Research On Synchronization Stability And Reactive Current Control During Low Voltage Ride Through Of Variable-speed Wind Farm

In the grid code for wind power generation system,wind farms are required to keep connected to the grid and supply some reactive current during the grid fault time.At present,in order to ensure that the wind farms can provide sufficient reactive current,grid operators always request the wind farms to configure a certain capacity of reactive power compensation devices.The most commonly used devices are static var compensator(SVC)and static synchronous compensator(STATCOM),and STATCOM is widely used since its output current has low harmonic and its current output capability has nothing to do with the grid voltage.In this paper,a variable speed wind farm with STATCOM is studied.We first carry out detailed mathematical modeling analysis of different types of wind turbines,including the mechanical model and electrical model of variable speed wind turbines,and the control systems of different types of units.Based on which,the low voltage ride through methods of different types of units are elaborated.In the MATLAB / Simulink simulation software,the transient characteristics of the wind turbines under different dips are simulated and analyzed.The results show that during the serious fault of the power grid,the output frequency of the variable speed wind turbine will deviate from the frequency of the grid voltage by using the traditional control method,so the phenomenon of synchronous instability occurs.For this reason,the equivalent model of variable speed wind farm is established to analyze the phenomenon and mechanism of synchronous instability of wind farm,and the conditions of ensuring the stability of the wind farm are pointed out.Based on which,a coordinated current control method for variable speed wind turbine and STATCOM is proposed,which controls active current output of the variable speed wind turbine to ensure the synchronization stability of wind turbine and STATCOM.Taking into account the current stress limit of the converters in variable speed wind turbines,the configured STATCOM can output part of the reactive current to assist the wind farm in meeting the requirements of the grid code.The method distributes the active and reactive components of the output current of the wind farm according to the ratio of the equivalent transmission impedance of the power system,which can realize the synchronous stability of the wind farm during the low voltage ride through and satisfy the demand of the reactive current of the grid.Then the current control strategies for their respective characteristics of the doubly fed induction type and permanent magnet direct drive type wind turbines are proposed.Finally,the validity of the proposed control method and the robustness to the system parameters are verified by MATLAB / SIMULINK numerical simulation.