This paper presents a three-phase Active Power Filter (APF) to improve grid power quality. Starting from indirect control strategy which was used only for three legs - three wires it is proposed in this paper to be used a bridge with three legs - four wire. The new control strategy it was named extended indirect control strategy. The extension consists in passing from the control of a three wire active power filter to the control of a four wire active power filter. The proposed active power filter can compensate the current harmonics and correct the power factor. Nloreover, the proposed control strategy allows the line current at the point of common coupling (FCC) to be balanced and sinusoidal even when the load is unbalanced. Consequently the voltage at the PC becomes balance. Simulation results show the validity of the proposed control strategy.