Power reversal strategies for hybrid LCC/MMC HVDC systems

Power reversal control strategies for different types of hybrid line-commutated-converter (LCC)/modular multi-level converter (MMC) based high-voltage direct-current (HVDC) systems have been proposed with the consideration of system configurations and MMC's topologies. The studies show that the full-bridge (FB) MMC gives better performance than halfbridge (HB) MMCs in terms of power reversal in hybrid LCC/MMC systems. The modulation method employed in this paper can achieve a smooth online polarity reversal for hybrid LCC/FB-MMC HVDC systems. Additional DC switches and/or discharging resistors may be needed to reverse the DC polarity of LCC/HB-MMC HVDC systems. Based on the proposed strategies, the power reversal processes of the studied systems can be accomplished within several seconds. The speed can be changed according to system operation requirements. The effectiveness of the proposed control strategies has been verified through simulations conducted in PSCAD/EMTDC.