Model based controller for an LCL coupling filter for transformerless grid connected inverters in PV applications

Voltage source inverters (VSI) connected to the grid by means of LCL filters offer benefits in the injection of active power into the grid, since they provide low harmonic distortion and a small size of the output filter. However LCL grid connected VSI must be designed in order to provide damping at the filter resonance frequency. This paper presents the design and analysis of a model based controller for an LCL coupling filter to inject active power into the grid for single phase photovoltaic transformerless applications. The control objectives are to provide closed-loop stability and to assure that the current to be injected follows a reference which is proportional to the grid voltage. Therefore, these objectives guarantee the active power injection to the grid. The proposed output feedback controller is based on the error model of the LCL filter and makes use only of the measurements of the output current of the inverter and the grid voltage.