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当电网阻抗较高时，并网光伏系统会变得不稳定；特别是连接在弱电网上的大型光伏电站，稳定性问题更为突出。为研究电网阻抗对大型并网光伏系统稳定性影响，该文建立了三相逆变器并联系统的诺顿等效模型，推导了逆变器并网电流、并网公共点电压表达式以及大型并网光伏系统的闭环特征方程，采用根轨迹法分析了电网阻抗对大型并网光伏系统稳定性的影响，最后建立系统仿真模型进行验证，结果表明，三相逆变器的诺顿等效模型相间不存在耦合效应，三相逆变器并联系统的特性分析可在单相逆变器并联系统中进行；在一定参数范围内，电网阻抗会造成大型并网光伏系统出现不稳定现象。“,”The grid-connected photovoltaic (PV) system will become unstable when the grid impedance is large, especially when the large-scale PV plant was connected to a weak grid. In this paper, a Norton equivalent model of the three-phase paralleled inverters was established to study the stability of the large-scale grid-connected PV system including the grid impedance. The expression of the grid-connected current, the point of the common coupling (PCC) voltage and the closed loop characteristic equation of the system were derived. Then the stability of the large-scale grid-connected PV system was analyzed by the root locus. Finally, the study was validated through simulations. Studies show that there is no inter-phase coupling effect in the Norton equivalent model of the three-phase paralleled inverters, and the three-phase paralleled inverters can be converted to the single-phase paralleled inverters in order to study the characteristics. The large-scale grid-connected PV system will be unstable in some parameter regions depending on the grid impedance.