Three-phase multilevel voltage source converters with low switching frequencies and less-distorted input voltages

A novel, modified neutral-point-clamped (NPC) converter is proposed. It generates quasi-18-step input voltages and gives nearly sinusoidal input currents at a switching frequency of triple the power line frequency. Theoretical and experimental work has shown that the proposed converter is suitable for large-capacity power converters for electric utility applications, because it makes the most of the circuit characteristics of high-voltage converters, i.e., the series connection of switching devices, and it produces fairly good input voltage and current waveforms at low switching frequencies. The authors present the operating principle, the analysis and synthesis of the input voltage waveforms in a single and a double converter system, and experimental results