• • The oscillation transfer model's amplitude-frequency characteristic at the oscillation frequency directly reflects the strength of oscillation transfer between electrical quantities, with experimental results confirming this correlation; this enables quantitative assessment of coupling effects that traditional stability margins overlook, providing a diagnostic tool for SSO mitigation in GFM-VSG systems.
• • Sub-synchronous oscillations in GFM-VSG systems are not exclusively caused by insufficient stability margins; they can also arise from oscillation transfer effects due to coupling among electrical quantities, as demonstrated by the proposed framework, which identifies coupling as a distinct instability mechanism requiring separate consideration in system design.
• • The gain ratio of the oscillation transfer model under different operating conditions can be used to judge the oscillation amplitude of corresponding electrical quantities, offering a practical metric for condition-dependent stability assessment and control tuning in grid-forming inverter deployments.
• • Experimental validation confirms the effectiveness of the proposed modeling method and evaluation framework, with the amplitude-frequency characteristic at the oscillation frequency matching experimental observations, thereby establishing the framework's credibility for industrial application in renewable-rich power systems.