Impedance Analysis Considering Unstable Subsystem Poles for MMC-HVDC-based Wind Farm Integration System

Employing the AC/DC impedance analysis method, extensive research has been conducted on issues pertaining to harmonic resonance observed in wind farm-connected MMC-HVDC systems over the past 5 years. In general, this method divides the studied system into two subsystems at AC and DC sections. Subsequently, the impedance produced by these subsystems can be used to determine the system stability by implementing Nyquist criterion or generalized Nyquist stability criterion. However, the existing impedance analysis methods usually assume the subsystems to be stable and disregard certain special cases with potentially unstable subsystems, where the unstable poles of these subsystems must be evaluated before applying Nyquist criterion. In view of this consideration, a simplified method for identifying unstable subsystem poles is presented in this paper. Hence, the shortcoming of existing impedance analyses is rectified by considering the unstable subsystem poles. The proposed method and the stability analyses are validated by the electromagnetic transient simulation performed in PSCAD/EMTDC.