Online Hierarchical Aggregate Power Flexibility Characterization of EV Charging Stations with Disaggregation Feasibility Guarantee

With the increasing integration of electric vehicles (EVs), there is a growing recognition of the contribution of EVs to the flexibility of distribution networks. However, quantitative methods for precisely assessing the flexibility that EVs can provide to the upstream power grid have not been fully explored. To fill the research gap, this paper proposes an online hierarchical aggregate flexibility characterization method, which considers the distinct roles of distribution system operator (DSO) and charging station operators (CSOs). Under this framework, each CSO first characterizes the aggregate EV power flexibility of its charging station as a time-decoupled flexibility region. We prove that any trajectory within the region can be disaggregated into feasible individual EV charging strategies. Upon receiving the aggregate EV power flexibility regions submitted by all CSOs, the DSO solves an optimization problem to derive the aggregate power flexibility region of the distribution network, and we also prove that any power level within this region can be disaggregated into a feasible solution satisfying the network constraints. Furthermore, to account for the impact of the latest dispatch order on future EV power flexibility, real-time feedback is designed and integrated into the hierarchical framework. Numerical experiments on a multi-phase distribution network with 123 buses demonstrate the effectiveness and advantages of the proposed method. It is worth noting that the proposed online method outperforms the offline benchmark with perfect information, due to the consideration of dispatch signals.