State observer for grid-voltage sensorless control of a converter under unbalanced conditions

This paper deals with grid-voltage sensorless synchronization and control under unbalanced grid conditions. A three-phase grid-connected converter equipped with an LCL filter is considered, and no signals other than the converter currents and the dc-link voltage are measured for control. An augmented adaptive state observer is proposed for estimation of the positive- and negative-sequence components of the grid voltage. Dynamic performance limitations of the proposed method and effects of parameter errors are analyzed. The proposed observer is tested as part of a sensorless control system. Experimental results show that the proposed method works well even under highly unbalanced grid conditions.