Stability improvement of MMC-based hybrid AC/DC grids through the application of a decentralized optimal controller

Interconnection and expansion of AC networks through high-voltage direct current grids based on modular multilevel converters to form a multiterminal hybrid AC/DC grid can pose stability issues. These challenges can arise from dynamic interactions between/within AC and DC subgrids due to poorly damped modes that are potential sources of persistent and disruptive oscillations. This paper aims to ensure the stability of multiterminal hybrid AC/DC grids via a decentralized optimal controller. The proposed methodology analytically and simultaneously identifies both the decentralized optimal controller and the worst-case perturbation scenario under the grid control inputs' and state variables' constraints, without the need for detailed and time-consuming dynamic simulations of all possible scenarios. Eigenvalue stability analysis and time-domain simulations show that the proposed controller can efficiently enhance the test grid stability margins and reduce the oscillations, not only under the worst-case perturbation scenario (increasing damping ratios of the two pairs of least damped modes by 2.34 and 4.05 times) but also under other critical fault conditions. Furthermore, the controller's superior performance is validated through comparison with the power system stabilizer and modular multilevel converter droop controller under small and large disturbances, and its robustness is assessed against parametric uncertainties.