Simplified digital-analogical control for shunt active power filters under unbalanced conditions

In this paper, a digital-analog control scheme for a three-phase shunt active power filter is presented. Three algorithms for determining the active filter compensating current under several mains conditions are investigated and compared. The three single-phase method, the power balance concept and the extension of the p-q algorithm gives good steady-state and transient performances for both harmonics and reactive power compensation under balanced sinusoidal or nonsinusoidal conditions. However the p-q algorithm is still ineffective under unbalanced voltage conditions. The three-phase digital PLL system allows to operate satisfactorily in a wide range of mains frequencies. The close agreement between simulation and experimental results proves the validity of the optimized DC capacitor design and the feasibility of the proposed control methods.