Design of a novel robust current controller for grid-connected inverter against grid impedance variations

In this paper, a simple low order robust current controller with capacitor-current-feedback active damping is developed to reject grid impedance variations. Two control parameters among five are identified as the parameters that affect the system's stability significantly. Thus, the goal of the robust controller design is to fix the three control parameters, varying only the other two, and to determine the robust stability region in the two parameter plane by using D-decomposition method under the specified uncertainty region. The D-decomposition method maps the roots of closed-loop characteristic polynomials onto the graphic control parameter plane since certain relationship exists between the value of the control parameters and the value of the characteristic roots. By constraining all the closed-loop poles inside the unity circle in z-plane, the two control parameters are then the solutions to the characteristic polynomials which mathematically determines the robust stability region. Any parameters selected from this region can reject the specified grid impedance uncertainty. A robust performance region (settling time and damping ratio) is further determined inside the robust stability region, thus achieving robust stability, fast response and high quality current injection. All the analytical studies and simulation results verified the control design. The developed simple low order robust controller has been proven to be able to guarantee robust stability and achieve desired robust performance with the specified grid impedance uncertainty.