Design of a grid adaptive controller for PWM converters with LCL filters

The system resonance frequency of a grid connected inverter with LCL filter depends on the filter parameters and the time varying grid impedance. To obtain stability and to preserve good dynamic behavior of the system, grid impedance adaptation of the current controller is proposed. The design of the current controller is described and the adaptation strategy to prevent the excitation of the system resonance frequency during grid-impedance changes is introduced. The actual grid impedance is estimated by an extended Kalman filter (EKF) and a Luenberger observer allows to use the same number of voltage and current sensors as in conventional PI controllers. The proposed control method provides the possibility to tune the controller dynamics independently from the grid impedance. The theoretical analysis is validated with laboratory measurements.