Passivity-Based Stabilization of a Grid-Connected Three-Phase Inverter System with Feedforward Control

Abstract

Instabilities due to mutual interactions among inverters and grid systems are sometimes observed. This paper proposes an optimal digital controller design method for passive stabilization of a grid-connected three-phase inverter with LCL filter to improve output frequency impedance characteristics which sufficiently satisfy the Nyquist stability criterion. The principle is that the output impedance can be designed to be passive with the designed feedforward loop characteristics and the system is assured to be stable even if the grid system is weak. The proposed three-phase control consists of the following three steps. In the first step, the state feedback control gains for a single-phase control subsystem are designed to have a good sinusoidal tracking characteristic according to the optimal control. The designed characteristic of the output impedance may not be passive at a certain frequency band. In the second step, a resistive characteristic is added to the band by a new feedforward control design based on a fitting method to make the total characteristic passive and to ensure stability. In the third step, combining two sets of the designed single-phase control subsystems, a three-phase control system is derived. The principle and the design process are described and the designed system is investigated and validated for stability and response characteristics through simulation and experimental results.