A comprehensive review of HVDC transmission systems: From fundamental concepts to emerging technologies

High Voltage Direct Current (HVDC) transmission systems have emerged as a transformative solution for modern power grids, offering efficient long-distance power transfer with reduced transmission losses. This review presents a comprehensive overview of HVDC technologies, beginning with their historical development and underlying principles. It examines various system configurations and highlights key applications, including offshore wind integration, multi-terminal networks, and grid stability enhancement. Converter technologies are extensively discussed, with a focus on Line-Commutated Converters (LCC), Voltage Source Converters (VSC), and Modular Multilevel Converters (MMC). Recent innovations such as hybrid HVDC systems and ultra-high voltage transmission advancements are also explored, reflecting the rapid technological evolution in this field. Furthermore, the review addresses the technical, economic, and regulatory challenges of HVDC deployment, and outlines future directions involving advanced control strategies, improved insulation materials, and harmonic mitigation techniques.