光伏级联DC/DC变换器模式切换稳定性分析及降压运行策略

在光伏直流升压汇集系统中，光伏侧DC/DC变换器通过级联拓扑可实现光伏功率升压直流送出。当光伏输入侧功率差异较大时，级联变换器会在最大功率点跟踪模式和定电压模式间切换，模式切换控制会引起系统稳定性变化。文章以级联隔离升压全桥变换器为研究对象，建立变换器在电导增量法-最大功率点跟踪控制、定电压控制下的小信号输出阻抗模型和级联系统等效阻抗模型，根据阻抗稳定性判据评估控制模式切换下系统稳定性差异。当变换器由最大功率点跟踪模式切换为定电压模式时会降低系统稳定性能，且随着切换控制模式模块数量的增多稳定性显著减小。在此情况下，可采用母线电压降压运行的方案，尽可能使光伏侧DC/DC变换器工作在最大功率点跟踪模式，以维持系统的稳定性。在Matlab/Simulink中搭建级联系统仿真模型，验证了理论分析的正确性。

Mode Switching Stability Analysis and Step-down Operation Strategy of Photovoltaic Cascaded DC/DC Converters

In the photovoltaic DC boost collection system,DC/DC converters on the photovoltaic side realize power transmission through the cascaded topology. However,when the input photovoltaic power is different greatly,the cascaded converter switches between the maximum power point tracking( MPPT) mode and the constant voltage mode,and the mode switching control causes the system stability change. In this paper,the isolated boosting full-bridge converter is taken as the research object,and the small-signal model of the converter output impedance under the control of incremental conductivity method-MPPT and constant voltage is established,and then the equivalent impedance model of cascaded system is deduced.According to the impedance stability criterion,the difference of system stability under control mode switching is evaluated.The results show that the stability of the system is reduced when the converter is switched from the MPPT mode to the constant voltage mode,and the stability is significantly deteriorated as the number of switching control mode modules increases. Therefore this paper proposes to decrease the bus voltage to maintain PV-side DC/DC converter within the MPPT control mode. Finally,a simulation model of the cascaded system is built in Matlab/Simulink to verify the correctness of the theoretical analysis.