Novel Control Approach for Modular Multilevel Converter Based on $\alpha \beta 0$ Reference Frame Without PLL

Modular multilevel converter (MMC) has become the basic building block for multi-terminal direct current (MT-DC) system and dc grids. To effectively control the power flow of the ac-dc MMC systems, the dual-loop power decoupling control based on dq reference frame is employed, which needs accurate phase information of ac grid through phase-locked loop (PLL). However, the control system may fail due to unstable PLL. In this paper, a novel power flow control approach based on αβ0 reference frame without PLL is proposed, which consists of an inner loop for power decoupling and an outer loop for tracking power reference. Nonlinear state feedback is employed to eliminate time-varying and nonlinear terms through full-state linearization for constructing the inner loop. PI controller and feedforward compensation are used to build classical second-order damping elements for keeping fast dynamic response and tracking power reference in the outer loop. Performance of the proposed grid-connected control strategy for an high voltage MMC station is evaluated based on simulation studies in the PSCAD/EMTDC software environment. The study results illustrate the effectiveness of the proposed strategy for controlling the grid-connected MMC.