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Role of External Grid Representations in Frequency Stability Studies of Power Systems with High Non-Synchronous Generation

I. Taczi(1), I. Vokony(1), G. M Peter(2)

1. Department of Electric Power Engineering, Budapest University of Technology and Economics, Budapest, (Hungary)

2. E.ON DSO Hungary, Gyor, (Hungary)

2026-06-27

The transition toward power systems with high shares of non-synchronous generation has made the quality of dynamic system modeling increasingly important for credible stability assessment. Particular sensitivity arises at the boundaries of the study area, where the external grid is commonly represented by reduced or equivalent models rather than by a full dynamic description of the interconnected system. This manuscript addresses the role of external grid representations in frequency stability studies, with emphasis on the physical meaning of the equivalent, its parameters, and the consequences of parameter uncertainty. The discussion is centered on frequency stability, while its relation to other calculation types and stability classes is also clarified. A critical review of literature is used to show that the influence of external grid representation is recognized, but still insufficiently structured for present-day low-inertia systems. A simulation-supported case study based on a Hungarian islanded microgrid model and the IEEE 9-bus system is then used to illustrate how external-grid inertia assumptions affect the frequency nadir. The manuscript shows that the external grid can no longer be treated as a purely technical closure of the model. In low-inertia conditions, it becomes an active determinant of the computed dynamic behavior, and its parameterization must therefore be treated as a modeling problem of first-order importance.

Key words: Frequency stability, external grid equivalent, non-synchronous generation, low-inertia power systems

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