Transient Stability Analysis of MMC-HVDC Connected DFIG-based Wind Farms in the Electromechanical Timescale

This paper investigates the transient stability of doubly-fed induction generator (DFIG)-based wind farms connected via a modular multilevel converter (MMC)-based high-voltage direct-current (HVDC) transmission system in the electromechanical timescale. During faults, the MMC tends to be switched to the current-limiting mode, which further interacts with the control systems of DFIG-based wind farms, thereby complicating the transient stability problem. In this paper, a large-signal model is constructed for transient stability analysis under this condition. The transient stability mechanism is revealed, and a transient stability index that considers the impact of a phase-locked loop in the electromechanical timescale is derived. The proposed index quantitatively assesses the instability risk arising from the coupled interaction between the DFIG-based wind farms and the MMC. In addition, the impacts of the current- limiting mode and other parameters (i.e., speed control loop gains, rotor shaft damping, inertia coefficient and fault location and severity) are analyzed. Based on the results of the parametric analysis, a suite of targeted countermeasures is proposed to enhance the transient stability of the power system. Finally, a theoretical analysis is demonstrated and verified through hardware-in-the-loop simulations using the Modeling Tech microgrid real-time simulation platform.