Observer-based State Feedback Control of an LCL-filtered Grid-connected Inverter for Distributed Power Generation System

The LCL filters are being more and more preferred over the L filter 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, an improved current control scheme in which the controller design is accomplished based on the state-space model of inverter system is proposed for an LCL filtered grid-connected inverter. Furthermore, to reduce the steady-state error in output currents, an integral 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, an observer which uses the measured grid-side currents, grid voltages, and control inputs is constructed 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 excellent performance of the proposed control scheme.