Charge accumulation has always been a problem for the safe operation of gas insulated switchgear (GIS)/gas insulated transmission line (GIL) both under AC and DC. It is of great significance to investigate the behaviour of surface charge under high voltage. In this study, the charge distribution characteristics and accumulation mechanism on insulator surface in air and heptafluorobutyronitrile/carbon dioxide (C 4 F 7 N/CO 2) mixtures under AC voltage are studied via the means of surface potential measurement and inversion algorithm, combined with the improved method of controlling the truncated phase of AC voltage. The results show that under the needle-plate electrode structure, charge distribution on the insulator surface presents a three-tier concentric circle structure both in air and C 4 F 7 N/CO 2 gas mixtures, and the charge composition of the innermost circle is closely related to the truncated phase of AC voltage. Under the same amplitude of voltage, the range of charge distribution on insulator surface in C 4 F 7 N/CO 2 mixtures is smaller than that in air, with the negative charges domination. It is suggested that the characteristic of charge distribution in C 4 F 7 N/CO 2 mixtures is related to the larger electron attachment crosssection of C 4 F 7 N gas in a wide range of electron energy distribution.

In this work, we apply a molecular dynamics simulation of Ag-nanoparticle-doped epoxy resin to analyze in depth the micro-mechanisms in this nanodielectric. The simulation results show that when Ag nanoparticles with a radius of 10 Å are used as dopants, the periodically arranged atoms in a ∼4.5-Å-thick surface layer of the nanoparticles have become amorphous. This modification of the interface depends on temperature and nanoparticle size and leads to an interface polarization layer that changes the relative permittivity of the epoxy matrix. Moreover, a simulation indicates that doping with Ag nanoparticles can improve certain thermal and mechanical properties. However, the interface properties have little effect on the thermal and mechanical properties of nanodielectrics, which may depend only on the thermal and mechanical properties of the doped material itself, the doping concentration, or the microstructure of the nanodielectrics. The innovation of this article lies in the study of the microstructure characteristics of the nanodielectric and the changes of some key physical parameters at the nanoscale by means of molecular simulation. It provides a more efficient research idea for the traditional, experimental-based nanodielectric field. Our results may help in the analysis of nanodielectrics and insulating materials, and they suggest that doping with Ag nanoparticles may improve the thermal and mechanical performance of dielectrics.INDEX TERMS Interface properties, metal nanoparticles, molecular dynamics simulation, nanodielectric, key physical properties.