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 MMCHVDC 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.

Stability Investigation and Improvement for DC Cascade Systems with Simplified Impedance-based Stability Criterion

In DC distributed power systems(DPSs),the complex impedance interactions possibly lead to DC bus voltage oscillation or collapse.In previous research,the stability analysis of DPSs is implemented based on mathematical analysis in control theory.The specific mechanisms of the instability of the cascade system have not been intuitively clarified.In this paper,the stability analysis of DPSs based on the traditional Nyquist criterion is simplified to the resonance analysis of the seriesconnected port impedance(Z=R+jX)of source and load converters.It reveals that the essential reason for impedance instability of a DC cascade system is that the negative damping characteristic(R<0)of the port the overall impedance amplifies the internal resonance source at reactance zero-crossing frequency.The simplified stability criterion for DC cascade systems can be concluded as:in the negative damping frequency ranges(R<0),there exists no zero-crossing point of the reactance component(i.e.,X=0).According to the proposed stability criterion,the oscillation modes of cascade systems are classified.A typical one is the internal impedance instability excited by the negative damping,and the other one is that the external disturbance amplified by negativity in a low stability margin.Thus,the impedance reshaping method for stability improvement of the system can be further specified.The validity of the simplified criterion is verified theoretically and experimentally by a positive damping reshaping method.