Generalized Impedance Analysis and New Sight at Damping Controls for Wind Farm Connected MMC-HVdc

The small-signal stability of wind farm (WF) connected modular multilevel converter (MMC)-based high voltage direct current (MMC-HVdc) systems has been widely studied by impedance analyses. However, it is generally difficult to calculate the poles of WF-connected MMC-HVdc due to complex internal dynamics. Thus, the existing impedance analysis methods usually assume that the studied subsystems are stable. In some special cases, the MMC-HVdc itself may be unstable but the WF-connected MMC-HVdc may be still stable. If the poles of the subsystem are not calculated, these phenomena cannot be well revealed by impedance analysis. In order to address these special issues, this article divides the WF-connected MMC-HVdc into three subsystems and proposed a 3-order minor loop-gain matrix to investigate the unified stability of both ac and dc systems. Based on the proposed model, the impact of ac/dc current-based damping controls in WF side MMC on the system stability are investigated. As a result, the proposed model makes up for the deficiencies in ac/dc impedance analysis method and provides a new sight at evaluating the impact of active damping controls on system stability. The simulation results in PSCAD/EMTDC validate the proposed models and stability analyses.