Grid-Forming Control for DFIG Based Wind Farms to Enhance the Stability of LCC-HVDC

This article presents the grid-forming control (GFC) for doubly-fed induction generator (DFIG)-based wind farm to enhance the stability of line-commutated converter (LCC) based high voltage direct current (HVDC) transmission system. GFC is a grid friendly control strategy to build or follow the grid voltage and provide inertia support for power grid. The GFC for DFIG can not only improve the stability of HVDC sending-terminal system, but also reduce the dependence of synchronous generator (SG) in HVDC sending-terminal system. Impedance analysis is used to analyze the stability of HVDC sending-terminal system. The impedance modeling process of the GFC for DFIG is introduced in detail. Based on the impedance analysis, the GFC for DFIG shows the better stability than the conventional vector control (VC) for DFIG in HVDC sending-terminal system. When the capacity of SG decreases and transmission line impedance increases, the GFC for DFIG can keep stable, while the VC for DFIG will lose the stability in HVDC sending-terminal system. Vector graph of impedance characteristic is applied to illustrate the mechanism of instability. The time domain simulations are presented to verify the impedance analysis. Moreover, the simulation in islanded condition is also given to illustrate the stability and effectiveness of the GFC for DFIG.