Optimal Scheduling of Electric Vehicles in Hybrid Alternating Current/Direct Current Charing Stations Considering Three-Phase Unbalance Constraints

Inhalt:
Plug-in electric vehicles (EVs) is growing rapidly nowadays. Uncoordinated charging of a large number of EVs challenges the safe and reliable operation of electric power distribution networks. In this paper, an optimal scheduling strategy is proposed for EVs in hybrid AC/DC charging stations taking into account the possible three-phase load unbalance caused by the unequal three-phase charging power brought by the AC charging piles at the charging station. A global scheduling scheme is firstly formulated to track the reference power curve set by the upper-level power grid, while fulfilling EV owners’ charging requirement, guaranteeing safe operation of distribution transformers and reducing three-phase load unbalance degree. Both slow AC charging piles and fast DC charging piles are integrated and the three-phase current unbalance factor is transformed into an equivalent format using three-phase load power. Then a rolling scheduling scheme is proposed to cope with random arrival of EVs in realistic situations. The multi-period rolling scheduling model is dynamically updated and solved at the beginning of each period such that the requirements of currently charged EVs are always satisfied. Simulations show that the proposed real-time rolling scheduling scheme provides close results compared with those obtained by the global scheduling strategy. The total load curve is flattened and the load unbalance among three phases is greatly reduced compared to scenarios where EVs are charged without coordination.