Discrete-Time Control Design for Three-Phase Grid-Connected Inverter Using Full State Observer

LCL filters are being more and more preferred over the L filter in a grid-connected inverter due to the smaller physical size and better harmonic attenuation characteristics. However, additional control loops are often required to control inverter-side current as well as capacitor voltage. These additional control loops complicate the controller design process. To overcome such a limitation, a discrete-time control design for a three-phase grid-connected inverter using a full state observer is presented in this paper. The controller design is accomplished based on the state-space model of the inverter system. Furthermore, to reduce the steady-state error in output currents, a discrete-time integral state feedback controller is employed. Generally, all state variables should be available to implement a state feedback controller. For the purpose of reducing the number of sensors in a practical system, an observer which uses the measured grid-side currents, grid voltages, and control inputs is constructed in discrete-time domain to predict the inverter-side currents and capacitor voltages. As a result of using both the feedback controller and the observer, the proposed control scheme provides a better control performance in a systematic design approach. Simulation results are given to demonstrate the feasibility and performance of the proposed control scheme.