This work presents an observer design for grid current and capacitor voltage of voltage source pulse-width modulation (PWM) converters with LCL filter. Theoretical aspects including the mathematical LCL filter system observability, observer placement strategy and practical discretization implementation. It gives insight to mathematical modelling of the line filters dynamics. By the limitations of how the components in the line filter operates, the Kalman filter is adjusted accordingly. The strategy for designing the Kalman filter is presented. A time-varying KF is developed, benchmarked and implemented in simulator. Through an explanation of the magnetic field fundamentals, a nonlinear model of the inductors is modeled and used. An observer scheduling development has been implemented on the nonlinear system. The effect of sampling frequency is studied for KF and for the observer as well. At last the results are presented and analyzed
