并网VSC系统正阻尼时的一类切换型振荡分析

遭受大扰动作用后具有正阻尼特性的并网电压源换流器(VSC)系统仍存在振荡失稳风险，即当电流达到限幅值时会引发一类切换型振荡。为此，结合降阶模型分析其振荡原因。首先建立了并网VSC系统的简化状态空间模型，并分析了该模型的小扰动特性与该模型参数对小扰动特性的影响。其次分析了d轴电流达到限幅值对该系统正阻尼下动态的影响，发现大扰动下并网VSC系统会因d轴电流达到限幅值产生切换型振荡。再次从电流达到限幅值获得的降阶模型角度，揭示了该切换型振荡的产生原因在于物理降阶系统出现负阻尼振荡。最后讨论了物理降阶系统与原系统中振荡产生原因的关联性，可以推测物理降阶系统发生负阻尼振荡是原系统发生切换型振荡的必要条件。

Analysis on category of switched oscillation for grid-connected VSC system with positive damping

The grid-connected VSC(Voltage Source Converter) with positive damping characteristics still has the risk of oscillation instability after suffering large disturbance, that is, a category of switched oscillation will be caused when the current reaches its limitation. Therefore, combined with the reduced-order model, the oscillation reasons are analyzed. Firstly, the simplified state-space model of grid-connected VSC system is established, and its small disturbance characteristics and the influence of model parameters on its small disturbance characteristics are analyzed. Secondly, the influence of the d-axis current reaching limitation on the dynamics of the system under positive damping is analyzed, and it is found that switched oscillation occurs under large disturbance when the d-axis current reaches its limitation. Thirdly, from the prospective of the reduced-order system obtained when the current reaches its limitation, it is revealed that the switched oscillation is caused by the negative damping oscillation in the physical reduced-order system. Finally, the relevance of oscillation reason between the physical reduced-order system and the original system is discussed, and it can be inferred that the negative damping oscillation occurred in the physical reduced-order system is a necessary condition for the switched oscillation occurred in the original system.