Modular converter structures for statcom application under unbalanced conditions

Abstract

The current society is facing several challenges in terms of energy, such as the high dependency on fossil fuels, the high rate of reliance on imported energy, the constant increment in the energy consumption, and the environmental problems caused by these factors. The integration of distributed generation systems, mainly based on renewable energy sources (RES), is the most interesting solution to address these issues. However, the penetration of RES into the power grid implies certain challenges due to their non-dispatchable nature compared to conventional generation systems. Such systems often use power electronic converters for grid connection instead of synchronous generators, making power systems become converter-dominated. As the power electronic converterbased grid makes it increasingly necessary to employ power equipment with higher voltage and power rating, since their scalable attributes, multilevel Voltage Source Converters (VSCs) based on modular structures are envisioned as the most promising alternatives for that purpose. In this framework, Flexible AC Transmission System (FACTS) devices, and in particular Static Synchronous Compensators (STATCOMs), play a key role in the evolution of the modern power grids to the future smart grids. The STATCOM, especially the one which employs multilevel VSCs based on modular structures, is applied in an increasingly wider variety of scenarios in which the operation under negative-sequence voltage and/or current conditions stands out. From the power converter manufacturer point of view, it is not obvious which VSC topology should be used. That is why the purpose of the thesis is to contribute to the body of knowledge in the field of the design of VSCs, with the aim of facilitating the manufacturer to compare and select the most appropriate topology, to design it properly, or to adapt it in order to face the demanding operating requirements that a converter-dominated power grid implies. Among these, this work is mainly focused on the STATCOM application of multilevel VSCs based on modular structures operating under unbalanced voltage and/or current conditions. To achieve this goal, the operation of the most commonly used high power – MV VSC topologies and their main modulation strategies are first reviewed. Secondly, a methodology which provides valuable information that allows the converter manufacturer to compare and select the most suitable VSC topology for any required application is proposed. Then, a comprehensive review about the operation of multilevel VSCs based on modular structures for different and equivalent unbalanced operating scenarios is presented, in which the STATCOM functionality is included. Having identified the research gap, the reactive power limits of different VSC topologies for STATCOM application operating under unbalanced voltage and/or current conditions are studied and compared. Finally, the control implementation of one of the most interesting topologies for the application under study is evaluated, comparing the performance of different regulators that could be employed. Experimental results obtained from a real-scale 100 kVA prototype in a MV laboratory validate the carried out studies.