Improved Dual Closed-Loop Control for Modular Multilevel Converters Based on Voltage Drop on Grid Impedance

[Objective] As renewable power bases gradually expand from load centers to remote areas such as deep offshore， deserts， and barren lands， using grid-forming voltage source converter-based high voltage direct current （VSC-HVDC） transmission technology is a promising solution for integrating renewable power to the grid. Considering power quality and fault current limitations， current grid-forming control usually adopts a dual closed-loop structure of AC voltage and AC current. This control structure relies on the voltage-current integral relationship of the AC-side filter capacitor and grid current feedforward to achieve stable control of the AC voltage. However， the AC side of the modular multilevel converter （MMC） usually lacks a centralized filter capacitor， and obtaining AC-side current feedforward to decouple from the grid is difficult， leading to degraded AC voltage control performance. [Methods] To solve this problem， this study proposes a novel dual-closed-loop AC voltage-control strategy for grid-forming VSC-HVDC converters. The strategy does not rely on the AC capacitance but adjusts the AC voltage of the PCC by controlling the voltage drop on the grid impedance. [Results] Simulation verification based on PSCAD/EMTDC showed that， under the proposed control mode， the grid-forming MMC exhibited good operational stability and active/reactive power dynamic control performance under different grid conditions. [Conclusions] The proposed control strategy can effectively improve the AC voltage-AC current dual-closed-loop control performance， operational stability， and grid adaptability of grid-forming MMC.