Multilevel Converter Topologies for STATCOMs

Abstract

To implement the STATCOMs for transmission and distribution applications, high power converters are needed. A multilevel converter is a logical choice for such applications as it can meet the imposed line side current and voltage harmonic distortion standards without extending its device ratings. Several types of monolithic multilevel converters have been proposed based on different structures of a DC link voltage (or current) to generate staircase output voltage (or current) levels. The multilevel current source converters (CSC) are also a potential configuration for STATCOM. Multilevel CSCs using fully controllable switches have the advantages of low output harmonics and controllable active and reactive power. During the last decade, modular multilevel converters have shown a breakthrough and have made their way to commercial high power applications. Modularity, in general, refers to a technique to develop comparably large systems by combining smaller subsystems. For power converter topologies, this means, a cascaded connection of converter cells seems to be an interesting solution to reach high voltage and high quality waveforms with a minimum complexity. This chapter provides an overview on monolithic and modular multilevel converter topologies, targeting high-power applications and in particular STATCOM. Modular multilevel converters can be built either straightforward by applying the modular building block cells or by a combination of cells with monolithic multilevel topologies. The monolithic and modular CSCs can also be realized by employing the duality concept on each corresponding Voltage Source Converter (VSC). This chapter summarizes the most recent developments made for multilevel converter topologies, by covering new promising topologies and operational issues. In addition, emerging trends, challenges, and possible future directions of the multilevel converter technologies are outlined.