Low Voltage Ride Through Control Of Wind Power System Based On Variable Frequency Transformer

Squirrel cage induction generator(SCIG) is variable speed constant frequency(VSCF) wind power generation system, which with characteristics of simple structure, low cost, low fault and less maintenance,but its capability of Low Voltage Ride Through(LVRT) is poor. Doubly fed induction generator system(DFIG) is the mainstream of VSCF wind power generation system,which with characteristics of wide range of speed and high efficiency, could realize independent control of active and reactive power.Since the stator windings are directly connected to the grid and the control ability of small capacity excitation converter is limited,DFIG system are vulnerable to grid voltage sags fault.With the increased installation scale of wind turbines, and the characteristics of the wind turbines while faults occur become more and more significant to the grid. Therefore,it is very important to enhance the LVRT capability of wind turbines during grid voltage sags fault.The traditional vector control are designed by Third-order model,which ignored the transient excitation current of the stator,the control effect is poor during grid voltage sags fault. In order to enhance the LVRT capability of DFIG system during grid voltage sags fault, establishing the mathematical model with considering the transient excitation current of the stator. From the viewpoint of magnetic linkage in the case of a three-phase short-circuit fault occurring at the terminal of DFIG,the analytical expression of the short-circuit rotor current were deduced together with the estimated value.With the short current and DC bus withstand voltage, the series resistance range were deduced in Crowbar circuit. A study on the LVRT effect of DFIG at different quitting time and resistance of Crowbar in Matlab/Simulink simulation platform.Simulation results show that:in order to avoid Crowbar circuit again operation when grid voltage recovery,the scheme can be taken until the grid fault is cleared off before removing the Crowbar circuit;Selecting the larger resistance in the setting range helps to accelerate transient current decay and enhance the LVRT capability of DFIG.A new LVRT control scheme was proposed for SCIG wind turbine by adding a variable frequency transformer (VFT) between SCIG and grid to enhance the LVRT capability of SCIG.With analyzing the mathematical model of SCIG, the cause of the instability of rotational speed and terminal voltage of SCIG was studied during grid voltage sags fault. The rotary speed of VFT was controlled to regulate the frequency of SCIG during LVRT. The acceleration of wind turbine was decreased and the slip ratio of SCIG was limit below the critical value to realize LVRT by controlling the pitch angle. The simulation results show that the LVRT performance of SCIG wind turbine can be further enhanced with the coordination control scheme of rotary speed of VFT and pitch angle of wind turbine during grid voltage sags fault.By analyzing the characteristics of DFIG system during the operation of crowbar protection circuit. With the increase of the slip of DFIG,the vast grid reactive power would be absorbed by the crowbar-based DFIG wind turbine when grid voltage sags fault. A new LVRT control scheme was proposed for DFIG wind turbine by adding a VFT between DFIG and grid. With Matlab/Simulink simulation platform, the simulation results show that the amount of reactive power absorbed by DFIG system from grid can be decreased and the slip ratio of DFIG can be limit below the critical value with controlling the rotary speed of VFT during grid voltage sags fault, which contributes to the recovery of the grid voltage and enhances the LVRT performance of DFIG wind turbine.