In conventional hybrid energy storage systems, two storage units complement each other. One
low-capacity and fast-response unit as a power supplier,
and one high-capacity and low-response unit as an energy
supplier. The power supplier mitigates fast fluctuations in
generation or demand by transferring energy over seconds
or minutes, and the energy supplier transfers energy over
hours for managing energy. According to this concept,
this paper presents a new model of hybrid energy storage
systems, where three energy suppliers are considered as a
three-level hybrid energy storage system. Energy storage at
level 1 shifts energy from off-peak (or low-cost) hours to the
on-peak (or high-cost) hours during one day, the storage
unit at level 2 transfers energy from off-peak (or low-cost)
days to the on-peak (or high-cost) days for the period of one
week, and level 3 transfers energy from off-peak seasons
to the on-peak seasons through one year. The proposed
planning results in a large-scale optimization programming
that optimizes large numbers of design variables at the
same time. In order to increase the flexibility of the planning, the initial energy of the storage units is also modeled
as a design variable and optimized. The uncertainty of
loads is modeled and a stochastic planning is carried out
to solve the problem. The introduced three-level hybrid
energy storage planning is simulated on two test systems,
and the results demonstrate that the proposed planning
can reduce the planning cost by about 1.8%.