Stability Study of the Grid-Forming Wind Turbine System with Mixed Potential Function Under Large Disturbances in the LCC-HVDC Transmission System

With the large-scale integration of grid-forming wind turbines via LCC-HVDC transmission, the large-disturbance stability of such systems has become a critical concern due to complex interactions among multiple power electronic converters. This paper proposes a unified stability analysis framework based on the mixed potential function theory for a VSG-DFIG fed LCC-HVDC system under AC voltage sags at the point of common coupling. By incorporating the physical dynamics and constant-current control of the rectifier, and equivalently representing the PI controllers of the VSG-DFIG as RLC circuits, an equivalent circuit model of the entire system is established. The large-signal stability criterion is then derived, which explicitly reveals the influences of control parameters, circuit parameters, and the depth of AC voltage sags on the stability boundary. To enhance system stability, a virtual resistor strategy is introduced in the LCC-HVDC control loop. Finally, time-domain simulations in Simulink are carried out, and the results fully validate the theoretical analysis as well as the effectiveness of the proposed virtual resistor scheme.