阻抗分析法是目前研究新能源并网系统小信号稳定性的常用方法。阻抗建模中，通常假设网侧交流电压三相对称，从而用二阶矩阵即可描述换流器的端口阻抗特性，然而在不对称工况下换流器呈现更复杂的频率耦合特性，模型不再适用。基于此，首先考虑了脉冲宽度调制（pulse width modulation，PWM）的边带效应以及采样环节的频率混叠效应、锁相环动态、直流电压控制、正负序电流内环控制等影响因素，建立了适用于三相不对称工况的并网换流器小信号模型；接着给出了一种不对称工况下多输入多输出（multi-input-multi-output，MIMO）系统降阶为单输入单输出（single-input-single-output，SISO）系统的方法，并通过PSCAD/EMTDC验证了理论分析的正确性；进一步地，定义并网换流器系统的不对称度并分析了控制参数对系统不对称度的影响；在此基础上根据最大峰值奈奎斯特判据分析了不对称工况下控制参数对并网系统稳定性的影响。

The impedance analysis method is common for understanding the small signal stability of new energy grid-connected systems. In impedance modeling works, the three-phase symmetry of the grid side AC system is assumed, and the port impedance characteristics of the converter can be described using a second-order matrix. However, under asymmetric conditions, the converter exhibits more complex frequency coupling characteristics, and the model is no longer applicable. This paper firstly considers the sideband effect of a pulse width modulation（PWM） modulation and the frequency aliasing effect of sampling link, phase-locked loop dynamics, DC voltage control, positive and negative sequence current inner loop control, and other influencing factors, and establishes a small signal model of grid-connected converter suitable for three-phase asymmetric conditions. Then, a method for equivalenting a multi-input-multi-output （MIMO） system to a single-input-single-output （SISO） system under asymmetric operating conditions is proposed, and the correctness of the theoretical analysis is verified through PSCAD/EMTDC. Furthermore, the asymmetry of the grid-connected converter system is defined and the influence of control parameters on the system asymmetry is analyzed. Finally, the stability of grid-tied systems under asymmetry operation condition is analyzed based on the maximum peak Nyquist criterion.