System-oriented efficiency optimization of variable speed drives

Low-voltage variable speed drive (VSD) systems are among the most important consumers of electrical energy in industry. Such systems are subject to tremendous cost pressure and are traditionally built with the simplest possible power electronic converter satisfying the requirements. Enabling a bidirectional power flow, the 2-level voltage source back-toback converter was and still is the standard industry choice due to its low purchase costs. Recently, there is a trend towards increasing electrical energy prices because of declining fossil energy reserves and the subsidies accelerating the change towards renewable energy sources. Therefore, the operating expenses of a VSD system become more and more important and lead to a reconsideration of the purchase costs, making systems with higher energy efficiencies more competitive. The main goal of this thesis is to give a comprehensive, system oriented efficiency analysis and optimization of a modern low-voltage VSD system. This holistic approach includes all components of such a system, namely the EMI input filter, the power electronic converter, the load machine, possible interactions among them and impacts of control strategies. Consequently, loss models of all subsystems are developed and interconnected in order to consider mutual interactions. State-of-the-art converter topologies that are uncommon in lowvoltage applications, such as the 3-level T-type topology, or the 3-level NPC topology, have advantageous properties concerning converter efficiency and have additional beneficial impacts on the surrounding, such as harmonic losses in the load machine and in the filtering components. A detailed analysis reveals that despite the increased initial costs, these alternative topologies enable to build a modern VSD system outperforming the standard industry solution in several aspects, namely the system energy efficiency, the necessary semiconductor chip area and