Pre-charging Control Strategies of Modular Multilevel Converter for VSC-HVDC

Abstract

Based on a novel multilevel voltage source converter(VSC) for HVDC,the pre-charging dynamic process of the converter is analyzed at full length,so as to search a suitable pre-charging control strategy.For a single station,the process is divided into two stages,the mechanism and the concerning factors of the transient over current during,in particular,the unlock charging course are analyzed.To ensure the effective charging current control for modular multilevel converter(MMC),an optimized multilevel modulation algorithm is presented,making it suitable for both the pre-charging process and the normal operation.When the MMC-HVDC supplying for a passive network,or used as a black start power source,the AC network of the active side will be the only energy source for MMCs.Based on the pre-charging control approach for a single MMC,an effective pre-charging coordinated control strategy for both ends of MMC-HVDC is introduced,solving the problem of the capacitors undercharged of the inverter side.Finally,a digital and a physical MMC-HVDC simulation systems supplying for passive network are established,and the simulation results verify the feasibility and validity of the control strategies mentioned.