Transient Stability Analysis of Grid-Following/Grid-Forming Hybrid Converter Systems Considering Capacity Ratio

Abstract

Hybrid systems comprising grid-following voltage-source converters (GFL-VSCs) and grid-forming voltage-source converters (GFM-VSCs) are increasingly adopted for renewable-energy integration. However, the influence of installed-capacity allocation on their coupled transient synchronization stability remains insufficiently characterized. This paper develops a capacity-ratio-dependent nonlinear transient model under a fixed total installed capacity. A synchronous-machine-like analytical framework is established to elucidate how the capacity ratio modifies GFL current injection, GFM power support, and current-limiting behavior, as well as the electrical coupling between the two converters. The critical clearing time (CCT) is employed as a quantitative measure of transient stability, and the reduced-order model is validated against a detailed MATLAB/Simulink model under three-phase and two-phase-to-ground faults. In addition, the sensitivities to key system and control parameters are investigated. The results demonstrate that transient stability varies non-monotonically with the capacity ratio. A low GFL capacity limits its fault-current contribution and alters the coupling dynamics, whereas a low GFM capacity weakens voltage-forming support and reduces the available margin before current limitation becomes dominant. The proposed analysis provides a quantitative basis for capacity allocation and stability-oriented design of hybrid converter-based power systems.