Control optimization of the offshore HVDC grid based on modular multilevel converter for improving DC voltage stability

The aim of this paper is to reduce the voltage oscillation of the HVDC cable in an offshore point-to-point configuration based on the modular multilevel converter. An optimization methodology based on semidefinite programming is proposed, which improves the DC voltage stability under the worst-case initial perturbation scenario. This goal is achieved by implementing a centralized optimal linear feedback controller, which guarantees Lyapunov stability margins while considering constraints on control inputs and state variables. The scope of this work is twofold. First, to analyze small-signal eigenvalue stability of the modular multilevel converter-based HVDC grid to identify the source of the oscillations. Second, to use this analysis as a basis to design a centralized optimal linear feedback controller to improve stability margins. The linear feedback controller is first applied as an additional control layer to the standard control of the offshore point-to-point HVDC grid, and then it is implemented to substitute the droop control function of the grid. Finally, the applicability and validity of the results are verified by time-domain simulations.