Voltage-Source Converter High-Voltage Direct Current (VSC-HVDC) technologies have seen rapid development with the emergence of multilevel VSCs, especially with the invention of modular multilevel converter by Professor R. Marquardt. MMCs have been widely adopted in many current generation VSC-HVDC projects all over the world due to their modular structure, easy scalability, low-expense redundancy design and high-quality output voltage and current. However, the continuous expansion of large-scale renewable energy systems and power networks increases the capacity requirement of VSCs. The capacity of a single VSC for VSC-HVDC systems currently in planning exceeds 3 GW.

In such extra high-power applications, the drawbacks of MMC become apparent in terms of large submodule capacitances, high volume, high cost etc. Meanwhile, multi-terminal VSC systems and hybrid topologies combining both multilevel VSC and line-commutated converter (LCC) have also attracted interest in these scenarios. Circuit topologies, their characteristics, operating principles, and control strategies, protection schemes and converter coordination of such HVDC systems requires further investigation. This special session will focus on improved and emerging technologies in multilevel VSCs for HVDC systems and applications.