电压源变换器接入电网的小扰动稳定机理分析

研究了基于矢量控制的电压源变换器(VSC)接入电网的小扰动稳定问题。基于VSC接入无穷大系统的详细模型，针对不同控制模式，分别对平衡点的存在性、稳定性进行了分析，系统地总结了VSC小扰动失稳的不同机理。系统中发生鞍结点分岔会导致平衡点消失而失稳，且存在以下几种机理：输出电流过大会导致锁相环(PLL)失去平衡点，对应PLL失去同步，单独的PLL失去同步可能发生在切除外环控制、采用内环定电流控制的情况下；输出有功过大会导致功率外环失去平衡点，当无功外环采用定无功功率、定交流电压控制时，分别对应电网的静态电压、功角失稳，而且失稳后电流增大一般也会引发PLL失去同步。在平衡点存在的情况下，系统振荡模式中包含低频振荡模式和次同步振荡模式，系统也可能发生Hopf分岔而出现振荡失稳。低频振荡模式主要由外环控制主导，次同步振荡模式则由PLL、电流环和线路动态主导。平衡点的存在性不受VSC控制参数的影响，只受网络参数、VSC工况的影响，而平衡点的稳定性和VSC控制参数有关。

Analysis of small disturbance stability mechanism for grid-connected voltage source converter

The problem of small disturbance stability for grid-connected vector control-based VSC(Voltage Source Converter) is studied. Based on the detailed model of VSC connected to infinity system, the existence and stability of the equilibrium point are analyzed for different control modes. Different mechanisms of VSC small disturbance instability are summarized. Saddle node bifurcation might cause the loss of equilibrium point and result in instability. This can be illustrated by several mechanisms. The excessive output of current can cause PLL(Phase Locked Loop) losing its equilibrium point, which corresponds to the loss of synchronization of PLL. Meanwhile, the loss of synchronization of PLL may occur individually when the outer loop control is cut off and the inner loop constant current control is used. Additionally, the excessive output of active power may cause the power outer loop losing equilibrium point. When adopting constant reactive power/constant AC voltage control in outer loop that corresponds to the static voltage/power angle instability of power grid. In addition, the increase of current after the instability generally results in the loss of synchronization of PLL. When the equilibrium point exists, the system oscillation mode generally includes low-frequency oscillation mode and subsynchronous oscillation mode. The system may also suffer from Hopf bifurcation, which further results in oscillation instability. The low-frequency oscillation mode is mainly dominated by the outer loop control, while the subsynchronous oscillation mode is dominated by PLL, current loop and line dynamics. The existence of the equilibrium point is generally not affected by VSC control parameters, but only determined by the network parameters and VSC operating conditions, while the stability of balance point is associated with VSC control parameters.