Three-level neutral-point-clamped voltage source converters for renewable energy systems : modeling, operation and control

Renewable energy sources are attracted a great deal of interest in recent years since they are clean, endless and always available. Widely used power converters in renewable energy sources are back-to-back converters included a AC-DC side, a DC-link capacitor and a DCAC side. Multilevel voltage source inverters are used in the DC-AC side that can reach high voltages with low harmonics. By using appropriate modulation schemes, they can provide higher performance and higher e ciency than the traditional converters. Power losses are a serious subject in voltage source inverter. In order to improve the e ciency of systems, we should decrease the power losses in the semiconductor devices. The thesis nds a mathematical model of conduction and switching power losses in diodes, IGBTs and MOSFETs. Afterwards, we give an overview on two-level and three-level neutral point clamped converter topologies and try to model properly these converters.Then, instantaneous power losses in both inverters are investigated. The next step is to give an overview of modulation schemes such as pulse width modulation (PWM) and space vector modulation (SVM). Then we try to improve SVM to reduce switching power losses by nding the time intervals that the phase currents reach their maximum values and prevent switching during these time intervals. At the end, the models are simulated and the results are compared with each other.