Re-examination of small-signal instability in weak grid-connected voltage source converters

Weak grid sub-synchronous oscillation (SSO) is an important research topic. Although some analysis and miti-gation schemes have been conducted, misperceptions still exist. This paper first aims to update the understanding of weak grid instability of voltage source converters (VSCs) and then points out that instability risks originate in the super-synchronous frequency range rather than in the sub-synchronous frequency range. Moreover, the resonance frequency can be even larger than the double fundamental frequency under certain parameter con-ditions. This paper also refines the impacts of VSC control parameters on weak grid instability mechanism and resonance frequency. Based on the results, the classification of this phenomenon should be revised.