High voltage power module packaging insulation endures unipolar electric stress, but its space charge phenomenon has never been paid enough attention. The space charge behavior and trap energy distribution of silicone material for packaging were investigated at various temperatures in this paper. The space charge and isothermal relaxation current of silicone samples were tested at 40, 60, and 80℃ under the applied electrical field from 3 to 15 kV/mm by simultaneous measurement system of space charge and trap energy states. The results show that the space charge accumulation is getting worse with the increase of temperature and applied field. There are charges with the same polarity accumulated near the electrode, which distorts the local electrical field distribution. Furthermore, a modified isothermal relaxation current analysis method based on non-negative least-squares iterative algorithm was proposed, and the depolarized relaxation currents at various temperatures were analyzed to provide the trap energy distribution from 0.8 to 1.2 eV, which was beneficial for understanding the mechanism of space charge behavior. The accumulation of space charges results in local field distortion, and the distorted field can cause partial discharge and insulation failure of packaging. Therefore, it is suggested that the space charge problem should be taken into account at packaging design of high voltage power modules.

In order to study the reliability of nitrile butadiene rubber (NBR) sealing element under the action of extremely low temperature and oil medium, the mechanical properties and microstructure of sealing rings in high-voltage transformers in extremely cold regions were studied. The mechanical characteristics, morphology change, chemical structure characteristics, and thermal response characteristics of the sealing ring after serviced for one year in extremely cold environment were obtained by tensile test, micron indentation test, scanning electron microscope (SEM), infrared spectrum (FTIR), and differential scanning thermal analysis (DSC). The results show that the elastic modulus and hardness of NBR sealing ring decrease in extremely cold environment, which leads to the loss of sealing effect. This is because the surface of sealing ring swell under the joint action of low temperature and oil medium, and micro-cracks appear. Oil immersion in NBR makes the molecular chain spacing increase, and the compliance of main chain of NBR is improved, meanwhile, the molecular structure of NBR changes due to the hydrolysis reaction of cyanide group.

In order to study the performance changes of irradiation crosslinking low-smoke halogen-free flame-retardant insulation layer and sheath layer for photovoltaic cables before and after getting moistened, the insulation layer and sheath layer of photovoltaic cable with various water-immersion time were conducted mechanical properties, thermal extensibility, dynamic thermomechanical analysis (DMA), Fourier transform infrared spectroscopy (FTIR), and volume resistivity tests. The results show that the long-term water-immersion causes hydrolysis of molecular chains in the material, and the entanglement of molecular chains weakens, which lead to decrease of tensile properties of samples. After water-immersion, the initial storage modulus of the insulation layer and sheath layer samples decreases, the adhesion force between filler particles and matrix resin decreases, and the mechanical loss peak of the insulation layer presents double peaks characteristic. With the increase of moistening degree, the EVA in the polyolefin matrix swells, and the free volume increases. Some flame retardant fillers are separate from the binding of matrix and become impurity ions, making the carrier concentration and mobility increase, which leads to continuous decrease of volume resistivity.

Aiming at the contaminant composition of transmission line insulator in Ningxia Yellow River irrigation area are not clear, and the degree of environmental impact is unknown, we subdivided the environment of Ningxia Yellow River irrigation area into 5 categories: industry, agriculture, urban residential area, desert, and grassland, and insulator contaminants samples in the corresponding environments were taken to conduct microscopic morphology, element content, crystal composition, anion and cation composition and content detection and analysis. Meanwhile, the pollution accumulation characteristics of insulators in irrigated and non-irrigated areas were compared and analyzed. The results show that the soil composition in the Yellow River irrigation area determines the basic composition of insulator contaminants. The industrial and agricultural environment have bigger influence on the micro-structure and different ingredients content of the insulator contaminants, and the urban residential area is followed. The pollution accumulation degree of insulators in irrigated areas was obviously higher than that in non-irrigated areas. In order to further improve the operation reliability of the transmission lines in Ningxia Yellow River irrigation area, some suggestions for anti-pollution flashover operation and maintenance are put forward, which can be use as reference for relate operation and maintenance units.

The faults caused by internal interface defects of composite insulators is a great threat to the safe and stable operation of power grid. In order to study the influence of different shapes of defects and their sizes and moisture immersion on the electric field distribution of insulators, an isosceles triangle defect model including tip curvature radius parameter was proposed. A 3D model of 500 kV composite insulator was established, and the electric field of defects with different shapes and sizes and moisture ratio were simulated and compared by the COMSOL. The results show that the isosceles triangle defect model can better reflect the tendency of defect developing to the low voltage side. The tip curvature radius of defect has no obvious effect on the air gap, but there is a point effect in the water gap. With the increase of moisture content in the defect, the maximum electric field strength increases gradually. Under the action of point effect and moisture immersion, the electric field strength at the tip of low voltage side is distorted severely, which increases the possibility of partial discharge and accelerates its development to the low voltage side.

