Discrete-time backstepping-state feedback approach for current control of LCL grid-tied converters

Abstract

Voltage source converters connected to the grid through an LCL filter effectively attenuate the high frequency harmonics generated by the PWM modulation. However, due to the intrinsic resonant peak of this filter and due to the discretization process, the controller design is commonly a challenge and it may not guarantee robustness against abnormalities of the grid. In order to overcome these problems, this paper proposes a new discrete-time current control scheme applied to grid-tied converters based on a backstepping algorithm and state feedback controller in a multi-loop approach. The tracking of converter-side current for a given reference, which is generated by the outer loop, is performed by the backstepping algorithm. In addition, the controller gain design methodology is presented taking into account an extended model, that includes the delay resulted from the discrete-time implementation. An analysis of the gain choice effect on the system closed-loop eigenvalues is demonstrated, which allows the designer to select an appropriate set of controller gains. As outer loop, a state feedback method combined with a state-space resonant controller is used for the grid-side current tracking. In this structure, the harmonic compensation is easily preformed, in addition, the feedback gains are chosen using an optimum algorithm. The robustness of the multi-loop backstepping based control structure is improved in comparison with a full state feedback controller. Simulation and experimental results are obtained to demonstrate the effectiveness of the proposed control scheme under different test conditions, as reference changes, operating during electric faults and grid impedance variation.