Asymmetric Mixed Modular Multilevel Converter Topology in Hybrid Bipolar HVDC Transmission Systems

In this paper, a hybrid bipolar HVDC system consisted of Line Commutated Converter (LCC) and asymmetric mixed MMC is introduced. Among the various MMC topologies for hybrid bipolar HVDC, the asymmetric mixed MMC has advantages in terms of reduced number of switching devices and lower losses. Also, the asymmetric mixed MMC can regulate its DC voltage flexibly from null to the rated value so that it can provide DC fault ride-through capability. Especially, at the post-fault, DC bus voltage of the asymmetric MMC can increases in slow ramp-up manner, which is important to prevent over-voltage and travelling wave generation along the transmission line. Finally, the full-scale simulation result and the down-scaled experimental results demonstrate the validity of the asymmetric mixed MMC based hybrid bipolar HVDC system.