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Gas insulated switchgear may be exposed to AC-DC mixed voltages in bipolar high voltage direct current (HVDC) systems. To improve the insulation performance of the epoxy insulator, this paper develops the multi-dimensional functionally graded materials (MFGM), which combines the bulk permittivity graded materials (ϵ -FGM) and the surface nonlinear conductivity materials (SNCM). The ϵ -FGM bulk and the SNCM layer of the MFGM insulator were prepared by the layer-by-layer pouring method and the air spraying technology. Results show that the electric field strength of the MFGM insulator around the HV triple junction is reduced by over 37% and 39% under AC and DC voltages. Compared with the uniform insulator, the AC and the DC flashover voltages of the MFGM insulator are raised by 17.5% and 27.5%, respectively. The MFGM insulator can adapt to both stationary and transient conditions, and thus has a promising application in HVDC compact gas-insulated apparatuses. [ABSTRACT FROM AUTHOR]

This study discusses the multidimensional functionally graded materials (MFGMs) for spacers in high voltage direct current gas-insulated transmission lines and switchgears (HVDC GILs/GISs). The effects of surface conductance graded materials ($\sigma $ -SFGMs) and bulk permittivity graded materials ($\varepsilon $ -FGMs) on the electric field distributions along the basin-type spacer are investigated under dc stationary and transient conditions. Results show that the $\sigma $ -SFGM spacer can effectively regulate the steady-state electric field, but the regulation effect on the transient electric field is limited when its time constant is longer than the transient time of the applied voltage. The $\varepsilon / \sigma $ -MFGM spacer with both bulk permittivity and surface conductance gradients has the same relaxation effect on the stationary electric field as the $\sigma $ -SFGM spacer and the same effect on the transient electric field as the $\varepsilon $ -FGM spacer, which can coordinately regulate the electric field distribution under multiple operating conditions in HVDC GIL/GIS. Compared to the uniform spacer, the $\varepsilon / \sigma $ -MFGM spacer can reduce the maximum electric field strengths under steady state, switching-on, and dc-impulse conditions by 38.9%, 18.6%, and 28.2%, respectively. [ABSTRACT FROM AUTHOR]

In this article, the influence of polishing orientation on surface charge accumulation and flashover characteristics of the basin spacer in a downsized dc-gas insulated transmission line (GIL) are explored by experiments. Results show that the surface charge distribution of the polished spacers is different from the untreated one. Heteropolar charges can be observed in the polished area and outward expansion area under prestressing condition. The flashover voltages of all polished spacers are decreased. The radial polishing (RP) has the greatest impact on the flashover voltage, and the flashover voltage of the radially polished spacer is decreased by 11% under positive prestressing condition. The metal dust adsorptions of the spacers are also measured, which shows that the radially polished spacer is easier to absorb metal dust and the metal dust diffuses in the radial direction. SEM and energy spectrum scanning of different spacers show that there are more exposed alumina particles on the surface of the polished area. The microdischarge induced by alumina particles is the source of heteropolar charges. When the polished area of the spacers is repaired with epoxy paint, heteropolar charges disappear on the surface of the repaired spacer. It is confirmed that charge accumulation and flashover along the spacer surface are mainly affected by the change of surface morphology on the polished area. [ABSTRACT FROM AUTHOR]

This study discusses the application of the surface conductivity gradient materials ($\sigma $ -SFGMs) for relaxing the electric field concentration around the epoxy-based spacer in high-voltage direct current gas-insulated transmission lines (HVDC GIL). Based on the ±500-kV HVDC GIL model, the iterative method is applied to optimize the thickness gradients of the $\sigma $ -SFGM layers on convex and concave surfaces of the basin-type spacer. It is proven that the $\sigma $ -SFGM layers can significantly suppress the electric field distortion around the high voltage electrode by increasing the homopolar charges and reducing the heteropolar charges on both spacer surfaces. To fabricate the $\sigma $ -SFGM spacer, a thickness-graded layer of high conductivity is formed on a truncated cone-type spacer by electrospinning. Experimental test results show that the application of $\sigma $ -SFGM exhibits a promoting effect of about 11.5% on the dc flashover voltage of the cone-type spacer. The $\sigma $ -SFGM spacer can withstand over ten thermal-shock cycles, indicating that the $\sigma $ -SFGM layer fabricated by electrospinning is more reliable than that fabricated by dip coating. [ABSTRACT FROM AUTHOR]

• A new general strategy to prepare MBGFs and MMBGFs. • The compositions and functions of the MMBGFs can be controlled. • MMBGFs have good biocompatibility. Bioactive glass fibers (BGFs) are potential materials for preparing orthopedic implants due to their special fibrous structure. Electrospinning is the main method to prepare BGFs with a small diameter. Mixing surfactants and metal salts with a BGF precursor is a popular method to obtain mesoporous BGFs and improve their properties. However, this kind of precursor cannot be used for electrospinning because charged surfactants and metal ions would disturb the electric field. Thus, synthesizing a metal-doped mesoporous BGF (MMBGF) is still a big challenge. In this work, a general strategy for preparing MMBGFs is designed. Four kinds of BGFs including mesoporous bioactive glass fibers (MBGFs), Mg-doped mesoporous BGFs (Mg-MBGFs), Zn-doped mesoporous BGFs (Zn-MBGFs) and Fe-doped mesoporous BGFs (Fe-MBGFs) are prepared by replicating an electrospinning porous polystyrene fiber template. The mesoporous structure of the PBGFs is confirmed suitable for drug loading and releasing; the Mg-MBGFs show the best bioactivity among four samples; the Zn-MBGFs show excellent bactericidal performance; and the Fe-MBGFs show a magnetic property. These results confirm that varieties of MMBGFs can be prepared by precipitating metal salts and BG precursors of different concentrations and compositions into porous PS fibers and calcinations, which enriches materials for constructing fibrous orthopedic implants. [ABSTRACT FROM AUTHOR]