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Charging of insulators modifies local electric field distribution and increases potential threat to the safety of the gas insulated equipment. In this paper, surface charge tailoring techniques are classified and reviewed by introducing a Dam-flood model. Technical solutions of different charge tailoring methods are compared and discussed. The outlook of potential solutions to suppress charge accumulation is recommended and discussed based on industrial consideration. This paper serves as a guide handbook for engineers and researchers into the study of charge tailoring methods. Meanwhile, we hope that the content of this paper could shed some lights upon charge-free insulators to promote the industrial application of HVDC GIL/GIS. [ABSTRACT FROM AUTHOR]

Surface charging of spacers is one of the main factors restricting the industrialization of DC gas insulated lines (GILs). The effects of ion mobility, gas pressure and temperature on surface charge behavior are important issues to be clarified. In this paper, a selection method of ion mobility is reviewed and reconsidered. A multiphysics-coupled model considering ion mobility, species of ions, temperature, and gas pressure is investigated. A mathematical model of ion mobility under low and high electric fields is presented and the critical electric field strengths are calculated, based on the current density vs electric field strength. The results show that the two critical electric field strength have different characteristics with gas pressure and temperature. The ion mobility in SF 6 gas has a completely different calculation model in high electric field and low electric field, and these two ion mobilities are equal only at a specific temperature and gas pressure. This paper can provide a reference for the selection of ion mobility in DC GILs under conditions of temperature and pressure. [ABSTRACT FROM AUTHOR]

Examines the synthesis of fractal signals with wavelet bases. Formula for time-average spectrum; Analysis of wavelets; Dependencies among wavelet coefficients.

The electric field distortion along the gas-solid interface on insulators in GIS or GIL greatly threatens their long-term insulation performance. In recent years, with the developments of field grading materials, their application on GIS/GIL insulators has been considered as an effective approach to suppressing the field distortion and enhancing the insulation property. Many reviews about field grading materials had been published. However, these articles rarely focus on the nonlinear materials and their unique characteristics when used in gas insulated equipment. Compared with traditional applications, such as cable accessories, generator coils, and motor stator bars, nonlinear materials in GIS/GILs are mainly used for grading the surface electric field and avoiding unwanted charge accumulations. Hence, there are two characteristics of the material that need to be paid special attention to-the surface insulation performances and the charge release functions-which are not so critical in previous applications. In this paper, first, types and principles of several commonly used nonlinear grading materials are summarized briefly. Then, the existing application cases of nonlinear materials in gas insulation equipment are sequentially introduced from the optimization ideas, material selection, manufacturing process, and using effects. Finally, the problems encountered in the application of field grading materials in practice are discussed. Suggestions for the follow-up studies are given in order to guide the application of nonlinear materials in GIS/GIL insulators. [ABSTRACT FROM AUTHOR]

Charges due to ionization at conductors at low electric field serves as the main source of charge carriers that is responsible for surface charge enhancement, especially for hetero-polar charges on the convex surface side of spacers inside DC gas-insulated transmission lines (GILs). In this paper, simulation results of charge generation processes from gas side, including natural ionization, micro-discharge and field emission, are presented. The results show that for the case where the electric field strength is -6.548 kV/mm and the gas pressure is 0.4 Mpa, when conductor roughness Ra is lower than 24.16 µm, natural radiation ionization is the main cause of the dark current. When Ra is between 24.16 µm and 188.59 µm, the dark current is dominated by micro-discharge. The Fowler-Nordheim (F-N) field emission occurs when Ra exceeds 188.59 µm. Considering the roughness of the conductor in the actual GIL, the increase in the dark current due to local roughness is mainly due to micro-discharge rather than F-N field emission. The results in this paper provide an important part for further determining of conductor surface conditions in high voltage direct current (HVDC) GILs and serve as an important reference for completion of the surface charging model of spacers under DC. [ABSTRACT FROM AUTHOR]

This work investigates the effect of graphene filler in epoxy resin with the purpose of developing an improved thermally conductive epoxy encapsulation material for electronic devices. The findings in this work show that the thermal conductivity increases by a factor of about 1.6 times with 0.1 wt% loading of graphene in epoxy resin. The increase in the rate of surface charge verifies an improved anti-static property. On examining the phase resolved partial discharge patterns, it is observed that the inception voltage and the number of partial discharge pulses is lower for the filled epoxy compared to that of the neat epoxy. The increase in the rate of surface charge decay indicates an improved anti-static property. The findings in this paper provide a reference in the development of materials with improved thermal conductivity and anti-static properties for graphene/epoxy materials for use in the packaging of electronic devices. [ABSTRACT FROM AUTHOR]

Surface charge pattern analysis on HVDC insulators is a difficult task, which requires a comprehensive understanding of the physical mechanisms of surface charging. This paper provides a review on surface charge patterns and related models published within the past 30 years. In addition, based on the field-dependent charging theory, the research differences of various surface charge patterns obtained by previous researchers are explained. Difficulties and future prospects in modeling of surface charging phenomenon are also discussed. This paper may be used as a reference for the analysis of charge accumulation patterns and it would be helpful to researchers to compare and understand charge accumulation results published by earlier researchers. [ABSTRACT FROM AUTHOR]

A theoretical model considering the dynamic change of surface charge accumulation in transient and steady states is proposed. The surface charge accumulation under transient and steady states are simulated using three model spacers with different volume conductivities. The results show that on the convex surface of a spacer, when the volume conductivity is lower than 2 × 10−18 S/m, the hetero-polarity charge accumulated on the spacer surface is dominant, while the homo-polarity charge accumulated on the surface is dominant when the conductivity is higher than 6 × 10−17 S/m. The surface charge density is non-uniformly distributed, and the time duration of the transient process of the surface charge density at different positions on the spacer surface is different, due to the distribution of electric field lines. It is hopefully that the content of this paper can provide reference for the mechanism study of gas-volume interface charge transport and the preparation of HVDC spacers. [ABSTRACT FROM AUTHOR]

The dynamic behavior of surface charging on spacers in DC-GILs can be influenced by multi-factors including the non-uniform distributed electric field as well as the time-varying temperature gradient. In this paper, the time-varying effect of surface charging phenomenon on spacers is studied and a time-varying mathematical model is established, based on the influence of temperature and electric field on the ion mobility at the gas phase and the bulk conductivity in the solid phase. The results verify that the bulk conductivity can be greatly influenced by temperature, which leads to an increase in the surface charge density on the spacer. This allows the surface charge accumulation to stabilize more quickly. However, the ion mobility from the gas phase is less affected by temperature. When the non-uniform distributed electric field changes from 1.3 to 6.4 kV/mm, ion mobility is less influenced and the surface charge density on the spacer varies slightly. In this case, the effects of the non-uniformly distributed electric field in surface charge density variation is much smaller and can be ignored. [ABSTRACT FROM AUTHOR]