This paper presents a comprehensive analysis of the insulation breakdown phenomenon in epoxy resin insulation induced by the degradation of electrical trees, with reference to both domestic and international studies. It elucidates the mechanisms of electrical tree initiation and degradation in epoxy resins by drawing on the theories of space charge accumulation and migration, local electric field formation, ultraviolet radiation, and mechanical stress chain breakage. The paper also discusses the operational conditions of the electrical equipment, detailing the effects of superimposed electric fields, temperature gradients, mechanical stresses, magnetic gradients, and humid environments on the insulation degradation of the epoxy resin. Furthermore, it summarizes the correlation between tree growth characteristics and charge transport behavior under the influence of multiple physical fields. To enhance the insulation performance of epoxy resins and ensure the safe and reliable operation of electrical equipment, methods to inhibit electrical tree formation are discussed. These methods include the regulation of the manufacturing process, inorganic doping, and the use of self-repairing materials. The inhibition mechanisms are analyzed at the molecular configuration level and microstructural scale, and from a macroscopic standpoint. This study aimed to provide recommendations for future research on the growth and suppression of electrical trees in epoxy resin based on the current state of knowledge.

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.