Performance evaluation of an enhanced grid-forming control mode for grid-tied DFIG wind turbines compared with conventional grid-forming and grid-following controls

This paper addresses the grid-forming control (GFM) of DFIG-based wind turbines (WTs) using a PLL-less virtual synchronous generator (VSG) technique, not for the purpose of participating in frequency control and providing inertia, but rather for power injection under weak grid conditions of the DFIG. However, existing PLL-less GFM strategies, including conventional VSG and droop-based controls designed to emulate inertia and provide frequency support, often suffer from power deviations when grid frequency deviates from its nominal value. To overcome this shortcoming, this paper proposes an enhanced VSG technique, including two auxiliary compensators, known as power correction and phase angle compensators, to ensure that the power injected by the DFIG is not sensitive to frequency variations and does not change with deviations of the grid frequency from the reference value. The output signals of these auxiliary compensators, by correcting the reference power and the phase angle, attempt to ensure that the stator power fully tracks the reference power under frequency variations. In this way, reduced-order small-signal models are developed for compensators design and stability analyses of the system with enhanced VSG. Simulation results demonstrate that the proposed GFM control with enhanced VSG, compared to the traditional VSG method, effectively suppresses power fluctuations caused by grid frequency deviations and significantly enhances system robustness in weak grid scenarios.