The Causes And Suppression Of Low Frequency Oscillation In Three Lecel Grid-connected Converters

The grid-connected converter is a typical power electronic device.As a key part in new energy generation,the grid-connected converter has complex dynamic behavior.Current low frequency oscillation could happen,which could seriously affect the safe and stable operation of the power grid.Therefore,a three-level grid-connected converter system is taken as the research object in this paper.The causes of its low-frequency oscillation are deeply studied.The reasonable controller of the system was designed to avoid and suppress the oscillation,and to improve the stability.Firstly,a discrete iterative mathematical model of the diode point-clamp three-level grid-connected converter system is established based on the stroboscopic mapping method.Based on the discrete mathematical model,the reduced-order Jacobian matrix method is used to design the stable range of controller parameters.The conventional Jacobian matrix method and the bifurcation graph method are used to verify the result.Then the correctness of the theoretical analysis is confirmed by the folding diagram method and PSIM circuit simulation.They show that the low frequency oscillation will be occurred in the improper proportional coefficient in the grid-connected converter system.In addition,other causes leading to low frequency oscillation of the system are analyzed in details.for example,grid-connected inductance,non-ideal power grid impedance and digital filter.All of them will make the grid-connected system come to low frequency oscillation,which will lead to the poor stability and robustness of the system.Secondly,The delay washout filter is designed to repress the low frequency oscilliation of the system.The discrete mathematical model of the three-level grid connected converter system with the delay washout filter is built at first.Then the control parameters of the delay washout filter are designed by using the reduced order Jacobian matrix and bifurcation diagram.The correctness and effectiveness of the delay washout filter parameters are comfirmed by folding diagram and circuit simulation.Finally,based on 7.5kW diode clamped three-level grid-connected converter experimental platform,the causes of low-frequency oscillation of three-level grid-connected converter and its suppression experiments are completed.The effectiveness of the controller design and suppression strategy is proved.