Abstract:  A dual current closed-loop control method for grid-connected flywheel energy storage system is proposed. To track the given sinusoidal current and simultaneously improve system stability and power quality， proportional resonant (PR) controller is introduced into the grid-side control loop and flywheel-side control loop. Capacitor current inner-loop feedback control can suppress the LCL filter resonance peak and improve the power factor. At charge stage， the control of grid-side converter works on an outer voltage loop， while the flywheel-side converter adopts an outer speed loop. At standby and grid-connected stage， the control of grid-side converter adopts direct power control strategy based on a grid-side current loop cascaded with an inner capacitor current loop；the control of flywheel-side converter is based on an outer DC-bus voltage loop cascaded with an internal current loop， and maintains the DC-bus voltage. The parameter of grid-side controller is designed using the generalized root locus method. The grid-connected control model of the flywheel energy storage system is built， and simulation results validate the effectiveness of the control strategy.

Key words: flywheel energy storage system, proportional resonant controller, capacitive current feedback, LCL filter, generalized root locus