Small-signal stability analysis of Modular Multilevel Converters and application to MMC-based Multi-Terminal DC grids

The attempt to massively integrate renewable energy sources in the electric power system has encouraged major interest in High Voltage DC systems (HVDC). Furthermore, Multi- Terminal DC grids (MTDC) appear as an attractive solution for the connection of offshore energy sources. Among different types of Voltage Source Converter (VSC), one of the most promising topologies for future MTDC grids is the Modular Multilevel Converter (MMC), which are proposed by the main European vendors. Up to now in Europe, a single vendor is in charge of the converter stations for each HVDC project, but future DC grids are likely to be multivendor schemes, where different control strategies may be applied to each converter station depending on its provider. However, due to intellectual property, manufacturers do not provide the complete control structure in open source but rather as a black-box models or control cubicles replicas. For this reason, it is impossible to use a formal mathematical approach to perform inter-operability analysis. Hence, a more formal approach is needed to cope with the different issues that may arise due to the impact of the MMC controllers on the MTDC grids. This thesis deals with the modeling and control of MMCs in the context of MTDC. The first objective is to obtain an MMC model in dq frame which can reproduce accurately the AC- and DC- interactions, while representing at the same time the internal dynamics which may interact with the rest of the system. This model is suitable for linearization and stability studies, among other linear techniques. Then, based on the developed dq model, different control strategies are developed based on state-of-the-art MMC controllers. Since the dimension of the system is a limiting factor for studying MTDC grids with many MMCs, different reduced-order models are presented and compared with the detailed dq model. Finally, the developed MMC models with different controllers are used for the MTDC studies. The impact of the selected controllers for each MMC is evaluated to highlight the potential issues that may occur in multivendor schemes. (author)