Oscillation suppression strategy for direct drive wind power system based on voltage feedforward compensation for phase locked loop

Due to the fact that the impedance of grid interacts with that of direct-drive wind turbines under weak grid, the direct-drive wind power system (DDWPS) possesses the risk of power oscillation. Given that phase-locked loop (PLL) is the crucial factor that hinders DDWPS stability under weak grid, the following has been achieved: firstly, potential factors influencing PLL output phase angle stability by using linearized PLL model are found; secondly, for PLL output phase angle, a voltage feedforward compensation method based on the relationship between the PLL output phase angle and grid voltage is proposed, which effectively suppresses power oscillation in DDWPS under weak grid; thirdly, the sequence admittance model of DDWPS adopting the proposed voltage feed-forward compensation based PLL (VFC-PLL) is established. Comparative analysis of the stability with the DDWPS using traditional PLL (T-PLL) demonstrates that the proposed VFC-PLL significantly improves DDWPS stability. More importantly, this method does not require complex parameter design and measured grid impedance information, so under weak grid, it is easy to popularize and apply in DDWPS. Finally, the experiment testifies to the validity and feasibility of the proposed VFC-PLL.