Research On Operation Analysis And Overvoltage Suppression Strategy Of DFIG-Based Wind Power System Connected To Sending AC Grid Of HVDC System Under Commutation Failure

Large scale development of wind power will help China achieve carbon peaking and carbon neutrality.The wind power resources and power load in China have the characteristics of reverse distribution.The dominant form of wind power resource development in China is to build large-scale wind farms and then transmit power over a long distance.Ultra high voltage direct current system(UHVDC)transmission technology has many advantages,such as high voltage level,large transmission capacity and small transmission loss.Therefore,the UHVDC transmission technology has become the main cross-regional long-distance transmission technology in China.The increasing capacity of wind power also brings new challenges to the power grid.The high proportion of wind power connected to the sending alternating current(AC)grid presents a new characteristic of voltage amplitude first decreases then increases,which is continuously changing under commutation failure of the UHVDC system.The voltage amplitude fluctuation of the sending AC gird poses new requirements for wind turbines connected to the grid.Especially the overvoltage of the sending AC gird under commutation failure seriously threatens the grid connection capacity of wind turbines.Besides,the overvoltage of the sending AC gird limits the transmission capacity of the UHVDC system,and restricts the ability of consume wind power across regions through UHVDC system.In view of the above problems,this paper studies the mechanism of over-voltage caused by commutation failure and the improvement strategy of wind power system to suppress overvoltage under commutation failure.The main conclusions and innovations of this paper are summarized as follows:1.There is an overvoltage problem in the sending AC grid of the UHVDC transmission system under commutation failure.Based on the piecewise linearization of DC current in UHVDC system under commutation failure,a mathematical model is established to quantitatively describe the voltage amplitude fluctuation characteristics of the sending AC grid under commutation failure.A calculation method for calculating the peak overvoltage of the sending AC grid under commutation failure is proposed in this paper,which taking into account the transmission capacity of the UHVDC system and the grid strength of the sending AC grid.Further,the quantitative relationship between the reactive power of the wind farms connected to the sending AC grid and the overvoltage of the sending AC grid under commutation failure is established.Based on the relationship between the peak overvoltage of the sending AC grid under commutation failure and the transmission capacity of the UHVDC system,this paper quantitatively analyzes the effect of the increment of reactive power absorbed by the wind farms of the sending AC grid at the overvoltage stage on the transmission capacity of the UHVDC system.2.For the doubly fed wind power generation system(WPGS)connected to the sending AC grid,the transient mathematical model of the doubly fed WPGS connected to the sending AC grid under commutation failure is established.Several improved control strategies for suppressing the overvoltage of the sending AC grid under commutation failure is proposed from three aspects of control parameter optimization,voltage detection delay compensation and power command coordination optimization of the WPGS.A transient mathematical model of double-fed WPGS is established,which is suitable for commutation failure analysis and takes into account Crowbar circuit.A parameter optimization strategy for suppressing the overvoltage of the sending AC grid under commutation failure is proposed based on the mathematical model.Based on the analysis of the grid voltage detection link in doubly fed WPGS,an improved strategy of second order discrete differential is proposed to completely compensate the delay of voltage detection,which can decrease the grid overvoltage of the sending AC grid.Considering the limitation of the maximum stator current,maximum rotor current and the maximum current of GSC converter,an optimal power coordinated control strategy of power command for RSC and GSC in doubly fed WPGS is proposed to increase the reactive power absorbed by the WPGS at the overvoltage stage.The simulation and experiment results show that the control parameter optimization,voltage detection delay compensation and power command coordination optimization of the doubly fed WPGS can effectively suppress the overvoltage of the sending AC grid under commutation failure.The proposed strategies improve the security and stability margin of the sending AC grid,and also increases the maximum transmission capacity of the UHVDC system.3.Several strategies for suppressing the overvoltage of the sending AC grid under commutation failure are proposed from two dimensions: the improvement of doubly fed induction generator(DFIG)with virtual synchronous generator(VSG)control and the power distribution for multiple wind farms.A transient mathematical model of the DFIG with VSG control under grid voltage fault is established in this paper.Based on the analysis of the influence of the grid voltage detection delay on the reactive output of the DFIG with VSG control.In this paper,an improved VSG control strategy of adding voltage amplitude coupling compensation term to compensate virtual internal potential in reactive power loop is proposed.A simulation model of the sending AC grid with 100% renewable energy penetration is built to verify the effectiveness of the improved VSG control strategy to suppress the overvoltage of the sending AC grid under commutation failure.Further research has been carried out for the scenario that multiple wind farms are connected to the sending AC grid and transmit power through the UHVDC system.Considering the constraints of wind farm capacity and active power on the absorption of reactive power by wind farms,based on the construction of Kuhn-Tuck function,a power distribution method for multiple wind farms is proposed to suppress the overvoltage of the sending AC grid under commutation failure.The proposed method maximizes the total reactive power absorption capacity of the wind farms of the sending AC grid.Therefore,the overvoltage of the sending AC grid under commutation failure is suppressed and the voltage stability margin of the sending AC grid is improved.