Research On The Low Voltage Ride Through Of Permanent Magnet Direct Drive Wind Turbine Based On Super Capacitor Energy Storage

With the increasing of global installed capacity of wind power,grid-connected of wind power generation has more and more influence on the stability of power system.In order to ensure the safety and stability of power system,many countries have developed wind power grid-connected standards,requiring wind turbine with low voltage ride through capability.In order to solve the problem of low voltage ride through of the permanent magnet synchronous generator(PMSG),this paper proposes a hybrid energy storage method which can increase the rotor speed and increase the super capacitor energy storage to solve the Problem of PMSG low voltage ride through in low voltage fault of power.Unlike the DFIG,PMSG has no gearbox device,and the mechanical stability of the system is far superior to that of the DFIG.More and more researches have been made on the PMSG in recent years.Low-voltage fault on power grid in the daily operation and maintenance of wind turbines is the more common fault,and because PMSG has a relatively simple structure,it has obvious advantages in low voltage ride through compared to the DFIG.This paper chooses PMSG as a research object to do some intensive study.The main contents and results are as follows:1.Introduce the main components of wind turbine;establish the mathematical model of the PMSG system,which includes wind turbine mathematical model,the PMSG model,the mathematical model of generator side converter and grid side converter,and the working principle of super capacitor and equivalent circuit model.2.Analyze the control strategy of the converter of generator side and grid side,and improve the control of converter.Use zero d axis current control based on rotor field orientation,on the basis of this control strategy,the q axis current control is improved.When the grid side is fault,it can calculate a more accurate side of the active power reference value with referencing to the grid voltage drop and the normal working voltage combined with the wind speed and pitch angle,thus to control the wind turbine more precisely.The control strategy of the grid side converter is the voltage and current double loop control strategy based on the grid voltage orientation,and the reactive power compensation operation control of grid side converter.Through the decoupling control of voltage and current,when the power grid fault,reference value of active and reactive current recalculated by the double loop control,change the output power factor of grid side converter,so as to change the working state of the converter and realize the reactive power compensation.3.Focus on the energy storage unit control,analyze the energy flow when system failure,decompose the output power of the grid side converter,to control the output voltage of the converter to control the output power,establish the control model of the energy storage unit,analyze charge and discharge status of energy storage circuit,Optimal control strategy.Calculate the energy storage capacity of the generator rotor and the calculation formula of the capacity of the super capacitor used as the energy storage unit,in order to lay the foundation for the use of energy storage device during the process of LVRT.4.Focus on the simulation analysis of LVRT based on composite energy storage of DC side,modeling and simulating the modified control strategy proposed in this paper.Compare with the traditional Crowbar circuit control strategy through simulating and analyze the simulation waveforms.Comparing the DC voltage,the output power of grid side,the active and reactive power support in the process of voltage recovery,the simulation results show that the proposed scheme can stabilize the voltage better and ensure the safe and stable operation of the whole wind power generation system in the event of voltage drop fault.