Padé-Based Stability Analysis for a Modular Multilevel Converter Considering the Time Delay in the Digital Control System
Padé-Based Stability Analysis for a Modular Multilevel Converter Considering the Time Delay in the Digital Control System
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DOI：
10.1109/tie.2018.2868008
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发表时间：
2019-07
影响因子：
7.7
通讯作者：
C. Dong;Shunfeng Yang;H. Jia;Peng Wang
中科院分区：
文献类型：
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作者：
C. Dong;Shunfeng Yang;H. Jia;Peng Wang
模块化多电平变流器的数字控制系统通常伴随着开关动作。从效率的观点来看，MMC的开关频率通常被选择为尽可能低，这导致控制系统中不可避免的且相对长的时间延迟。这样的时滞可能会恶化MMC的性能，甚至通过将无穷多个不确定特征值引入系统状态函数来降低稳定裕度。如果没有适当的时滞模型和定量的系统稳定性分析，MMC可能会遭受不确定性和不稳定的风险，在他们的操作，特别是当开关频率较低。本文推导了数字控制系统中考虑时滞的MMC模型。提出了一种基于Padé的稳定性分析方法，该方法包括三个步骤：信息集成与临界特征值提取、轨迹可视化与临界角计算、定量评估与极值确定。该模型和方法可以有效地提取出系统的临界特征值。通过Hopf分岔，数字控制系统中的时滞在MMC系统中引入了一对临界共轭特征值，这是MMC系统不稳定的一个原因。在选择控制器参数和最小开关频率时，定义了CA和临界影响来定量评估稳定性。MMC稳定性评估的建议方法进行了实验验证。
The digital control system of modular multilevel converters (MMCs) normally synchronizes with the switching actions. The switching frequency of an MMC is generally selected as low as possible from an efficiency point of view, which leads to an unavoidable and relatively long time delay in the control system. Such a time delay might deteriorate the MMC performance and even reduce the stability margin by introducing infinite uncertain eigenvalues into the system state functions. Without a proper time-delay model and quantitative system stability analyses, MMCs might suffer from uncertainties and unstable risks during their operations, especially when the switching frequency is low. This paper derives an MMC model considering the time delay in the digital control system. A Padé-based stability analysis method is then proposed, which consists of three steps: information integration and critical eigenvalue extraction, and trajectory visualization and critical angle (CA) calculation, and quantitative assessment and extreme value determination. By the proposed model and method, the critical eigenvalue can be effectively extracted. With the Hopf bifurcation, the time delay in the digital control system induces a pair critical conjugate eigenvalues into MMC systems, which is detected as an instability cause. The CA and the critical impact are defined to quantitatively assess the stability, while selecting the controller parameters and minimum switching frequency. The proposed method for the MMC stability assessment is experimentally verified.