Unsymmetrical fault behavior of PLL based grid-connected converters

The fault behavior of converters is essential for stable operation of converter-driven power grids. This paper introduces a stability criterion of unsymmetrical faults for phase-locked loop (PLL) based grid-connected converters. The focus lies on the stability of the PLL considering the grid conditions and the converter feed-in power during the fault. For this purpose, the approach is based on the phase portrait method where several situations are examined in theory. Finally, the stability criterion is validated with real-time system experiments.