The Research Of Flywheel Battery Improves Gridconnected Performance Of Direct-drive Permanent Magnet Wind Power Generation System

The increase of wind power integration scale will have a significant impact on powernetwork, so wind farm was hoped to have the same operation ability as conventional powerfield. On the one hand wind power fluctuation needed to be restrained to ensure the qualityof power network, on the other hand wind turbine should survive when grid network hasinstantaneous voltage sag. In order to achieve these two goals, the main contents are asfollow:By studying the characteristics and operation mechanism of wind turbine andpermanent magnet synchronous generator, simulation model of D-PMSWG system wasbuilded using simulink toolbox of matlab. The simulation model includes wind turbinemodel, permanent magnet synchronous generator model, flywheel battery model, convertermodel, control system model and the corresponding drive system model. SVPWM controlstrategy was used in generator-side rectifier and grid-side inverter to control PMSG. Inorder to capture the maximum wind energy, a double closed ring control strategy of innerelectric current ring and outer power ring is introduced in generator-side. In order to adjustthe reactive power inputs according the demand of grid network, a double closed ringcontrol strategy of inner electric current ring and outer DC voltage ring is introduced ingrid-side. And a double closed ring control strategy of inner torque electric current ring andouter DC voltage ring is introduced to meet the demands of switching between chargingand discharging state and maintaining the stability of DC voltage in flywheel battery.According to the introduced control strategy and simulation model, the simulationabout maximum wind power tracking under a certain wind speed was carried on in thesimulink toolbox. The result shows that using the introduced control strategy can quicklyachieve wind power tracking and the flexibility of simulation result is verified.Due to the charactristics that specific frequency power fluctuations will cause systemfrequency deviation, the wind power should be restrained. In order to achieve this goal, theevaluation index of wind power fluctuation and the calculation function of flywheel batterycapacity are built. A second-order high-pass filter was constructed using Chebyshevapproximation method in matlab. At last, through a simulation example verified thesmoothing effect to wind power and the flywheel battery capacity are obtained.In order to research the effect in lifting LVRT performance of adding flywheel batteryinto D-PMSWG system, the control strategy of LVRT in the three-phase symmetric grid voltage sag was introduced. In order to make wind farm ride through the condition of gridvoltage sag, wind turbine generator’s output power was reduced by decreasing windturbine generator’s torque current component in generator-side. At the same time flywheelgenerator’s torque current component was increased to make flywheel battery restore alarge number of active powers in flywheel battery-side. The active powers that flowthrough inverter was reduced by decreasing the instruction value of active powers, soinverter have enough capacity to transfer a large number of reactive power. This providesreactive power support for the recovery of the grid voltage. Based on the control strategy acomparison was done between the two simulation results with and without flywheel batteryin the system. The comparison result shows that the flywheel battery certainly improvesthe LVRT performance of the system.