The high proportion of new energy access has attracted more attention to power system flexibility. Accurately quantifying the flexibility adjustment demand caused by the uncertain output of new energy and reasonably evaluating and improving the flexibility balance capacity of the system are necessary measures to improve the system's acceptance capacity of new energy.A novel modeling methodology is proposed to characterize renewable energy uncertainty

where flexibility demand is determined based on confidence intervals of forecasting errors.Concurrently

flexibility supply capacities of conventional units are systematically modeled.Comprehensive flexibility evaluation indices are developed from the perspective of flexibility supply-demand equilibrium.Then

an optimal scheduling model incorporating flexibility requirements is constructed

and the proposed index is applied to evaluate the current insufficient flexibility of the system.Finally

the flexible transformation path of thermal power and the future development of new energy installed capacity are simulated to verify the role of flexible transformation of units in improving flexibility.