In electric power systems delivering alternating current, it is essential to maintain its synchrony of the phase with the rated frequency. The synchronization stability that quantifies how well the power-grid system recovers its synchrony against such perturbation depends on various factors. As an intrinsic factor that we can design and control, the transmission capacity of the power grid affects the synchronization stability. Therefore, the transition pattern of the synchronization stability with the different levels of transmission capacity against external perturbation provides the stereoscopic perspective to understand the synchronization behavior of power grids. In this study, we extensively investigate the factors affecting the synchronization stability transition by using the concept of basin stability as a function of the transmission capacity. For a systematic approach, we introduce the integrated basin instability, which literally adds up the instability values as the transmission capacity increases. We first take simple 5-node motifs as a case study of building blocks of power grids, and a more realistic IEEE 24-bus model to highlight the complexity of decisive factors. We find that both structural properties such as gate keepers in network topology and dynamical properties such as large power input/output at nodes cause synchronization instability. The results suggest that evenly distributed power generation and avoidance of bottlenecks can improve the overall synchronization stability of power-grid systems.In modern society, power grids play an essential role as one of the most important infrastructures by providing electrical energy. As the structure and the operation strategy of power grids are becoming more complex, designing and managing the power grids for stable electricity supply are also becoming a harder challenge. The problem of course belongs to the field of electrical engineering with all of the complicated practical matters, but there have been significant endeavors to analyze it with only the most essential ingredients, starting from arguably the simplest one: the topology of power grids as mathematical objects represented by graphs or networks. Initiated solely from this structural aspect, the past decades have witnessed the drastic advancement in the field of power-grid research, powered by the theoretical and practical tools of network science combined with nonlinear dynamics. One of the most quintessential approaches is the stability analysis of synchronization among power-grid nodes. It is known that the transmission capacity affects the synchronization stability. In the light of the fluctuating transmission capacity in real power grids, we investigate what structural and dynamical factors make the dynamic stability of power grids resilient over a range of transmission capacity. a) hkim@utalca.cl b) mj.mijin.