Insulation Design of Gas–Solid Interface at HVDC Condition—Part II: Impacts of Metallic Charged Particles and New Challenges from Eco-Friendly Insulating Gases

High-voltage direct current gas-insulated equipment (HVDC GIE) is becoming a key component in DC transmission systems due to its compactness and reliability, particularly for cost-effective offshore wind power integration. However, its reliability is compromised by surface charge accumulation at the gas–solid interface—a problem intensified by metallic contaminants and the adoption of eco-friendly gas mixtures. Addressing this challenge requires not only material and design innovations but also the development of standardized testing protocols to validate insulation performance. This review systematically examines the critical, interconnected areas essential for advancing HVDC GIE, focusing on the dynamics of charged metallic particles and their role in charge accumulation, interface charge behavior under alternative gas environments, and experimental methodologies with evolving test standards. By synthesizing insights across these domains, this work aims to provide both theoretical insights and practical guidance for optimizing HVDC GIE insulation design formulating scientifically grounded operation and maintenance strategies.