电网不平衡下MMC多变量保护控制器参数设计研究

Design and Analysis of Control Parameter for Multi-variable Flexible Control in MMC Under Unbalanced AC Grid Conditions

电网不平衡不仅会导致模块化多电平变流器(multimodular converter,MMC)网侧电流波形质量变差,还会引起桥臂电压不平衡、桥臂环流增加等一系列问题。为避免MMC系统发生过电压或过电流的安全问题,考虑交流侧最大工作电流、桥臂最大电容电压纹波以及最大注入无功三个方面的工作条件限制,分别从理论角度推导出多变量保护控制算法下控制参数k的计算公式。首先分析了MMC系统电流方程、电压方程以及能量的动态方程。基于上述系统方程,考虑MMC系统允许的工作范围如交流侧最大工作电流、桥臂最大电容电压纹波以及最大注入无功三个方面,详细给出了参数k的设计原则和流程。最后,利用实时仿真平台和MMC样机对理论计算给出了仿真和实验验证结果。

In the grid-connected modular multilevel converters（MMCs） system, grid faults not only cause the quality deterioration of grid currents which may lead the MMC to shut down, but also bring unbalanced problems to the arm voltages. In this paper, a multi-variable protected control method which was employed in d-q coordinate was presented. Considering the permitted maximum of three-phase current, the permitted capacitor voltage ripple and the maximum reactive power set-point, the calculation of the parameter k was given respectively. The converter mathematical models which were based on the laws of Kirchhoff and energy balance were given first. Then the relationship between the system requirements, such as the maximum current and the permitted capacitor voltage ripple, and the parameter k was derived. Furthermore, the design principles of parameter k were deduced. Finally, the simulating results from real time digital simulator（RTDS）, and the experimental results from a scaled-down MMC prototype were presented to verify the theoretical analysis.