Modified dual second-order generalised integrator FLL for synchronization of a distributed generator to a weak grid

Performance of grid-connected converter relies on fast and precise estimation of phase angle and amplitude of the fundamental component of grid voltage signal to guarantee the proper synchronization of DG in modern power systems. However, the extent to which the frequency is estimated is dependent on the type of grid voltage abnormalities and structure of the phase-locked loop or frequency locked loop control schemes. Among various control schemes, second-order generalized integrator based frequency-locked loop (SOGI-FLL) is reported to have the most promising performance even under grid voltage abnormalities like sag, swell, harmonics, imbalance, frequency variations etc. except the dc offset. This paper presents a modified dual second-order generalized integrator frequency-locked loop (MDSOGI-FLL) for three-phase systems to cope with the non-ideal three-phase grid voltages having all type of abnormalities including dc offset. The complexity in control scheme is almost the same as the standard dual SOGI-FLL, but the performance is enhanced. Simulation results show that the proposed MDSOGI-FLL is effective under all abnormal grid voltage conditions. The results are also compared with that obtained by DSOGI-FLL to justify the superior performance.