A synchronous DQ frame controller via an LCL coupled filter under unbalanced three-phase voltage supply conditions

This paper proposes a synchronous DQ frame current controller for inverter based interfaced systems. The grid voltage may be unbalanced, possibly due to unequal line impedance. The output active and reactive powers oscillate at twice the fundamental grid frequency around the desired values, due to the negative sequence voltage. The proposed controller can eliminate the effect of the negative sequence component, using two separate PI controllers. The first is associated with positive sequence current control and the other is for the negative sequence. A modified phase locked loop is proposed and confirmed practically. Steepest descent adaptive control for grid voltage estimation is employed in the control system. The proposed controller produces high quality current injected into the grid, therefore, high power quality. Simulated and practical results verify the performance of the proposed control system.