Research On Low Voltage Ride Through Technology In Doubly Fed Induction Wind Power Generation System

With the continuous increase in the ratio of wind power connected to the grid,the impact on the power system cannot be ignored.Therefore,the requirements for the grid connected operation of wind power are more and more stringent.And the modern grid requires that the wind power generators must have low voltage ride through(LVRT)capability.That is to say,when the grid voltage undergoes a certain degree of sags,the wind turbine must remain be connected to the grid and could safely ride through the faults.In this paper,it researched on how to improve low voltage ride through technology based on the double fed wind power generation system.It has good performance under normal grid conditions with the traditional vector control.However,when the voltage of the grid voltage falls suddenly,the transient attenuation process lasts for a long time,and it will result in over-voltage and over-current on the rotor side,which seriously threatens the safe operation of the converter and the wind turbine.In order to study the low voltage ride through characteristics of doubly fed wind power generation system under the condition of grid voltage dip,several typical LVRT methods are analyzed.It is concluded that these methods are all implemented by controlling the rotor current command value.To accelerate the transient attenuation of the flux linkage,the compensation term of the transient component of the stator flux is added on the basis of the current loop reference value to improve the under-damping characteristics of the system.Considering that the PI controller's ability to adjust the AC quantity is limited in the method,the current feed forward compensation term is introduced to correct the PI controller and an improved damping control method is then proposed.The improved damping control method effectively limits the rotor terminal voltage,shortens the attenuation of the stator transient flux,and enhances the fault ride through capability of the DFIG.Aiming at the phenomenon of rotor-side converter overvoltage and slow transient attenuation in the grid voltage symmetric faults,a virtual transient flux linkage control method is introduced.Compared with the traditional demagnetization control method,there is no need for complicated observation technology.Compared with the virtual damping flux linkage control method,it does not need fault detection and mode switching device.The structure is simple and easy to implement,and it accelerates the attenuation of the transient flux linkage and improves the low voltage ride through capability of the DFIG.Experimental part introduces the hardware experimental platform and software program design of doubly fed wind generators.The power circuit of DSP core board hardware,AD sampling circuit,orthogonal coding circuit and IPM drive module are mainly analyzed in the hardware part.And the main program,PWM interrupt service routines,duty cycle calculation,sampling interrupts,and SCI communication interrupts are described in the software section.Finally,the vector control method is run on the experimental platform.