Weak Grid Characteristic Analysis and Operating Mode Selection for Voltage Support Enhancement of Wind Farms Connected to MMC-HVDC During Asymmetric Faults

The voltage support functions have been mandated in grid codes to help wind turbines (WTs) ride through AC faults. In a modular multilevel converter-based high voltage DC (MMCHVDC) system for wind farm integration, particularly in scenarios where there is no conventional synchronous generator at the sending end, the MMC adopts voltage-frequency control to form an AC grid. Such grid, however, is found to have a distinctive weak characteristic, which significantly alters the relationship between the point of common coupling voltage and the current injected by WTs and makes existing voltage support modeling methods and control strategies for grid-connected inverters inapplicable to the WTs connected to MMC-HVDC. To enhance the voltage support performance for the WTs, this paper aims to elaborate on the weak grid characteristic of the voltage formed by the MMC. To streamline the analysis, the concept of generalized fault impedance is first proposed to eliminate the need for numerous models built for scenarios with different network structures, negative sequence controls, fault types, and transformer winding configurations. Additionally, this paper categorizes the operating modes of the MMC and utilizes the piecewise modeling method to derive precise control equations in each operating mode, based on which the weak grid characteristic is thoroughly discussed. Subsequently, to avoid globally identifying the optimal voltage support point, an operating mode selection method prior to the voltage support realization is proposed. Finally, the above modeling method and analysis are verified through control-hardware-in-the-loop experiments.