Direct power control method for grid-connected flywheel array energy storage system

Flywheel energy storage system (FESS) can be used in reducing the negative effects caused by output fluctuation of renewable energy. But due to the confinement from rotor material, motor and unit building cost, FESS with a single flywheel module could not achieve the required MW power level so as to function properly in the power grid. By connecting the flywheel modules in parallel, a flywheel array energy storage system (FAESS) is built up to achieve higher power and energy capacity whilst keeping the cost at an acceptable level. The main contribution of this paper is to have proposed an optimized direct power control method for the FAESS to meet the demand from power grid. The proposed two-level direct power control method for FAESS consists of power allocation criterion and grid-connected control scheme. The power allocation criterion assigns the power command to each flywheel module according to the equal incremental principle that have considered all the main constrains and achieved the minimum total loss of grid-connected operation of the FAESS. The grid-connected control scheme of each flywheel module follows a three stage procedure in order to realize power transfer between grid and FAESS. The proposed two-level direct power control method is verified through simulation based on actual parameters of a 100kW FAESS prototype. The experimental results are also given to verify the grid-connected control scheme.