Control strategies for multilevel voltage source converters for medium-voltage DC transmission systems

In the past, the market of generation, transmission and distribution of electricity was dominated by large power utilities. At present, the market for electrical power is being liberalized in multiple regions of the world. To guarantee power quality, new power electronic systems are becoming more and more economic. These systems are usually based on voltage source inverter (VSI) topologies. Here, transformerless systems are considered to offer higher power density and lower cost. However, to provide high waveform quality without high switching frequencies, multilevel converter topologies are essential. The aim of this paper is to find an optimal control for a medium-voltage DC transmission system (MVDC). Different multilevel topologies are compared briefly and the most promising topology is chosen and examined in detail with respect to harmonic content of output voltages and dynamic response behavior during load changes