基于谐波传递矩阵的MMC换流站频率耦合特性建模与分析

Modeling and Analysis of Frequency Coupling Characteristic for MMC Station Based on Harmonic Transfer Matrices

模块化多电平换流器(MMC)应用于柔性直流输电系统可有效降低输出谐波和开关损耗,同时也带来突出的系统稳定性问题。阻抗稳定性分析方法可用来分析MMC系统运行时产生的稳定问题。现有文献建立的MMC阻抗模型均为正序阻抗和负序阻抗相互解耦的序阻抗模型,并通过单入单出的阻抗稳定判据分析系统稳定性。然而,MMC在低频段呈现较为显著的频率耦合特性,此时正负序阻抗不再解耦,仍使用单入单出的稳定判据无法精确判定系统稳定性。文中通过分析扰动分量与稳态谐波的交互作用,研究了MMC频率耦合产生机理,进一步采用谐波传递矩阵建立了定交流电压控制下MMC频率耦合模型,并分析了频率耦合特性的主要影响因素和对稳定性的影响。最后,基于MATLAB/Simulink搭建的仿真系统结果验证了所建立频率耦合模型精度和系统稳定性分析结果的正确性。

The output harmonics and switching losses can be significantly reduced with the application of modular multilevel converter(MMC) in high voltage DC transmission system, while it also brings prominent stability problems. Impedance-based stability analysis method can be applied to analyze the stability problems when MMC system operates. In the existing studies, the sequence impedance model of MMC is derived, which is decoupled into a positive-sequence impedance and a negative-sequence impedance, and the system stability can be determined by single-in-single-out stability criterion. However, MMC exhibits significant frequency coupling characteristic in the low frequency band, making the positive-sequence impedance and negativesequence impedance no longer decoupled, therefore the single-in-single-out stability criterion cannot accurately determine the system stability. In this paper, the frequency coupling mechanism of MMC is revealed by analyzing the interaction between perturbation and steady-state harmonics. Furthermore, the frequency coupling model of MMC with AC voltage control is derived based on harmonic transfer matrices. Based on the derived model, the influence of frequency coupling characteristic on system stability judgment and the main factor influencing frequency coupling characteristic are analyzed. Finally, the simulation results based on MATLAB/Simulink simulation system validate the correctness of the frequency coupling model and stability analysis results.