基于端口特性的并联下垂逆变器系统的小信号建模与稳定性分析

Terminal-characteristics Based Small-signal Modeling and Stability Analysis for Parallel Droop-controlled Inverter System

孤岛交流微电网中,并联下垂逆变器系统被广泛应用。基于有功功率-基波频率的下垂控制原理,逆变器之间的动态相互作用不仅体现在母线电压和输出电流之间,也体现在变化的系统基波频率上,从而引发小信号稳定性问题。建立了下垂逆变器的端口特性小信号模型,除传统的输出阻抗和输入导纳之外,提出一种新型端口特性以表征基波频率动态;进一步基于单个模块的端口特性,建立了多台逆变器并联系统的小信号模型;基于广义奈奎斯特稳定性判据,计算由阻抗导纳乘积与基波频率端口特性乘积之和组成的回率矩阵的特征轨迹,对并联系统的稳定性做出判断。最后,实验结果证明了所提小信号模型和稳定性判据的有效性。

Parallel droop-controlled inverter system is widely employed in islanded AC microgrids. Since the active power-fundamental frequency droop control principle is applied, the dynamic interactions among inverters exist not only in the bus voltage and transmitted current but also in the variable system fundamental frequency, thereby causing small signal stability issues. A small-signal model for the terminal characteristics of a droop-controlled inverter is established. Besides the conventional output impedance and input admittance, a novel set of terminal characteristics is proposed to characterize the dynamics of fundamental frequency. Furthermore, a small-signal model of parallel droop-controlled in verter system is constructed based on the terminal characteristics of individual inverters. According to the generalized Nyquist criterion (GNC), a stability prediction approach is proposed for the parallel system by calculating the characteristic loci of return ratio matrix, which is the sum of the product of impedance and admittance and that of fundamental frequency terminal characteristics. Finally, experimental results validated the effectiveness of the proposed small-signal models as well as the stability criterion.