考虑延时影响的MMC-HVDC系统高频振荡机理分析

基于模块化多电平换流器的高压直流（MMC-HVDC）输电系统的链路延时可能会引发系统产生高频振荡。文中通过Pade近似对延时环节进行等值，建立了基于四阶Pade近似等值延时环节的MMC-HVDC系统的状态空间模型。通过在阶跃工况下与PSCAD/EMTDC中详细电磁暂态模型动态特性的对比以及高频振荡的复现，验证了所建立的状态空间模型的正确性及四阶Pade近似等值延时环节的有效性。然后，研究了延时环节时间常数对系统特征根及高频振荡特性的影响，结果表明，基于特征值分析法对所建立的考虑延时环节特性的MMC-HVDC系统状态空间模型进行分析，可以得到引发系统高频振荡的临界延时时间。最后，通过参与因子分析法探究了引起系统发生高频振荡的主要因素，结果表明，系统的高频振荡现象主要是由交流系统特性与含有延时环节的MMC-HVDC系统特性之间的耦合作用引起。

Analysis on High-frequency Oscillation Mechanism for MMC-HVDC System Considering Influence of Time Delay

The circuit time delay of modular multilevel converter based high voltage direct current (MMC-HVDC) transmission systems may cause high-frequency oscillation of power systems. This paper establishes an equivalent model of the time-delay link based on the Pade approximation, and develops a state space model of MMC-HVDC system by equivalently representing the time-delay based on the fourth-order Pade approximation. By comparing the dynamic characteristics of the model with those of the detailed electromagnetic transient model in PSCAD/EMTDC under the step condition and the recurrence of high-frequency oscillation, the correctness of the developed state space model and the validity of the fourth-order Pade approximation for the time-delay equivalent are verified. Then, the influence of the time constant of the time-delay on the system eigenvalues and the high-frequency oscillation characteristics is studied. The results show that, based on the eigenvalue analysis method, the critical delay time that causes the system high-frequency oscillation can be obtained by analyzing the state space model of MMC-HVDC system considering the delay link characteristics. Finally, based on participation factor analysis method, the main factors that cause the system high-frequency oscillation are studied. The results show that the system high-frequency oscillation is mainly caused by the coupling between the characteristics of AC system and MMC-HVDC system with the time-delay link.