  In recent years, electric vehicle (EV) charging stations have witnessed a
rapid growth. However, effective management of charging stations is challenging
due to individual EV owners' privacy concerns, competing interests of different
stations, and the coupling distribution network constraints. To cope with this
challenge, this paper proposes a two-stage scheme. In the first stage, the
aggregate EV power flexibility region is derived by solving an optimization
problem. We prove that any trajectory within the obtained region corresponds to
at least one feasible EV dispatch strategy. By submitting this flexibility
region instead of the detailed EV data to the charging station operator, EV
owners' privacy can be preserved and the computational burden can be reduced.
In the second stage, a distributed coordination mechanism with a clear physical
interpretation is developed with consideration of AC power flow constraints. We
prove that the proposed mechanism is guaranteed to converge to the centralized
optimum. Case studies validate the theoretical results. Comprehensive
performance comparisons are carried out to demonstrate the advantages of the
proposed scheme.
