Key Takeaways & Executive Findings
- •• • 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.
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Abstract
The integration of high-penetration renewable energy sources into power grids has exacerbated deficiencies in system inertia and damping, precipitating sub-synchronous oscillation (SSO) instabilities. Grid-forming virtual synchronous generator (GFM-VSG) systems, which emulate the rotor inertia and damping characteristics of conventional synchronous machines, are increasingly deployed to provide grid support. This study establishes an oscillation transfer effect model and evaluation framework to elucidate the mechanisms by which oscillations propagate among electrical quantities in GFM-VSG systems. The analysis reveals that SSO in GFM-VSG systems does not solely arise from insufficient stability margins; it is also attributable to oscillation transfer effects between different electrical quantities. The proposed framework enables quantitative assessment of oscillation transfer effects. Experimental validation confirms the effectiveness and feasibility of the modeling methodology and evaluation framework. The oscillation transfer model's amplitude-frequency characteristic at the oscillation frequency directly reflects the strength of oscillation transfer between corresponding electrical quantities, and experimental results align with this characteristic. The findings demonstrate that conventional stability analysis models, which neglect oscillation transfer effects, are inadequate for capturing the coupling mechanisms that influence system stability. The proposed framework provides a systematic approach to quantify these effects, thereby enabling targeted suppression of SSO and enhancement of system stability.
1. Introduction
High-penetration renewable generation has eroded system inertia and damping, driving sub-synchronous oscillations (SSO) that conventional grid-following inverters cannot mitigate. Grid-forming virtual synchronous generators (GFM-VSG) emulate synchronous machine dynamics to provide inertia and damping support, yet they introduce new instability risks: SSO and oscillation transfer among electrical quantities. Existing literature predominantly addresses frequency stability, voltage stability, and power coupling, but the mechanisms by which oscillations propagate between active power, reactive power, voltage, and frequency remain inadequately characterized. This gap leaves system operators without quantitative tools to predict or suppress coupling-induced oscillations, particularly under varying grid conditions.
Prior studies have examined frequency stability via eigenvalue analysis and impedance modeling, voltage stability through adaptive control and data-driven methods, and power coupling effects on synchronous resonance. However, these approaches typically neglect the oscillation transfer effect—the phenomenon where oscillations in one electrical quantity excite oscillations in another through control coupling. The absence of a dedicated evaluation framework means that SSO events attributed to insufficient stability margins may actually stem from transfer effects, leading to misdiagnosis and ineffective mitigation. This paper addresses that bottleneck by constructing an oscillation transfer evaluation model for GFM-VSG systems, quantifying the transfer strength via amplitude-frequency characteristics at the oscillation frequency, and validating the framework experimentally. The proposed method enables targeted suppression of SSO by identifying dominant transfer paths, thereby enhancing stability in renewable-dominated grids.
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XIONG Xinhua, LI Chang, YANG Yaqian, YUAN Jun, ZHAO Chanjuan (2026). Oscillation Transfer Mechanism in Grid-Forming Virtual Synchronous Generator Systems. Acta Energiae Solaris Sinica. https://doi.org/10.19912/j.0254-0096.tynxb.202608_9713
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Frequently Asked Questions
What specific experimental evidence confirms that oscillation transfer effects, rather than stability margin deficits alone, cause sub-synchronous oscillations in GFM-VSG systems?
The experimental validation demonstrates that the amplitude-frequency characteristic of the oscillation transfer model at the oscillation frequency directly reflects the strength of oscillation transfer between electrical quantities. The observed oscillations matched the predicted transfer effects, confirming that coupling among electrical quantities can induce SSO even when stability margins appear adequate. This was verified across multiple operating conditions, with the gain ratio of the transfer model correlating with the oscillation amplitudes of corresponding electrical quantities.
How does the proposed oscillation transfer framework improve upon traditional stability analysis methods for GFM-VSG systems?
Traditional stability analysis models typically neglect oscillation transfer effects, focusing on stability margins derived from eigenvalue or impedance methods. The proposed framework explicitly quantifies the coupling-induced transfer of oscillations between electrical quantities, revealing that SSO can arise from transfer effects independent of margin insufficiency. This enables identification of dominant transfer paths and provides a quantitative metric—the amplitude-frequency characteristic at the oscillation frequency—to assess transfer strength, thereby allowing targeted suppression strategies that conventional methods cannot support.
What are the practical implications of the gain ratio metric under different operating conditions for GFM-VSG system design?
The gain ratio of the oscillation transfer model under varying operating conditions serves as a direct indicator of the oscillation amplitude of corresponding electrical quantities. This allows system designers and operators to predict which quantities will exhibit larger oscillations under specific grid conditions, enabling condition-dependent tuning of control parameters (e.g., inertia, damping) to minimize transfer effects. Consequently, this metric supports proactive stability management rather than reactive mitigation, reducing the risk of SSO-induced instability in renewable-rich grids.
What are the limitations of the proposed oscillation transfer model, particularly regarding its validation and scalability to multi-machine systems?
The model was validated on a single GFM-VSG system connected to a grid, as described in the experimental section. While the framework is theoretically extensible to multi-machine systems, the current validation does not cover interactions among multiple GFM-VSGs or with other grid-forming assets. Additionally, the model assumes a simplified network representation (e.g., neglecting filter capacitance effects at low frequencies), which may require refinement for systems with complex topologies or high-frequency dynamics. Scalability to large-scale networks remains an open area for further research.
How does the oscillation transfer effect interact with active and reactive power coupling, and what are the implications for control design?
The study reveals that active and reactive power coupling significantly influences oscillation transfer, as evidenced by the transfer model's amplitude-frequency characteristics. The coupling causes oscillations in one power component to excite oscillations in the other, potentially exacerbating SSO. This implies that control design must account for cross-coupling terms, for example by decoupling control loops or introducing damping that targets specific transfer paths. The gain ratio metric can guide such designs by quantifying the strength of transfer between active and reactive power under different operating points.
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