Contributions to the control of modular multilevel converters for medium-voltage applications

Resumen

Power quality is a key problem in the increasingly complex power grids. Grid interconnections as well as distributed generation are major challenges for governments and industry. In the last years, huge efforts are being made to improve the stability and reduce the disturbances in the grid. To that end, Flexible AC Transmission System (FACTS) devices are becoming a key part of the systems dedicated to control the grid quality. This Thesis is focused on optimizing the performance of the Modular Multilevel Converter to FACTS applications to improve the power quality. This Thesis covers several studies in areas related to the design and control of MMCs i.e. modulation techniques, control algorithms, power converter design and processing platforms. Moreover, this Thesis addresses the two main drawbacks of the MMC, the circulating current and the capacitor voltage unbalance. The circulating current is studied in depth and three ways to control this current are proposed. Regarding the capacitor voltage unbalance, this Thesis proposes a capacitor voltage balancing algorithm which can work even if very low switching frequencies are used. Along with the theoretical studies carried out, a `quasi-industrial¿ MMC prototype has been designed and built. The design and start-up of a prototype are challenging tasks which are full of technical complications. Furthermore, an industrial high-processing platform to control multilevel converters has been developed. The processing platform design follows a centralized controller approach. In addition, the processing platform can be used as central unit with the view to emulate decentralized control. With the objective to use the MMC in medium voltage FACTS applications, a STATCOM, an APF and a PCC voltage controller have been implemented in the designed prototype. These operation modes allow to control the reactive power, the voltage and the harmonics in the PCC improving the power quality and proving that this topology is suitable for FACTS applications.