Influence of Grid-Forming Converter Control Parameters on System Stability Boundaries Under Different Grid Strengths
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Abstract
The voltage and frequency support capability of grid-forming converters under weak-grid conditions makes them key equipment for power systems with a high share of renewable energy, yet the coupling between their control parameters and grid strength still lacks a boundary-oriented description suitable for parameter tuning. This paper establishes a reduced-order model of a grid-forming converter that includes the active-power synchronization loop, the reactive-power/voltage loop, power-measurement filtering and an equivalent inner-loop lag. Grid strength is characterized by the short-circuit ratio, and the minimum damping ratio and the critical clearing time are adopted as the small-signal and large-signal stability indices, respectively. Through parameter sweeps, the stability boundaries of the virtual inertia, damping coefficient, power-filter time constant, reactive-power droop coefficient and virtual reactance are obtained for short-circuit ratios from 1.2 to 10. The results show that the damping ratio of the dominant electromechanical mode decreases monotonically as grid strength increases: when the short-circuit ratio rises from 1.5 to 10, the minimum damping ratio under the base parameters falls from 0.285 to 0.090, and the damping coefficient required to keep the damping ratio above 0.1 increases from about 11 p.u. to about 31 p.u.; conversely, the transient stability margin shrinks as the grid becomes weaker, with the critical clearing time falling from about 516 ms to about 230 ms as the short-circuit ratio decreases from 10 to 1.2. When the power-filter time constant exceeds about 50 ms, the strong-grid case loses stability first, and the damping improvement provided by the virtual reactance is more pronounced in the strong-grid case. On this basis, a parameter tuning region that accommodates both strong- and weak-grid conditions is given. The conclusions are subject to the assumptions of the reduced-order model; electromagnetic-transient and hardware-in-the-loop validation shows that the damping ratio of the dominant small-signal mode deviates from the reduced-order model by about 6%–16% with a consistent trend, and that the critical clearing time with current limiting is about 17%–24% lower than the reduced-order upper-bound estimate, indicating that the reduced-order model is suitable for trend assessment and preliminary tuning, while the transient margin must be corrected for current-limiting conditions.
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