Principle And Control Technology For A Flexible Power Conditioner Based On Flywheel Energy Storage

Flexible Power Conditioner (FPC) is a large-capacity flywheel energy storage system. It consists of an energy storage machine and a voltage-source pulse width modulation (PWM) rectifier-inverter used as an AC exciter. Similar to that of the Superconducting Magnetic Energy Storage (SMES), the FPC can realize an independent active and reactive power control, so it can be used to improve the stability of power system as well as the quality of power supply. Additional advantage of the FPC over the SMES based device is that it is easy to develop and to operate. Therefore, the cost will be greatly reduced. The principle, modeling, control, simulation, development of the prototype for the FPC have been discussed in this dissertation.Based on the static equivalent-circuit of the FPC, the influence of the rotor AC excition on the operation of the FPC is analyzed. Then, the operating states of the FPC and the energy flow in all above states are studied. In order to solve the deficiency of the scalar analytical method in the practical control of the FPC, mathematic models of the energy storage machine in several coordinates are deduced. By the coordinate transformation, the multi-variable, high-order, nonlinear and close coupled machine model in three phase static coordinates is simplified in the synchronous rotary coordinates. .The mathematic models and control strategies of grid-side converter and rotor-side converter are investigated. Considerring the fact that the FPC might be operated in grid disturbance conditions, an improved stator flux linkage orientated control strategy is proposed. This strategy is robust for the grid voltage fluctuation and easy to realize. In addition, the rotor speed control strategy of the FPC and the synthesis schemes of the excitation system are discussed.The operational characteristics of an FPC are investigated by time domain simulations. The effectiveness of the operating principle and the proposed excitation control strategies of the FPC are verified. The effect of rotor current, voltage and power limitation on the power margin of the FPC is analyzed. It was found that power margin of the FPC is mainly determined by the rotor-side excitation current. Besides, the FPC excitation characteristics, the FPC control system stability and dynamic performance and the choice of the FPC steady operating point are also discussed in this chapter.Two kinds of start methods and corresponding control strategy are proposed. In addition, the potential special operating states of VSC converters are studied. For this basis, the ride-through control of the FPC during grid faults is proposed.A two-machine and infinite bus power system model including a synchronous generator and an FPC is established. The damping characteristic of the proposed system is analyzed by eigenvalue method. Research results indicate that the total damping of power system, especially the damping of the electromechanical mode, is increased with the FPC. And the ability to coordinate damping is enhanced by the FPC. As FPC can realize voltage control and damping control simutaniously, it can offer better control performance in multiple objective coordinated control of power system. Also, both the transient state stability and the small disturbance stability of power system can be improved by the FPC.The details of the development for a 10kW prototype FPC are given, which includes the main body of the energy storage machine, the frequency-converter with excitation control system and the real-time supervisory system of the prototype. Besides, the experiment test results of the prototype FPC are given in this chapter.