Improving performance of single-phase SOGI-FLL under DC-offset voltage condition

The second-order-generalized-integrator frequency-locked loop (SOGI-FLL) is typically employed to synchronize with the grid in the single-phase grid-connected converter. The SOGI creates two orthogonal signals from an input grid voltage, and estimates the magnitude while the frequency locked-loop identifies the grid frequency. If there is a DC component in the grid voltage due to the measurement process, it degrades the accuracy of SOGI-FLL estimates. The SOGI-FLL, therefore, cannot identify accurate information of the grid voltage. This paper introduces an improved control method for the SOGI-FLL when the grid contains a DC component. The value of the DC component voltage is estimated online and totally removed from the frequency control loop. The controller, thus, responses faster and more precisely detects both the magnitude and frequency of the input voltage. Simulation results are presented to verify the theoretical analyses.