Synchronizing stability of DFIG-based wind turbines attached to weak AC grid

This paper unfolds the issue of synchronizing stability in doubly fed induction generator (DFIG)-based wind turbines when connected to weak AC grid. In weak AC grid, the turbine´s terminal voltage vector (both amplitude and phase angle) is significantly influenced by the vectors of the grid voltage and DFIG´s internal potential that is contributed by both stator and grid-side converter (GSC). Currently, most developed DFIGs synchronize with grid by phase locked loop (PLL) which generates the phase angle by detecting terminal voltage. The complex dynamics of terminal voltage may deteriorate the performance of PLL and destabilize it under large-signal disturbance, which may lead to synchronizing instability of the DFIG system. In this paper, a DFIG-based wind turbine connected to infinite bus via high grid impedance is studied. The structure of PLL in weak AC grid is given and the equal-area method is adopted to investigate the large-signal stability of it conceptually. Simulation results verify the loss of synchronizing stability of DFIG.