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This paper presents a method to design a control system for a three-phase voltage source converter (VSC) that connects a renewable energy source to the utility grid through an output L-type or LCL-type filter. The well-known abc/dq transformation method creates coupling terms that are visible and can readily be canceled in the L-type filter. Such terms, however, are very complicated when an LCL filter is used. This paper, first revisits the derivation of the decoupling control method for an L-ype output filter and then, for the first time, derives the decoupling terms for an LCL-type filter. Having successfully decoupled the real and reactive power loops, feedback controllers are presented and designed to achieve desirable performance. The proposed controller provides active damping of the LCL resonance mode, robustness with respect to grid frequency, and impedance uncertainty. Moreover, a new controller is designed to improve the startup transient of the system. The methodology used in this paper is inspired from the feedback linearization theory and it provides a clear design method for the nonlinear systems. Simulation results are presented to confirm the analytical results.
Date of Publication: Oct. 2011