An Indirect Approach to Control an Active Capacitor

We propose an indirect approach to control an active capacitor, realized using a bidirectional converter, meant for dc voltage filtering (ripple elimination). This active capacitor can replace large, expensive, and unreliable electrolytic capacitors connected on a dc bus for power filtering. It can deal with a wide frequency band of harmonics so that it does not need a priori knowledge of the ripple frequencies. Thanks to the simple decentralized controller proposed in this paper, our active capacitor can be connected directly to the dc bus like a passive filtering capacitor, and no extra connections are needed. Compared with the various direct approaches, a unique feature of the proposed indirect approach is that the buffering capacitor energy V_S2 is directly regulated and much better dynamics of VS² under sudden load variations are obtained. The control algorithm has been verified by both simulation and experiments, with our circuit being connected to a 350-W commercial power factor compensator.