Low-Voltage Ride-Through Control Method of Doubly-Fed Wind Power Generation System Independently Linked to Flexible DC Grid

In the application background of single large-capacity doubly-fed wind generators (abbr. DFIG) connected to the medium voltage DC grid through a voltage source converter in all-DC wind farm, a DC low voltage ride-through control method was proposed based on voltage-frequency coordinated control at both ends of the machine and grid, to ensure the continuous controllable grid-connected operation of the DFIG after the voltage drop of the DC grid. Firstly, through theoretical analysis, it was explained that the rigid and non adjustable frequency after voltage drop during AC grid connection is the main reason for the transient DC component of the stator flux and the overcurrent generated by the stator and rotor windings, Furthermore, fully utilizing the flexible and adjustable characteristics of AC voltage and frequency within the DFIG system independently linked to flexible DC grid, the generation of transient DC components of stator flux was avoided through coordinated control of stator voltage and frequency between the machine and the grid, thereby eliminating the difficult to solve transient overcurrent phenomenon of stator and rotor during AC grid connection, and ensuring the continuous and controllable operation of the DFIG system. Simultaneously, the problem of DC overvoltage of rotor side converter was solved by adding DC unloading circuit. Finally, the effectiveness of the DC low voltage ride-through method based on voltage-frequency coordinated control was verified by the comparison of simulation of an AC grid-connected and flexible DC grid-connected single 5MW DFIG system.