Basin insulator is an important part of gas insulated closed combination switchgear (GIS) device, and its quality has an important impact on the construction and operation of power grid. In this paper, the failure causes of basin insulators were described, and the existing detection methods were compared in common cracking locations. According to the material characteristics of basin-type insulators and field test conditions, the probe frequency, K value, and chip size were calculated theoretically. An ultrasonic water immersion focusing oblique probe device was developed, a method of detecting cracks near the bolt holes of basin insulator by using the low-frequency dual-probe ultrasonic detection technology was proposed, and the test results were verified by laboratory penetration and ray detection. The results show that the ultrasonic refraction transverse wave with high signal-noise ratio can be generated in the basin insulator by using dual probes with 2 MHz 18×18 mm of model size and 24.8°-35.4° of angle (α_L) between the axis of the wafer emission acoustic beam and the normal line of workpiece interface. It is feasible to use special low frequency double probe ultrasonic detection technology to detect the defects of GIS basin insulators.

Taking 220 kV rod-shaped suspension composite insulator as an example, we conducted simulation and calculation on a 1/4 tower model for 220 kV line composite insulator by the COMSOL multiphysics software, and the influences of the grading rings with different types and structures on the electric field distribution of composite insulator were studied. The results show that the electric field uniformity effect of semicircular grading ring is slightly weaker than that of the tubular grading ring, and the voltage equalizing effect of bird-proof grading ring is basically the same as that of the tubular grading ring installed at low voltage end. The surface electric field intensity of composite insulator decreases with the increase of pipe diameter of high-voltage end or double-end grading ring, and the decline decreases gradually. When the diameter of high-voltage end grading ring increases, the surface electric field intensity of composite insulator decreases at first and then increases. When the diameter of double-end grading ring increases, the surface electric field intensity at high-voltage end decreases at first, then increases and finally decreases, while the surface electric field intensity at low-voltage end keeps increasing. Increasing the covering depth of grading ring at high voltage end can improve the surface electric field at the middle of the composite insulator, and lead to the increase of electric field at the connecting end, which is not conducive to the safe operation of composite insulator.

Due to the low thermal conductivity of XLPE main insulation and low temperature of seawater, it is easy to form a large temperature gradient in submarine cable insulation. The formation of temperature gradient will cause a radial difference in the aggregation and dielectric properties of XLPE, so as to influence the electrical treeing deterioration process. In order to study the electrical tree characteristics of XLPE under temperature gradient, a temperature gradient electrical tree experimental platform in the range of 10-90℃ was built, and the initiation and growth characteristics of electrical tree under different temperature gradient were measured. The results show that the changes of needle tip electric field and XLPE aggregation under different temperature gradient will affect the initial voltage of electrical tree. With the increase of temperature gradient, the main structure of electrical trees presents gradual change characteristics of rattan-branch tree, bush-rattan tree, and bush tree.

The carbon fiber composite mandrel was conducted thermal oxygen ageing tests at 170、185、200℃ for one month. Through the mechanical properties tests, Fourier infrared spectroscopy (FTIR), and scanning electron microscope (SEM) analysis of the mandrel sample at different ageing stages, the macro properties, micro characteristics, and the relationship between macro and micro properties were studied, and the change laws of mandrel properties with ageing time were obtained. The results show that the accelerated thermal oxygen ageing will further increase the curing degree of the mandrel and enhance its mechanical properties. While during the thermal oxygen ageing process, there are cracks, powders, and holes in the epoxy resin matrix in the outer layer of the mandrel. The interface adhesion between epoxy resin and glass fiber becomes worse, and the epoxy resin matrix becomes brittle, which will damage the mechanical properties. Under the action of these two factors, the mechanical properties of mandrel increase occasionally and then decrease as a whole.

Firstly, a corresponding simulation model was built on the basis of terminal structure of 220 kV high-voltage dry cable, and combined with the nonlinear conductivity equation of insulating materials, the electric field distribution and temperature distribution in the terminal with different reinforced insulating materials, ambient temperature, and applied voltage were studied. And then, the electric field distributions of the terminal with defects such as misplacement of stress cone installation, surface bulge, and bubbles in the reinforced insulation were compared and analyzed. Finally, the corner shape of stress cone and the distance between stress cone edge and reinforced insulation were optimized, the electric field distribution of the terminal after optimization was analyzed, and the optimal distance between stress cone edge and reinforced insulation was proposed. The results show that the nonlinear silicone rubber insulating material can well homogenize the electric field. The change of external ambient temperature will decrease the temperature difference between the internal core and the external umbrella skirt, and the maximum field strength in terminal increases significantly with the increase of ambient temperature. The stress cone installation dislocation makes the field strength at the three-phase junction increase sharply. The bulge on stress cone surface makes the local field strength in terminal increase sharply. When there are bubbles in reinforced insulation, the bubble size has little effect on the maximum field strength in the cable terminal. By changing the corner shape of stress cone into a circular arc shape, the field strength at the corner decreases by 75.26%. By increasing the distance between stress cone edge and reinforced insulation to 5 mm appropriately, the field strength at the stress cone corner decreases.

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.

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.

According to the characteristics of series resonance circuit, a new model for calculating the dielectric dissipation factor of high-voltage power cable was established, and a new dielectric loss factor calculating formula derived from the differential power of the series resonance circuit was proposed. On the basis of test model, the dielectric dissipation factor of XLPE power cable was calculated by the step-up or step-down process of the series resonance to solve the problem of dielectric dissipation measuring of high voltage and large capacity cable. The results show that the simulation calculating values of circuit parameters and field test results match with the actual values. Compared with conventional electric bridge method, the test method for calculating dielectric dissipation factor based on the resonance characteristic is more accurate to a certain extent. This provides a reference for testing the dielectric dissipation factor of high voltage and high capacity power cable.

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.

Three kinds of soluble polyimide (PI) resins were prepared from 4,4′-hexafluoroisopropylidene)bisphthalic anhydride (6FDA) and three kinds of romatic diamine monomer containing rigid-rod amide bonds in the molecular chains by one-step high temperature solution polycondensation reaction, respectively. And then three kinds of transparent PI films were prepared by the N,N-dimethylacetamide (DMAc) solution of the above resins. The films were tested by attenuated total reflection Fourier transform infrared spectroscopy, thermogravimetric analysis, dynamic mechanical analysis, thermomechanical analysis, and UV-visible spectrum, and their yellowness index was tested. The results show that the PI films exhibit good optical transparency, their ultraviolet cutoff wavelength is below 380 nm and the optical transmittance is over 80% at the wavelength of 500 nm. In addition, the PI films exhibit good thermal stability with the glass transition temperatures (T_g) higher than 320℃ and the 5% weight loss temperatures (T_5%) over 520℃. The linear thermal expansion coefficients (CTE) of the PI films are all lower than 50×10^-6/K, indicating that the introducing of rigid amide bonds can efficiently enhance the high-temperature dimensional stability of the solution-processable fluoro-containing PI films.