基于模块化多电平换流器的直流电网小信号建模

Small Signal Modeling of MMC-based DC Grid

该文提出交流系统三相平衡情况下基于模块化多电平换流器（modular multilevel converter,MMC）的直流电网小信号模型通用建模方法。基于模块式的建模思想,将整个系统划分成交流系统、MMC和直流网络3部分,易于扩展,适用于任意拓扑的直流电网。建模过程中考虑了桥臂环流、子模块总电容电压等MMC内部电气动态和锁相环、统一控制器、环流抑制控制器、信号滤波环节等控制系统的动态,为分析各种参数对系统小信号稳定性的影响提供了基础。根据MMC内部电气量和交直流网络的耦合关系,推导MMC的交流侧和直流侧戴维南等效电路,基于等效电路构建MMC与交直流网络之间的连接方程。通过线性化各子系统和连接方程,构建全系统的小信号模型,无需手工推导矩阵的每个元素,大大简化了基于MMC的直流电网小信号建模。以一个四端MMC直流电网为例,通过对比电磁暂态模型与该文方法建立的状态空间模型和小信号模型的阶跃响应,验证所提建模方法的准确性。利用根轨迹和特征值灵敏度分析不同控制方式下MMC控制器参数对该系统小信号稳定性的影响。

This paper proposed a generic small signal modeling method for DC grid based on modular multilevel converter（MMC） under balanced AC system.The entire system is divided into three subsystems： AC system,MMC and DC system,which makes it easily extendible to fit arbitrary topology of DC grid in a modular way.The internal dynamics of MMC including arm circulating current and total capacitor voltage,and the control system dynamics including phase lock loop,generic controller,circulating current suppression controller,and signal filters are modeled,which provides the foundation for analyzing the impacts of various parameters on the small-signal stability.According to the deduced coupling equation related to MMC internal electric quantities and AC/DC networks,the equivalent Thevenin circuits for MMC AC/DC-side are proposed.The link equations for MMC model and AC system and DC networks are then deduced.By linearizing each subsystem and link equations,the small-signal model for the whole system is automatically produced without too much manual burden,which simplifies the deduction of small-signal model for MMC-DC grid a lot.The correctness of the modeling method is validated by comparing the step response of the electromagnetic model,the deduced state-space model and the small-signal model based on a four-terminal MMC DC grid system.The impact of control parameters on the small-signal stability of the test system is analyzed through root locus and eigenvalue sensitivities.