Offset rejection for PLL based synchronization in grid-connected converters

Grid-connected converters rely on fast and accurate detection of the phase angle, amplitude and frequency of the utility voltage to guarantee the correct generation of the reference signals. An important issue associated with accurate grid voltage monitoring is the presence of an offset in the measured grid voltage. This voltage offset is typically introduced by the measurements and data conversion processes and causes errors for the estimated parameters of the grid voltage. Accordingly, this paper presents an offset rejection method for grid-connected converters based on a phase-locked-loop (PLL) technique. The offset rejection method relies on the second order generalized integrator (SOGI) having the advantages of a simple implementation independent of the grid frequency and avoidance of filtering delays due to its resonance at the fundamental frequency. Selected experimental results validate the performance and robustness of the proposed offset rejection method.