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Advanced Grid-Forming Control of HVDC Systems for Reliable Grid Restoration

Grid restoration strategies as emergency response gain increased attendance that arise from fundamental changes inside electrical grids. Due to the integration of fluctuating energy sources, the reduction of installed inertia as well as natural disasters a blackout might occur. High-voltage direct current (HVDC) systems open new possibilities to use electrical energy from a far distance and break new ground during grid restoration process through reliable power energy sources. In grid-forming control mode special attention has to be paid on converter current limitation and handle unbalanced grid conditions.In this paper a dynamic protection integrated model predictive control (DPI-MPC) concept using a generalized state space representation of a grid-connected modular multilevel converter (MMC) is derived. It enables to reconstruct an AC-grid after a blackout utilizing a VSC-HVDC system. With this approach, it is possible to restore a balanced voltage system even under unbalanced grid conditions and ensure functional protection of the grid-forming unit. This concept guarantees a more safe and reliable restoration process, which is validated in EMT simulations using MATLAB/Simulink. The results demonstrate that DPI-MPC and HVDC systems go together in a natural way offering novel possibilities during grid restoration process.