Magnifying Effect of Weak Grid Connection for a PMSG to Induce Torsional Sub-synchronous Oscillations under the Condition of Open-loop Modal Resonance

When open-loop modal resonance (OLMR) happens, a grid-connected PMSG may induce torsional sub-synchronous oscillations (SSOs) in a power system. This study examines the magnifying effect of a weak grid connection for the PMSG to induce torsional SSOs under the condition of OLMR. Theoretical analysis is carried out in this study to indicate that the weak grid connection magnifies the impact of OLMR, when the resonant mode of PMSG is dominated by the dynamics of a phase-locked loop (PLL) or the DC voltage control of the grid side converter. The magnifying effect of weak grid connection is analytically proved to have two aspects. First, the magnitude of the real part of the open-loop residue of the PMSG increases when the grid connection weakens. Second, the magnitude of the real part of the resonant open-loop oscillation mode of PMSG decreases under the condition of weak grid connection. According to the OLMR theory, the weakly connected PMSG is more likely to cause growing torsional SSOs when the grid connection is weak. This study helps to understand the mechanism of recent SSO incidents in a practical Chinese power system is analysed. Study cases are presented to demonstrate and evaluate the analysis and conclusions made in this study.