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Impedance analysis considering unstable subsystem poles for MMC-HVdc based wind farm integration system

Harmonic resonance issues of the wind farm connected MMC-HVdc system have been well investigated by ac/dc impedance analysis in past 5 years. These methods generally divided the studied system into two subsystems at ac or dc section. Then, the impedance of these subsystems can be used to identify the system stability by applying Nyquist criterion or generalized Nyquist criterion. However, the existing impedance analyses always assume subsystems are stable and overlook some special cases where subsystems may are unstable. In these special cases, the unstable poles of subsystems must be evaluated before applying Nyquist criterion. Based on this consideration, a simplified method for identifying unstable subsystem poles is proposed in this paper. With consideration of unstable subsystem poles, the gap of existing impedance analysis is filled. The proposed method and stability analyses are validated by electromagnetic transient simulation in PSCAD/EMTDC.