Control design for grid and energy/balancing controllers of modular multilevel converter based VSC HVDC systems

This paper provides a comprehensive control design approach for Modular Multilevel Converter (MMC) based Voltage Source Converter (VSC) High-Voltage Direct Current (HVDC) systems. Detailed control design strategies for power, voltage, energy and horizontal/vertical balancing controllers, as well as their subordinated current controllers, are carried out using the approaches of symmetrical and amplitude optimum respectively. Furthermore, the design of the filters for the balancing controllers is revealed. An energization and start-up process of the HVDC system is presented; subsequently, a power reversal process is performed and an AC voltage drop is applied in order to verify the accuracy and stability of the proposed control strategy and to illustrate the dynamic behavior of the investigated HVDC system. It is shown that the appropriate control design guarantees the stability of the HVDC system during the entire range of operation.