Potential of Nanocomposite Insulation for High Voltage Direct Current Cables: Advances, Perspectives and Future Outlooks

High-voltage direct-current (HVDC) transmission has become a key technology for long-distance, large-capacity power delivery, creating an urgent demand for insulation materials with superior dielectric performance, long-term reliability, and environmental sustainability. Conventional crosslinked polyethylene (XLPE) insulation exhibits excellent electrical properties but suffers from space charge accumulation, increased DC conductivity, electrical aging, and limited recyclability. Nanocomposite insulation has emerged as a promising strategy to overcome these limitations through the incorporation of functional nanofillers that regulate charge transport and enhance dielectric performance. This review presents a comprehensive and mechanism-oriented assessment of nanocomposite insulation for HVDC cable applications developed over the past decade. Inorganic, organic, hybrid, and grafted nanocomposite systems are systematically compared with respect to their effectiveness in suppressing space charge, reducing DC conductivity, and enhancing DC breakdown strength. Emphasis is placed on the underlying mechanisms, including charge trapping, interfacial polarization, crystallization behavior, and polymer–nanofiller interactions that govern dielectric performance. Furthermore, the most promising nanocomposite systems are critically evaluated through comparative analyses, summary tables, and trend figures, highlighting their relative advantages, limitations, and application potential. The review also discusses the major challenges hindering industrial implementation, including filler dispersion, large-scale manufacturing, long-term reliability, cost-effectiveness, compatibility with existing cable production technologies, and environmental sustainability. Finally, emerging research directions, including smart insulation materials, recyclable nanocomposites, and artificial intelligence-assisted material design, are discussed to provide practical guidance for the development and commercialization of next-generation HVDC cable insulation materials.