Grid-Connected Strategy For Direct-Drive Permanent Magnet Wind Power Generation System With Two-Stage Matrix Converter

Direct drive permanent magnet wind generators are occupying a growing market share with their superior performance,and are one of the main directions for the development of wind turbines in the future.The grid-connected control strategies for wind power systems based on dual PWM converters are complex,and the presence of DC capacitors can cause problems such as large volumes,frequent failures,and high maintenance costs.The two-stage matrix converter(TSMC)has an AC-DC-AC two-stage conversion structure,but no intermediate capacitance is needed,which can effectively avoid the disadvantages of dual PWM converters.It is with high research value that how to apply the TSMC to wind power systems instead of the dual PWM converter.The research work of this dissertation mainly focuses on the grid-connected control strategy of TSMC wind power system.While demonstrating the feasibility and superiority of TSMC wind power system,it further studies the grid-connected control of TSMC system when grid voltage is unbalanced,which can help to lay a theoretical foundation to broaden the application and development of this type of system.In order to track the maximum wind energy,open-loop control and closed-loop control of the power outer loop and current inner loop are performed on the TSMC rectifier and inverter stages,respectively,and the control strategy is simple and effective.In the Matlab/simulink environment,the two systems are simulated and compared.The results show that the system can achieve maximum wind energy tracking when the wind speed changes,and it is always running under the VSCF power generation mode,which verifies the feasibility of the control method.Besides,compared with the dual PWM system,the TSMC system has faster maximum wind energy tracking speed and better output power quality.When the grid voltage is unbalanced,2 times frequency oscillation occurs with negative sequence voltage and current,making it impossible for the wind turbine to track the optimal speed.In order to realize the control of negative sequence component,a mathematical model of the inverter stage grid-connected under the double-sequence dq axis was built,and a dual-current loop inverter-level grid-connected control method was proposed.Finally,the proposed control method was simulated and tested.The results verified that the control method can effectively eliminate the active power oscillation phenomenon on the grid side,and make the system can still track the maximum wind energy when the grid voltage is asymmetric.