The objective of this dissertation is the development of a novel two-degree-of-freedom control strategy, consisting of a feedforward controller and a state feedback controller, for a medium-voltage direct current (MVDC) system. The considered system comprises a meshed DC network interconnected with multiple AC networks via Modular Multilevel Converters (MMC). Due to the meshed structure, Power Flow Controllers (PFC) must be integrated at appropriate locations to ensure full controllability of the DC network. The dynamics of the overall system result from the current and energy dynamics of all converters, as well as from their interactions with the connected electrical networks. The feedforward controller calculates trajectories and the required input signals to achieve fast and smooth transitions between different steady states without inducing undesired transients or oscillations, even under certain fault conditions. A particular challenge in designing fast trajectories to new steady states lies in the nonlinear terms of the energy components of the MMC and PFC. In this work, two feedforward control approaches are presented: (i) an approach developed for the case of a minimal number of PFC to control the current dynamics in the meshed DC grid; (ii) an improved approach developed for the case of a larger number of PFC in the meshed DC grid. Due to the switching operation of the MMC and PFC, the calculated input signals are idealized and inevitably subject to discretization errors. Combined with external disturbances, this leads to deviations of the actual state variables from the desired trajectories. The state feedback controller corrects these deviations and ensures that the predetermined trajectories are tightly tracked. The proposed control concept also makes a new contribution to fault handling and provides a solution for uninterrupted operation of the system, particularly in the event of pole-to-ground faults, thereby eliminating the need for costly DC circuit breakers.      «

The objective of this dissertation is the development of a novel two-degree-of-freedom control strategy, consisting of a feedforward controller and a state feedback controller, for a medium-voltage direct current (MVDC) system. The considered system comprises a meshed DC network interconnected with multiple AC networks via Modular Multilevel Converters (MMC). Due to the meshed structure, Power Flow Controllers (PFC) must be integrated at appropriate locations to ensure full controllability of th...     »