The performance of hydropower plants (HPPs) is significantly influenced by the flow regimes of their respective river basins. In Vietnam, these flow regimes vary significantly across different river basins, and the sequence of flow events in each basin occurs at different times, creating a phase difference. Traditionally, the firm power of hydropower plants is calculated assuming independent operation, based solely on the flow of the individual river. This approach can lead to extreme scenarios in the power system when using hydropower for balancing purposes because it assumes that hydrological conditions with the same sequence occur simultaneously at all hydropower plants. In this paper, we propose a new calculation method to assess the impact of phase differences between river basin flows on the ability of hydropower sources to participate in the power system. By applying this method to 20 hydropower plants located in seven major river basins in Vietnam, the study demonstrates the substantial effects of these phase differences.