Study On Dynamic Voltage Restorer Based On Flywheel Energy Storage System

Dynamic voltage regulator (DVR) is a member of flexible AC transmission system; it can solve the voltage stability problem of power system effectively because of its dynamic performance and small capacitance. Flywheel energy storage (FES) is an advantageous type of late-model energy storage technology. As a pure electromechanical storage system, it has the advantage of high specific energy and specific power, non-pollution, long cycle life, has a great development in a short time. At present, flywheel energy storage technology has been successfully applied in many fields, such as uninterruptible power supply systems, power systems, and hybrid electric vehicles, etc. The performance of DVR can be improved by using FES as the energy storage device.When flywheel energy storage works in the charging mode, the flywheel motor runs as an electro motor. In discharge mode, the flywheel motor runs as a generator. This paper uses the permanent magnet synchronous motor as flywheel motor. In charge mode, the three-phase power grid's voltage is transformed into a stable DC voltage by PWM rectifier circuit. Then the motor is accelerated by the inverter. In discharge mode, the AC voltage generated by the generator is transformed into DC voltage by rectifier circuit.Compared to classical four-wire three-leg with split capacitor topology, the four-wire four-leg topology has the advantage of higher DC voltage utilization ratio, smaller capacitor used on DC side, and not need additional control to the voltage value of two DC capacitors. In this paper, four-wire four-leg inverter topology is applied to the main circuit of the dynamic voltage restorer. DVR with four-wire four-leg structure can compensate unbalanced voltage. It also can control the each phase's output voltage independently.The fault voltage detection method based on dq transformation can extract the positive sequency component of the system voltage. Then standard compensation voltage signal can be calculated. However, the using of low-pass filters leads a little time delay in this algorithm. When the system voltage doesn't contain harmonic component, a modified method can be used. This algorithm has no delay and has better performance.In this thesis, controllers are designed in every part of the system. Thereinto, the inverse system methods and sliding variable structure control theory are employed to improve the control system's performance. In order to verify the validity of this method, some simulations are preceeded in MATLAB/SIMULINK.