Research Progress on Functionally Graded Materials for Epoxy Insulators in HV GIL/GIS

Gas-insulated transmission lines and switchgears (GILs/GISs) are essential components that constitute ultra-high-voltage power transmission and transformation systems. Epoxy-based insulators, as core components, experience significant electric field distortions and consequent flashover faults. Since the 1980s, researchers have focused on utilizing dielectric functionally graded materials (FGMs) to improve the electric field distribution of insulators. The key research on FGMs for GIL/GIS insulators over the past half-century is reviewed. The development from bulk-FGMs to surface-FGMs, and eventually to multi-FGMs are outlined. Bulk-FGMs are typically used in AC systems. These materials provide a more uniform electric field distribution by creating a gradient in the relative permittivity within the insulator's bulk. Surface-FGMs are commonly employed in DC systems, and they regulate the electric field by designing a surface conductivity gradient, thus preventing internal breakdowns that result from bulk conductivity gradients. In practice, GIL/GIS insulators are exposed to complex operating conditions, including AC, DC, and transient voltages, resulting in the development of multi-FGMs. These combine both conductivity and permittivity gradients, thus providing a comprehensive solution for suppressing both dynamic and static electric field distortions. An effective reference for researchers and industry professionals to support the transition of the research on FGM insulators from laboratory studies to practical engineering applications is provided.