2. Soft-switching Multilevel Inverter with Flying Capacitors for Active Power Filter Applications

With significant development of power converter technologies, there has been a rise of interest in an active power filter (AF). It is now well known that an active filter can easily compensate for harmonic current contents in the load current by inserting negative harmonics into the power network [B3]-[B10]. Basic principle of an active harmonic compensation is shown in Fig. 2.1. The AF can cancel the load current harmonics generated by the static power converter, so that the ac source current is pure sinusoidal and is in phase with the input voltage. However, when the network voltage increases to a level between 2 kV and 4 kV, it is necessary to employ multilevel structures of the main circuit, even if they need double number of switches. The benefit of using a multilevel structure that uses a fraction of the high dc voltage is that it overcomes the limiting voltage and current ratings of the main switch device. Furthermore, since an AF is a type of boost converter, the high voltage is critical point at the design of the main circuit. Fig. 2.2 shows the sample current waveforms of a three-phase active filter. This indicates that the magnitude of current in the active filter depends highly on the distortion factor and power factor of the load current. The compensating current can be calculated by various approaches. Furthermore, since the active filter supplies only reactive power, all power of the inverter should be reactive so that the inverter only circulates the current in and out. With regard to reactive