Frequency adaptive observer to extract ac-coupled signals for grid synchronization

For three-phase three-wire grid, a method to synchronize the converter to the grid voltage, which was distorted by low-order harmonics and unbalances, has been widely described. In addition to filtering them, the DC rejection can be achieved together with a frequency adaptive observer in order to eliminate DC bias in the voltage. The state equation for the observer is newly established, and the observer gains are examined in terms of their influences to the performance. The effectiveness of the proposed method is verified with simulations and experiments.