Zero DC voltage ride through of a hybrid modular multilevel converter in HVDC systems

This study presents a comprehensive analysis and a suppression method of sub-modules' (SMs') voltage stress in the hybrid modular multilevel converters when riding through zero DC voltage faults in high-voltage direct-current (HVDC) systems. First, the general DC fault ride through (FRT) strategy considering the redundancy of the arm voltage generation scheme under a reduced dc-link voltage is derived. Then full-bridge sub-modules (FBSMs') voltage stress for the conventional DC-FRT schemes with and without common-mode voltage injection are analysed. Finally, on the basis of the available full-bridge SM capacitor energy control strategy, an improved method implemented by energy interaction between half-bridge submodules and FBSMs is presented. The proposed DC-FRT scheme can make all SM capacitor voltages balanced at their rated values during the zero DC voltage conditions. Simulated results are provided to demonstrate the validity of the analytical results and the feasibility of the proposed DC-FRT scheme.