Voltage source active power filter, based on multi-stage converter and ultracapacitor dc-link

A multi-stage inverter using three-state converters is being analyzed for active filter and static var compensator applications. Each phase of the converter is composed of four three-state converters, all of them connected to the same dc link and its output connected through output transformers scaled in power of three. The filter can compensate load currents with high harmonic content and low power factor, obtaining sinusoidal currents from the source. A 1F ultracapacitor is used in the dc link, making it possible to obtain a very stable voltage at the dc bus, even with highly contaminated currents. This high capacity also makes it possible to continue feeding the contaminating load during a voltage dip. The capacitor voltage is controlled simply by changing the phase angle of the converter, and thus changing the amount of active current flowing to and from the converter. The control is implemented with a non-linear PI gain and a modulation control to maintain a stable ac voltage during dc voltage drops. The great advantage of this kind of converter is the minimum harmonic distortion obtained. Simulation results for this application are shown and compared with similar results obtained with conventional PWM converters.