System Design for the World's First ±800 kV 8 GW VSC-UHVDC Transmission Project

Zhejiang VSC-UHVDC project faces challenges, including high sub‑module capacitive energy storage, multiple transient overvoltage, and AC short‑circuit current injection. To address these issues, this paper proposes a main circuit topology with series‑connected valve groups and a hybrid full‑bridge/half‑bridge MMC. For the first time in UHVDC, the negative‑voltage output capability of full‑bridge sub‑modules eliminates the need for transformer tap changers, while maintaining a reactive power output >35% at full active power. Arrester configurations suppress continuous overvoltage caused by valve‑side faults, reducing sub‑module overvoltage after valve blocking by 11%-32% and neutral‑bus switching overvoltage by 23%. A coordinated sending‑receiving end control strategy integrates dynamic voltage regulation with current limiting, achieving AC fault ride‑through without energy dissipation devices and lowering sub‑module overvoltage by 20%. Through these optimizations, transient stresses and equipment manufacturing difficulty are reduced, while reliability and economy are enhanced.