Improved sliding-mode control for MMC in DC power system

The modular multi-level converter based direct current (MMC-DC) power system is a dynamic equilibrium system, and the control of MMC is quite important to realise instantaneous balance of active and reactive power. Until now, the control architecture with outer-loop, inner-loop and circulating current controllers is widely used for MMCs in the DC power system, and these controllers are required to have excellent dynamic response and robustness. However, controllers based on existing proportional–integral (PI) control or conventional sliding-mode control (SMC) have unsatisfactory performances. Therefore, an improved SMC method is proposed in this study for inner-loop and circulating current controllers. Compared with PI and conventional SMC, the improved SMC has better dynamic response and robustness performance and can suppress chattering phenomenon substantially. The simulation model of a four-terminal MMC-DC system is established in power systems computer aided design/electromagnetic transients including DC (PSCAD/EMTDC), based on which, comparison between PI control, conventional SMC and the improved SMC under typical working conditions, e.g. steady state, large disturbances of power step, is conducted, and the feasibility and superiorities of the improved SMC are verified.