Filter design for single phase grid connected inverters

The filter selection and controller design is quite important in terms of the high quality energy transfer in grid inverters. These factors affect energy efficiency, power quality and system stability. Filter elements used between the inverter and the grid suppresses switching harmonics and decreases the THD value of the grid current. In this study, the selection of the LCL filter elements of a single phase two level inverter is analyzed. A computer program that works in MATLAB environment is developed to determine the values of LCL filter elements. The current control is performed with a proportional resonant (PR) controller. The PR controller is designed at discrete time. The behavior of the PR controller on the system stability is investigated. By means of the developed program, the total harmonic distortion (THD) of the grid current and the effective value of the inverter current are determined for different inductance, capacitor, switching frequency and reference current values. Characteristic curves are obtained for various LCL values. Filter elements can be selected by using characteristic curves. The solution sets formed by the L1, L2 and C1 values are found that providing of stable operation and THD<%5 condition at different switching frequency.