Research On The Flywheel Energy Stroage System Used In The DC Micro Grid

The flywheel energy storage system(FESS) is a novel electromechanical energy conversion and storage device, whose advantages are long service life, high conversion efficiency, high adaptability and pollution-free, etc.. FESS has enormous application foreground at the aspects of aeronautics and astronautics, distributed generation, power system peak load regulation and electrical vehicles. This article is focused on the FESS used in the DC micro grid. The research are mainly about the design of the FESS machine, design of the FESS power converter、the FESS machine losses caculation, and the energy management of the PV-FESS DC micro-grid, etc..Based on the application and design requirements of energy storage system, the external rotor ironless brushless DC machine(BLDCM), which is suitable for use in FESS, is designed and further explored. The finite element method is adopted to do computational research on external rotor ironless BLDCM. Performance improvement and loss minimization of the machine are achieved through o ptimizing the magnetic pole and the winding. The research results show that the copper loss of the machine can be reduced by appropriate selection of magnetic steel thickness even under the condition that the main parameter of the flywheel machine is set. Proper choice of magnetic steel polar-arc coefficient is beneficial to reduce the flux leakage on the surface of the rotor. The width of air gap flux density can be increased by applicable selection of magnetic poles and magnetizing style. The flywheel energy storage machine prototype is fabricated based on the optimized design of the machine. The experiment results show that the test results almost coincided with the design values.According to the characteristics of the flywheel machine and the performance requirements of the FESS, the FESS power converter is designed and further researched. Firstly, the converter topology is selected on the basis of comparison of applicable topologies of FESS converters, which are chosen based on the selection of flywheel machine. Secondly, the operating principles and control strategies in different working modes(energy storage mode and energy release mode) are analyzed. Then, the equivalent circuit model of the converter is built, selection of converter main parameters is discussed in detail. At last, the simulation model of the FESS is built to verify the design.The losses of flywheel energy storage machine is explored by computational method. Ironless BLDCM losses mainly consist of stator winding copper losses and rotor eddy current losses. In terms of stator winding copper losses, the effects of winding diameter, current and machine speed etc. on winding AC copper loss are analyzed. The winding circulating current equivalent calculation model is built, circulating current loss is calculated by simplified 3D finite element method; The winding rebuilding method to reduce circulating current loss is explored. In view of rotor eddy current loss, the co-simulation method is adopted to calculate permanent magnet eddy current loss and iron core eddy current loss under FESS charging mode, FESS discharging mode and standby mode, respectively. The research results show that the winding diameter has a huge effect on the winding AC loss in ironless BLDCM. The change of winding current has small effect on windi ng AC loss. The shunt wound structure with multiple wires method can reduce winding AC loss effectively. However, due to induced voltage and impedance unbalance, additional circulating current loss would be induced. Winding rebuilding method can effectively reduce additional circulating current loss.According to the application of FESS, a DC microgrid is constructed mainly based on photovoltaic power generation system, flywheel energy storage system array and local load. The unit coordination control strat egy and energy management technology of each subsystem are analyzed, the cooperative control method of flywheel energy storage system arrays is also explored. The DC microgrid operating mode switch principle and system energy management strategy are gained through analyzing DC microgrid effective steady state operating mode and DC microgrid subsystem working mode. The feasibility of each subsystem control and entire energy management strategy are verified by simulation.