Recently, scientists have been facing major obstacles in terms of improving the performances of dielectric materials for triboelectric nanogenerators. The triboelectric nanogenerator (TENG) is one of the first green energy technologies that can convert random mechanical kinetic energy into electricity. The surface charge density of TENGs is a critical factor speeding up their commercialization, so it is important to explore unique methods to increase the surface charge density. The key to obtaining a high-performance TENG is the preparation of dielectric materials with good mechanical properties, thermal stability and output performance. To solve the problem of the low output performance of PI-based triboelectric nanogenerators, we modified PI films by introducing nanomaterials and designed a new type of sandwich-shaped nanocomposite film. Herein, we used polyimide (PI) with ideal mechanical properties, excellent heat resistance and flexibility as the dielectric material, prepared an A-B-A sandwich structure with PI in the outer layer and modified a copper calcium titanate/polyimide (CCTO/PI) storage layer in the middle to improve the output of a TENG electrode. The doping amount of the CCTO was tailored. The results showed that at 8 wt% CCTO content, the electrical output performance was the highest, and the open-circuit voltage of CCTO/PI was 42 V. In the TENG, the open-circuit voltage, short-circuit current and transfer charge of the prepared sandwich-structured film were increased by 607%, 629% and 672% compared to the TENG with the PI thin film, respectively. This study presents a novel strategy of optimizing dielectric materials for triboelectric nano-generators and has great potential for the future development of high output-performance TENGs.

Owing to its lack of crosslinking, polypropylene (PP) is considered an environmentally friendly alternative to crosslinked polyethylene as high-voltage direct current (HVDC) cable insulation. However, pure PP can accumulate space charges under a HVDC, and thus must be modified for use as an insulating material for HVDC cables. In this study, 4-methylstyrene is grafted onto PP using an aqueous suspension grafting method to improve its properties. The effects of the swelling time, reaction time, and 4-methylphenylene concentration on the reaction were investigated. The optimum process conditions were determined, including an optimum grafting ratio of 0.97%. The volume resistivity, ability to suppress space-charge accumulation, and DC breakdown strength of modified PP were also studied. Modified PP with a grafting ratio of 0.88% showed optimal space-charge suppression and the highest volume resistivity and breakdown strength. The work will facilitate the design and development of more efficient insulation materials for HVDC cables.

Debye and its modified models are widely used to explain the relaxation polarization process in dielectrics at present. However, such models require prior knowledge of the number of polarization branches in them, so it is difficult to build an equivalent circuit model that matches the actual characteristics of relaxation polarization. Given this issue, a new method is proposed to analyze the relaxation polarization process in dielectrics by the distribution of relaxation time (DRT). In this paper, cross-linked polyethylene (XLPE) cables are selected as the research object and firstly aged at 160 °C for 24, 72, 144, 240, and 360 h, and the corresponding frequency domain spectrum (FDS) curves are obtained by experiment. Then, the DRT curves are solved by mathematical transformation, and the characteristic parameters are extracted. Finally, the quantitative relationship between aging duration and characteristic parameters was established by fitting a Logistic model. The results show that DRT can adequately reflect different types of polarization information, and its characteristic parameters can be applied to study the aging state of XLPE cables. The innovation of this paper is to propose a possible method based on DRT to study relaxation polarization.

Under the combined effect of the electric and temperature fields, bubbles will be formed on the transformer oil-paper insulation system, which will seriously endanger the insulation strength. However, the current study usually divided the formation and motion of bubbles into independent parts, lacking research on the whole process of bubble evolution. This paper experimentally investigated and theoretically analyzed the whole process of the formation, deformation, and motion of bubbles in the oil-paper insulation system, and explored the influence of the extremely inhomogeneous electric field (EIEF) on bubbles evolution. The results show that with the enhancement of the electric field, the bubble inception temperature (BIT) and bubble size increase significantly, while the number of bubbles decreases. Under the stretching and pushing effect of the electric field force, the bubbles will deform and move toward the low electric field region. In addition, this paper studies the influence of bubbles on the transformer electric field distribution through multi-physical field simulation.

The frequency domain spectrum (FDS) method can effectively evaluate the aging state of cross-linked poly-ethylene (XLPE) cable insulation. To more accurately achieve a quantitative analysis of information in the FDS, this paper combines the concepts of fractional element (Cap-Resistor) and fractional calculus, inspired by the Debye model and the multi-branch improvement method of the Cole-Cole (C-C) model, and establishes an improved fractional Zener model based on the existing Cap-Resistor model, and combines the improved fractional Zener model with the Elite Genetic Algo-rithm (EGA) to propose a new method for analyzing the re-laxation polarization information of XLPE dielectric. Six sets of XLPE cable samples with different aging times (including a validation set) are prepared at an aging temperature of 150 °C, and the FDS of the thermal aging samples are meas-ured. Then the FDS are fitted using a modified fractional Ze-ner model: firstly, the least squares method is used to estab-lish the optimized objective function, and the characteristic parameters are extracted and preferentially selected using EGA for analysis, and the fitting equations between aging time and characteristic parameters are established, and the validation results manifest that the error of the characteristic parameters on the aging time characterization of XLPE cable insulation samples is 0.18465 %, which is excellent. The eval-uation method’s accuracy has been validated. This paper in-troduces a feasible approach for analyzing relaxation polari-zation information in XLPE dielectric using an improved frac-tional Zener model and EGA.