Nonlinear Materials Applied in HVDC Gas Insulated Equipment: From Fundamentals to Applications

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.