Sub-synchronous Oscillations Caused by Grid-connected PMSGs under the Condition of Near Strong Open-Loop Modal Resonance

Wenjuan Du, Xiao Chen, Qiang Fu, Haifeng Wang*, Tim Littler

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


This article examines the modal condition under which a grid-connected PMSG may cause the sub-synchronous oscillations (SSOs) in a power system. The examination is based on a closed-loop interconnected model of the power system, wherein, the PMSG and the remainder of power system (ROPS) are modeled as two separate but interconnected open-loop subsystems. The model condition being examined is that an SSO mode of the open-loop PMSG subsystem is close to an SSO mode of the open-loop ROPS subsystem on the complex plane. Analysis in the article indicates that the modal condition is the near strong open-loop modal resonance (NSOMR), under which damping of one corresponding closed-loop SSO mode may very likely degrade. The damping degradation can be estimated by an SSO index derived in the article. Hence, study presented in the article reveals the mechanism about why the SSOs may be caused by the grid-connected PMSG from the perspective of the NSOMR. In addition, it is proposed that by applying the modal analysis to the open-loop subsystems, the SSO instability risk brought about by the PMSG can be detected and the risk-contributing PMSGs and synchronous generators in the power system can be identified.

Original languageEnglish
Pages (from-to)1731-1743
Number of pages13
JournalElectric Power Components and Systems
Issue number19-20
Publication statusPublished - 19 Mar 2020


  • grid-connected PMSG
  • sub-synchronous interactions (SSIs)
  • Sub-synchronous oscillations (SSOs)
  • sub-synchronous oscillations (SSOs)
  • torsional dynamics of synchronous generators (SGs)

ASJC Scopus subject areas

  • Energy Engineering and Power Technology
  • Mechanical Engineering
  • Electrical and Electronic Engineering


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