Research On Tie Line Power Fluctuation Suppression Strategy In Synchronous Grid Connection Based On Impedance Analysis Method

With the continuous expansion of interconnected power grids,there is a problem of tie-line power fluctuation in the process of grid connection.When back-to-back MMC-HVDC grid-connected devices are used for grid connection,the power of inter-regional tie lines will fluctuate greatly due to the influence of power command and controller performance.In severe cases,it will affect the stability of AC power grids on both sides.Therefore,a control strategy based on impedance analysis is proposed in this paper,which can reduce the influence of tie-line power fluctuation on the grid-connected process and improve the grid-connected reliability.In this paper,the back-to-back MMC-HVDC grid connected device researched by the research group is taken as the research object,the mathematical model of the back-to-back MMC-HVDC grid-connected composite device is established.And the operation principle of the device is analyzed through the four-quadrant operation mechanism.The control strategy of converters on both sides of the device is determined,and the feasibility and effectiveness of the control strategy are verified by PSCAD/EMTDC simulation.On this basis,the impedance model of inverter side converter of back-to-back MMC-HVDC grid-connected composite device is established.The impedance model of MMC converter without considering control link and considering phase-locked loop is deduced through the relationship between coordinate transformation and main control loop.The impedance characteristics of MMC converter are analyzed by Porter diagram,and it is concluded that the power fluctuation on tie line can be reflected by the impedance characteristics of Z_dd in impedance matrix.The impedance stability criterion is proposed to judge the influence of transmission power change on grid-connected process,and the correctness of theoretical analysis is verified by simulation.Aiming at the problem of large fluctuation of tie line power in the process of grid connection,a control strategy based on impedance analysis is proposed to suppress it.In the process of grid connection,the disturbance of power is equivalent to the change of power matrix G_pq,while the disturbance of power will cause the change of D-axis current instruction i_dref.On the premise of not changing the impedance characteristics before power disturbance,this paper takes the modified i_dref as the breakthrough point.By modifying the actual voltage value u_d of D axis,and then modifying i_dref.Finally,the impedance characteristics of the converter in the grid-connected process are modified,and the influence of tie-line power fluctuation on the grid-connected process is reduced.The correctness and effectiveness of the proposed control strategy based on impedance analysis are verified by PSCAD/EMTDC simulation model.Transient simulation further verifies that the proposed control strategy has a good suppression effect on power fluctuation of tie line.