Abstract:  ABSTRACT：  This paper proposes a series hybrid HVDC system, whose rectifier side adopts the line commutated converter （LCC）,and the inverter side is composed by LCC in series with the full bridge submodule based modular multilevel converter （FBMMC）. Firstly, we construct the mathematical model of the hybrid HVDC system, design the corresponding coordinated control strategy to ensure the safe and stable start-up of the system, and proposes a three step start-up procedure for the hybrid HVDC system with LCC in rectifier side and LCC in series with FBMMC in inverter side （LCC-LCC+FBMMC）. Phase 1, both the LCC and FBMMC are locked, and the DC side voltage charging process of FBMMC is uncontrolled with a current limit resistor in series. Phase 2, the current limit resistor is in bypass and FBMMC in inverter side is unlocked； the rated DC voltage of FBMMC is gradually built with the DC voltage controller. Phase 3, both the LCCs in the two sides are unlocked, and the rated DC current and DC voltage are gradually built by the DC current controller in rectifier side and DC voltage controller in inverter side. So far, the start-up process is completed. Finally we establish the simulation model for LCC-LCC+FBMMC based hybrid HVDC system in the PSCAD/EMTDC, and verify the effectiveness of the proposed start-up strategy for hybrid HVDC system.

Key words: hybrid HVDC system, line commutated converter （LCC）, full bridge submodule based modular multilevel converter （FBMMC）, full bridge sub module（FBSM）, start strategy