Functional Description and Control Design of Modular Multilevel Converters: Towards Energy Storage Applications for Traction Networks

The security of supply is becoming an important concern in the energy supply of the railway networks operated at 16.7 Hz. This situation calls for the improvement of the interconnection with the 50 Hz grid and creates a need for ancillary services, especially in relation to power quality. To these ends, Modular Multilevel Converters (MMC) are highly attractive as they are ideally suited for the corresponding voltage and power levels and are sufficiently versatile to adapt easily to the numerous applications affected by these needs. Besides, there attractiveness can be further increased if energy storage is embedded inside the converters, which are then able to provide a broad range of ancillary services. Practically, such a perspective is enabled by the fact that the submodules can directly integrate storage in a distributed manner, essentially limiting design concerns to control issues. By 2014, MMCs have been adopted by most major power electronics manufacturers (ABB, Siemens, Alstom, etc.) and constitute a rapidly expanding topic, drawing the attention of numerous academic research groups worldwide. This illustrates the breakthrough represented by this technology, which is notably due to the fact that their modular nature imposes operating principles that are fundamentally different from those of conventional structures, but what also allows unprecedented flexibility and scalability. Being given the numerous ongoing industrial projects in relation to railways, the relevance of MMCs in these applications is already largely proven. However, the integration of split storage in MMCs has still received only little attention. This thesis will even propose the use of hybrid split storage, which has apparently not been studied at all. In both cases, the development of energy management mechanisms has apparently not been addressed yet and the control design as a whole is also limited to few developments only. On the other hand, there are already numerous control solutions for the cases without storage, among which it is sometimes difficult to make wise choices. In this context, before adding energy storage (and its associated management mechanisms) to already complex control problems, it is important to rely on a sound basis, what is the main motivation for this thesis to develop a set of tools that can allow to take a step back on the control design in general. Firstly, this thesis proposes different representations of MMCs, providing a macroscopic view of their behavior and allowing a new interpretation of their principles of operation. As it will be seen, these results are useful to both control design and system engineering purposes. Secondly, using the principles of the Energetic Macroscopic Representation (EMR), this work presents a methodology for the systematic control design of MMCs, based on the functional inversion of a system model. Finally, the obtained results are validated on known structures before being extended to other converter systems including energy storage. In parallel to these developments, several digressions are also made to comment on the issues related to the control hardware and on the possible applications of energy storage in railways. In the end, these developments are expected to contribute to improve the modularization of the control in general, which is one of the possible ways to provide maximum flexibility, speed and effectiveness in the overall design of MMC systems for all types of applications.

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