BALANCING AND HARMONIC ANALYSIS OF FLYING CAPACITOR MULTILEVEL CONVERTERS

To supply medium-voltage AC machines, DC-AC converters working with a high bus voltage are required. By using multilevel converters, these high voltages can be supplied without the need for switches with a high voltage rating. Moreover, the output voltage waveform can be improved considerably. The flying capacitor multilevel converter is a recently developed converter topology assuring a flexible control and modular design. However, the flying capacitor multilevel converter requires a balanced DC voltage distribution. This can be realized by using a special control leading to natural balancing or by measuring the voltages and selecting the appropriate switching state. The balancing is influenced by three factors, namely the harmonic content of the reference waveform, the switching frequency and the load impedance. In addition to the voltage balancing of the flying capacitor multilevel converter, the output voltage must ensure the control of the load, e.g. a three phase AC machine.