Nonlinear Decoupling Control of Two-Terminal MMC-HVDC Based on Feedback Linearization

The traditional proportional-integral control for a modular multilevel converter (MMC) design based on a two-level voltage source converter does not consider the dynamics of arm circulating currents and sub-module capacitor voltages, which will lead to the incomplete decoupling between powers and circulating currents for an MMC. In view of this, a nonlinear decoupling control strategy, which applies for two scenarios, e.g., active/reactive power control for a power controlled station (PCS) and dc voltage/reactive power control for a dc-voltage controlled station (DVCS), is proposed for an MMC station based on feedback linearization. First, the nonlinear feedback laws for PCS and DVCS are derived based on the nonlinear equations of an MMC. Next, the complete nonlinear decoupling control strategies for PCS and DVCS are developed. Following that, the zero dynamics stabilities of the two control schemes for PCS and DVCS are investigated. In addition, the method to design parameters of the outer linear controller is also proposed to satisfy the requirement of dynamic performance. Taking a two-terminal MMC-HVDC for example, the simulation in Simulink validates the good transient performance of the proposed nonlinear control, and its robustness is also validated against the changing of system parameters.