To investigate the partial defects positioning method of low voltage cable, frequency domain reflectometry (FDR) was used to test the low voltage cable with defects, and the positioning results were analyzed in this paper. Three partial defects of D1, D2, and D3 were made successively in phases A, B, and C of 14.1 m long low voltage cable. The D1 defect was a 20 mm × 2 mm of rectangular insulation (expose cable core) in phase A made at 8.7 m away from the head-end of cable and was paralleled resistors with different resistance to simulate high and low resistance faults. The D2 defect was a longitudinal knife mark defect with 5 mm of length in phase B made at 4.3 m away from the front end of cable, while D3 defect was a V-shaped knife mark defect in phase C made at 11 m away from the head-end of cable. The A phase samples without defects and A, B, and C three phase samples with defects were tested by FDR instrument. Then, the D3 defect was immersed in saturated salt water and the three phases end of cable was applied 1 kV voltage for accelerated ageing. The ageing time was four weeks. After ageing, the three phases A, B, and C were detected by FDR. The waveform characteristics in frequency domain and time domain of different defects were analyzed according to FDR detection results. The results show that the peak amplitude in frequency domain of the sample without defect decreases, but the peak amplitude at the defect of the sample with defects increases and is larger than the waveform amplitude on both sides, and the peak amplitude increases more significantly when there is a low resistance fault. In addition, the time domain waveform of high resistance fault shows a positive wave, but the peak is not significant. The low resistance fault presents a significant negative wave. FDR waveforms in frequency domain and time domain can be used to locate low voltage cable faults effectively, and have higher sensitivity to locate low resistance fault.

In DC transmission lines, cable line is easy subjected to the impact of operating overvoltage and lightning overvoltage, which leads to cable line superimposed by impulse voltage on the basis of DC voltage, making the space charge accumulation on insulation layer change. In order to study the effect of impulse voltage superposition on the charge accumulation in the operating DC cable lines, a PEA method space charge measurement system under DC superimposed impulse voltage was developed through improvement on the traditional PEA method, combining with equivalent circuit formula deduction and Pspice software simulation, and reasonable selection of RC-component in measurement system. The space charge characteristics of XLPE samples under DC superimposed impulse voltage were measured. The results show that the coupling efficiency of impulse voltage and DC voltage is very high, and the space charge characteristics show that the superimposing the same polarity of impulse voltage can promote the injection and migration of the same polarity charge more compared with superimposing different polarity of impulse voltage.

With the development of UHV DC transmission system engineering in China, the voltage level of converter transformers is getting higher and higher, and insulation problems are becoming more and more prominent. The study of partial discharge is of great significance to the insulation condition judgment and equipment life evaluation of converter transformer. In this paper, the insulation characteristics, the process of field tests, and the detection methods of partial discharges and their precautions of converter transformer were introduced, the research achievements of experts and scholars at home and abroad on the partial discharge of converter transformer were reviewed, and the research directions of partial discharge of converter transformer were prospected, which provided a reference for the operation, maintenance, and research of converter transformers.

Because the terahertz wave has strong penetrating ability, high imaging resolution, and strong anti-interference ability, the non-destructive testing technology based on terahertz reflection imaging has become a research hotspot in the field of insulator defect detection. In this paper, the propagation characteristics of terahertz wave in medium were firstly introduced, and a propagation model was constructed to analyze the terahertz wave refraction and reflection phenomena at the defect interface caused by the discontinuity of medium existing a defect. Then the finite difference time domain (FDTD) method of electromagnetic wave was elaborated, and geometric modeling and simulation of silicone rubber material was conducted by using a full wave 3D electromagnetic simulation software on the basis of the equation that electric field varies with time based on FDTD theory. It was found that the distribution difference of terahertz waves observed on the observation surface which was 2.5 mm away from the center of model, can reflect the size and location of internal defect, which provided a simulation method for theoretical studies. Finally, the academic cases of using terahertz imaging technology to detect internal defects in insulators were reviewed, and the problems that the detection distance affects the sensitivity, the background noise affects the imaging resolution, and the identification of defect types were analyzed and prospected.

Biaxially oriented polylactic acid (BOPLA) films were prepared by twin-screw extruder and static stretching machine, and they were buried in outdoor soil. The natural degradation process and mechanism of the BOPLA films were studied by analytical balance, Fourier transform infrared spectoscopy (FI-IR), scanning electron microscope (SEM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). The results show that the BOPLA film degrades slowly due to its dense surface at the initial degradation stage. After degrading for 10 months, the dense layer on the surface is eroded seriously, and the degradation goes deep into the film, the degradation is accelerated. The crystallinity, grain size, glass transition temperature, and other properties, which affected by the degradation of crystalline and amorphous regions, reach the peak value after degrading for 3 months, and then show a fluctuating downward trend.