Grid-Forming Inverters Under Disturbance: Current Limiting and Nonlinear Synchronization

Grid-forming (GFM) inverter research has accelerated as inverter-based resources (IBRs) become dominant. Unlike grid-following (GFL) units, GFM inverters establish their own voltage and frequency references, enabling operation at high IBR penetration. System operators, research institutions, and industry are therefore calling for GFM capabilities in grid-connected converters. Yet the transition from synchronous-generator (SG) dominated systems to GFM-rich grids remains challenging: open questions persist regarding desired GFM behavior during disturbances (balanced/unbalanced faults, frequency or phase jumps, and overloading), the system-level impacts on stability and protection, and the resulting control design implications. This talk addresses two of these <a href="http://questions.

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Part I examines the consequences of strictly limited overcurrent capability in power-electronic converters. To protect hardware, GFM inverters must implement fast current limiting; once engaged, however, the inverter departs from ideal voltage-source behavior and can destabilize the system. The presentation will review widely used current-limiting schemes, introduce a frequency-stabilization strategy tailored for large disturbances, and present a recently proposed equivalent-impedance framework that quantifies the resulting stability margins. A complementary small-signal model then explains voltage-oscillation mechanisms arising from limiter-stabilizer interactions. The analysis reveals key design trade-offs and leads to a practical control-tuning <a href="http://workflow.

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Part II explores using Stuart–Landau nonlinear oscillators as the synchronizing core of GFM control to enhance reliability, disturbance rejection, and dynamic performance under weak, non-ideal, or unbalanced conditions. A cascaded architecture is adopted: an outer GFM layer sets voltage/frequency references, while a nonlinear, passivity-based inner loop enforces them, providing global asymptotic stability, stronger damping, and faster transients than conventional linear approaches. To capture magnetic saturation in interfacing elements, a gray-box model combines physics-based relations with measured characteristics; a high-gain observer estimates nonlinear inductance online, enabling adaptive gain scheduling across operating points. MATLAB/Simulink and PLECS studies demonstrate robust regulation, strong disturbance rejection, and resilience to load changes, parameter uncertainty, and weak-grid <a href="http://scenarios.

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Speaker(s): Bowen Yang, Vikram Roy Chowdhury

Virtual: https://events.vtools.ieee.org/m/514490