz ultra-low frequency voltage, the PDIV under the oscillating wave voltage is lower, the PD quantity is larger, and the number of partial discharge pulses is more. The difference in the voltage distribution mechanism and the voltage recovery rate at the defect location is the main reason for the above-mentioned difference.