Decentralized Control of an Active Capacitor with NDO-Based Feedforward Compensation

This paper is about decentralized control of an active capacitor, which is intended to store a wide range of unknown low-frequency ripples. Only the local dc bus voltage is used for the controller design. The control algorithm is based on the indirect approach, with the buffering capacitor energy that is directly regulated, which in turn influences the dc bus voltage indirectly. With the proposed feedforward compensation on the reference signal, the attenuation around hundreds of hertz is further improved, resulting in a larger emulated capacitance. The feedforward signal is generated from a reference model of the active capacitor with disturbance current on the dc bus and dc bus voltage as its inputs. Rather than direct measurement via a current sensor, the disturbance current is estimated by a nonlinear disturbance observer (NDO). With the saved current sensor, the cost and volume of the active capacitor can be reduced. The control algorithm has been verified by both simulation and experiments, with the active capacitor being connected to a 350-W commercial power factor compensators (PFCs).