Modular multi-level converter based multi-terminal HVDC system for offshore wind power transmission

Driven by the rise of renewable power technology, especially long distance wind power and solar power, high-voltage DC (HVDC) transmission has become increasingly popular due to the relative lower cost than the AC system, when applied to long distance transmission. Point-to-point HVDC technology is well developed and has been implemented widely for long-distance power transmission applications. However, the limitation of existing point to point high-voltage DC connections for offshore wind farm transmission is that it does not support power exchange or trading between two or multiple DC systems. However, interconnected voltage-source converter based multi-terminal high-voltage DC (MTDC) systems provide better system redundancy, higher flexibility and capability of exchanging power between multiple areas. Recent developments in modular multi-level converter (MMC) technology makes multi-terminal HVDC transmission more promising than before. Its robustness, low harmonics distortion, flexibility and scalability makes it a perfect fit for MTDC transmission systems. However, higher power losses and more complex control systems bring new challenges to the MTDC systems at the same time. Therefore, it is meaningful to discover new opportunities and address those challenges that comes with MMC-based MTDC transmission technology.This thesis presents an in-depth study of several key technical and operational aspects of multi-terminal high-voltage DC systems for large scale offshore wind farm transmission. It addresses contemporary challenges for multi-terminal high-voltage DC systems by doing studies of multi-terminal HVDC system typologies, modelling of MMC based multi-terminal HVDC systems, transient stability analysis during AC side fault, MTDC power flow and transmission loss analysis, and how MTDC system would provide inertia support for weak AC grids. Contributions of this thesis include improving system transient stability under unbalanced grid condition by using symmetrical components control methods, and introducing a novel way of determining and calculating the transmission line losses for different MTDC network topology configurations. Different MTDC system topologies are investigated. Simulated case studies are used to observe the system power flow and transmission losses for different DC network topologies. Finally, a novel control strategy is proposed to provide frequency support to low-inertia grid from the MTDC network, which improves the frequency response and the stability of the grid