Partial Discharge (PD) is the main sign of the insulation deterioration, therefore the online PD localization of the power transformers can provide the equipment degradation characteristics and has great value for the power grid. Because of some unique merits, acoustic PD detection is one of the best options for such application. However, when discharges happen inside the windings, precise PD localization by acoustic method becomes very challenging. As the acoustic PD signal can be reverberated and distorted along its propagation, the type and number of sensors, with their installation, should be carefully studied and selected. In this paper, the acoustic wave propagation inside the transformer is explored via numerical simulation. Based on the understanding of wave propagation, a novel-structure fiber-optic acoustic sensor array is properly designed and installed into one phase winding from a real 35 kV transformer. Experimental verification shows that, by using this proposed design, PD localization with less than 5 cm error can be achieved. Therefore, the novel sensor array developed in this paper, together with the installation and localization method, unveils a novel access to accurate online PD localization, especially for those PDs which happen inside transformer windings.

A novel extrinsic Fabry–Perot interferometric (EFPI) ultrasonic sensor is developed that can detect ultrasonic signals of partial discharge (PD) in high-voltage electrical equipment under different temperature and pressure conditions. The unique structure of the novel sensor is able to balance the pressure difference between the two sides of the quartz diaphragm driven by the temperature and the density of the insulating medium, and the inner surface of the quartz diaphragm and the end face of the optical fiber in the sensor are coated, thus considerably improving the sensor sensitivity. Experiments are performed on the novel sensor at different temperatures and measurement distances, and the results verify the temperature, frequency, sensitivity and other types of detection performance, showing that the proposed sensor provides a novel means for detecting partial discharge of high-voltage electrical equipment.

Polypropylene (PP) is regarded as an environmentally friendly cable insulation material to replace cross-linked polyethylene owing to its thermoplastic nature, high-temperature resistance, and outstanding dielectric characteristics. Grafting modification was applied to satisfy the demand for a higher-voltage power grid, and it was proved to be effective in enhancing the insulating performance of PP. The electrical tree is a common cause of cable insulation failure. This study examines the electrical tree initiation and growth characteristics of methyl methacrylate (MMA) grafted PP under different temperatures. Our results indicate that high temperature can promote the electrical treeing process, while grafting modification can hinder the process. The trap characteristics show that more deep traps are introduced in the grafted materials that can limit the motion of electrons and, thus, restrict the electrical tree initiation process. The crystallization morphology shows that the size of spherulites increases and the boundaries of the spherulites become blurry with higher MMA content. The compressed amorphous region and the tortuous boundaries of the crystalline can hinder the electrical treeing process. High temperature may improve charge carrier mobility and enhance the molecular chain relaxation, thus enlarging the free volume and finally accelerating the treeing process. The findings of this study would serve as a reference for designing grafted pP insulation for power cables.

Electric tree is a dendritic channel due to partial discharge, which is a more common form of failure in the insulation system of power electronic modules, and silicone gel is widely used in the package insulation of power electronic modules. On this basis, this paper conducts experiments on the growth of electric trees at different frequencies, investigates the initiation and propagation mechanisms of electric tree in silicone gels at high frequencies, and analyzes the mechanism of the effect of frequency on the growth characteristics of electric trees. Furthermore, the self-healing experiments of electric trees at different temperatures and component ratios were carried out to investigate the self-healing behavior of electric trees in silicone gels and its influence mechanism. The experimental results show that Maxwell stress is the main cause of electric tree initiation at high frequencies, and the propagation characteristics of electric trees in silicone gels are different from those of conventional dielectrics. Self-healing in silicone gels can be classified into types I and II, which occur respectively during electric tree propagation and after withdrawal of voltage, and type II self-healing can be further classified into physical and chemical self-healing. The effects of temperature and A-component content on the self-healing properties are also investigated in this paper.

Polypropylene (PP) represents an important development direction for recyclable cable insulation materials. This paper investigates the synergistic improvement mechanism of the mechanical and electrical properties of PP. The effect of elastomers on the aggregation structure and carrier migration of the samples was investigated by analyzing the microstructure, crystallization, and mechanical and electrical properties of PP/elastomer composites. The results show that elastomers can reduce the tensile modulus of PP, while improving the tensile strength and elongation at break, thereby enhancing the mechanical properties of PP. The elastomer and the PP matrix have better compatibility and introduce amorphous regions, generating a large number of crystalline–amorphous interfaces, which introduce deep traps and reduce the mobility of carriers. Meanwhile, with an increase in elastomer content, the breakdown strength and volume resistivity both increase and then decrease. Among the samples tested, PP/PBE-2 has the best electrical properties and is the insulation material with the best mechanical and electrical properties. This study provides an important reference for the comprehensive performance regulation of PP insulation materials for high-voltage cables.

Metallized film capacitors (MFCs) have been widely commercialized, and the insulation failure has become an important issue under high electric field. However, due to the self-healing characteristic, the MFCs offer a notable advantage in electrical insulation. This work aims to optimize the process of manufacturing in MFC in self-healing performance and explore the development process of self-healing in metallized film from experiment to theoretical model. In self-healing experience, the thickness of dielectric layer is negatively correlated with the performance of self-healing and winding tension in manufacturing is positive instead. The self-healing in metallized films have an incomplete but fierce combustion producing gas product like C $_{2}$ H $_{6}$ /CO and a large amount of H $_{2}$ . On the other hand, the self-healing process did not produce polar fragments/groups in dielectric layer, as indicated by Fourier transform infrared spectroscopy (FTIR). The SEM scan suggests a strong correlation between self-healing and carbonized area on the surface of dielectric layer. A small amount of metallic elements was found to remain in the self-healing area, with no metallic oxide particles. However, a large amount of graphite is composited in the clearing area along with a certain amount of element O. The simulation result of theoretical model built on plasma theory shows a depth of 0.28 $\mu$ m in carbon area, accounting for 5% of the 5.8- $\mu$ m thickness of dielectric layer. In addition, the radius of carbonized area in dielectric exceeds the evaporated area in metallized layer by 1.5 times.