Anlysis And Suppression On Sub-synchronous Oscillation Of Voltage Source Doubly-fed Wind Turbine

With the rapid development of clean and pollution-free wind power generation,the installed capacity of doubly-fed induction generator(DFIG)based wind turbines has been increasing in recent years.In order to increase transmission capacity of the transmission line,the compensation capacitors are often connected in series to the grid,and the sub-synchronous oscillation(SSO)problems caused by it is becoming more and more serious.The control technology of voltage source DFIG-based wind turbines has also made great progress in recent years.In this paper,it is divided into direct voltage virtual synchronous generator(VSG)control and indirect voltage virtual synchronous generator control according to whether there is voltage and current inner loop.Based on the impedance analysis method,this paper studies the SSO problems of the voltage source DFIG wind turbine in the series compensation grid.The main contents include the output impedance modeling of the voltage source DFIG-based wind turbine,the analysis of the grid connection stability in the series compensation grid and the research of the SSO suppression strategy.This paper first establishes the impedance model of the DFIG-based wind turbine controlled by the direct voltage VSG,and analyzes the stability of the DFIG-based wind turbine in the series compensation grid.The closed-loop output impedance model of direct voltage VSG control is established based on the mathematical model of DFIG in combination with the VSG control in dq frame,and the influence of control parameters on the closed-loop output impedance characteristics is analyzed.For the DFIG-based wind turbines controlled by direct voltage VSG in the series compensation grid,the influence of series compensation degree,inertia time constant,damping time constant and rotor speed on the stability of DFIG grid-connected system is analyzed by the generalized Nyquist criterion(GNC).Since the DFIG-based wind turbine controlled by the indirect voltage VSG has a voltage-current inner loop,the impedance characteristics are very different from the DFIG-based wind turbine controlled by the direct voltage VSG.Therefore,this paper establishes the impedance model of DFIG-based wind turbine controlled by indirect voltage VSG,analyzes the SSO of DFIG-based wind turbine in the series compensation grid,and proposes the additional damping control strategy to suppress the SSO.In dq frame,the impedance model of the DFIG-based wind turbine controlled by indirect voltage VSG is established by considering the voltage-current inner loop and the VSG outer loop,and the influence of control parameters on the impedance characteristics is analyzed.For the DFIG-based wind turbines controlled by indirect voltage VSG in the series compensation grid,the influence of series compensation degree,inertia time constant,damping time constant and voltage loop proportion coefficient on the stability of DFIG grid-connected system is analyzed by GNC.Based on the control structure characteristics of DFIG-based wind turbine,this paper adopts the additional damping control method to suppress the SSO,and analyzes the effect of the additional damping control on the system stability and output impedance.Compile the corresponding theoretical analysis program,and build the corresponding 2.1MW simulation model.The theoretical analysis and simulation results verify the correctness of the analysis and suppression of the SSO of the voltage source DFIG-based wind turbine connected to the series compensation grid.