The impedance-based stability analysis is widely used to study the grid-connected inverter system a weak grid. The advantage of the analysis is that without knowing the accurate parameters of an inverter, its equivalent output impedance can be obtained using some methods(e. g. the speciifc harmonic injection method). Then the stability of grid-connected system can be studied by the ratio of the grid impedance and the inverter output impedance. However, using the phase margin of the ratio to represent the phase margin of grid-connected system is inaccurate and incomplete. Also, only using the index to describe the system has its limitations. The paper will give a mathematical model of the system in the weak grid, that is to say, the grid impedance and the loads at the point of common couple which can not be neglected in weak grid will be contained in the control system. Hence the effects that the variations of the impedance and loads have on the control system will be explored more intuitively and accurately. On the basis of proving the limitations of impedance-based stability analysis, the conclusions will be validated by simulations.%当前对于弱电网的研究,主要的研究方法为利用阻抗分析法来对逆变器并网系统进行探讨.其优点在于在无需并网逆变器精确参数的情况下,利用某种方法(如特定谐波注入法)来获取并网逆变器的等效输出阻抗,利用电网阻抗和逆变器等效输出阻抗的比值研究并网系统的稳定性.但是,由该比值表征并网系统的相角裕度,是模糊和不精确的.并且仅由此指标来描述系统,也是片面的.本文将给出整个弱电网下系统的数学模型,即弱电网下不可忽略的电网阻抗和公共耦合点的负载,都将被归纳到并网逆变器控制系统中,从而更加直观和精确的研究电网阻抗和公共点负载变化时,对整个控制系统的影响.在证明阻抗分析法所述系统相角裕度存在误差的基础上,最后由仿真验证本文提出正确性以及阻抗分析法描述系统的局限性.
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