An optimum control method for active filters

Active filtering is the most demanding application from a control point of view compared to all the line side converter applications because many harmonics have to be controlled with minimum switching frequency. The theoretical limit of the highest order of harmonic that can be controlled is at half the switching frequency. In this paper, a new method is presented to achieve optimum performance or achieve control up to the theoretical limits. The control law is derived from the system equations. The proposed method uses differentiation in the feedforward and ways to overcome associated noise amplification problem are presented. By introducing a delay in the feedback loop, it is made sure that the feedback loop only acts when there is any error in system modeling in feed-forward path. The basic control philosophy is based on the synchronous reference frame (SRF). The proposed scheme can be easily implemented by using a digital controller for SRF based control with a fraction of additional computing power necessary to implement the proposed scheme. Both simulation and practical results are presented for a 50 kVA lab prototype.