Capacity Aggregation and Online Control of Clustered Energy Storage Units

With the growing penetration of renewable energy and gradual retirement of thermal generators, energy storage is expected to provide flexibility and regulation services in future power systems. Battery is a major form of energy storage at the demand side. To better exploit the flexibility potential of massive distributed battery energy storage units, they can be aggregated and thus get enough capacity to participate in auxiliary service markets or receive direct orders from the power system operator. This paper proposes an analytical method to determine the aggregate MW-MWh capacity of clustered energy storage units controlled by an aggregator. Upon receiving the gross dispatch order, a capacity-aware water-filling policy is developed to allocate the dispatched power among individual energy storage units, which is called disaggregation. The policy endeavors to track the dispatch order while reducing the difference among the storage level of each battery. It provides effective real-time power control strategies for a particular class of energy management problem without referring to the prediction of dispatch order, although storage operation must respect inter-temporal constraints. Case studies validate the proposed method through applications in wind power ramping alleviation and frequency regulation.