环形MMC桥臂电流应力分析

Bridge Arm Current Stress Analysis of Circular MMC

六边形模块化多电平变换器(modular multilevel converter,MMC)和九边形模块化多电平变换器是两种新兴的具有环形拓扑结构的直接AC-AC变换器,因其器件数量上的优势,可作为级联型矩阵变换器(modular multilevel matrix converter,M3C)的替代拓扑,在海上风电分频输电等领域具有广阔的发展前景。然而,两种拓扑均为桥臂复用型结构,这导致其单个桥臂应力过大,相对于M3C在器件数量上的优势不明显,同时这也降低了控制自由度,导致其无法通过传统方法进行应力优化。为进一步增强其器件数量上的优势,将六边形模块化多电平变换器和九边形模块化多电平变换器统一于环形MMC,建立了环形MMC桥臂电流的数学模型,探究系统参数对桥臂电流应力的影响。得出结论:环形MMC端口间相角差是影响桥臂电流应力的关键参数;环形MMC桥臂综合电流周期分量应力与端口间相角差呈周期性关系,通过调整九边形MMC同频端口间相角差的大小可达到降低桥臂综合电流周期分量应力的目的,经理论分析最大可减小29.25%。最后,在RT-LAB仿真实验平台搭建仿真模型验证了结论的正确性。

Hexagonal modular multilevel converter(MMC)and nonagonal MMC are two emerging direct AC-AC con-verters with a circular topology.Owing to their advantages in device count,they can be used as an alternative topology of cascaded H-bridge multilevel matrix converter,and have broad development prospects in the fields of offshore wind power fractional frequency transmission system.However,these two topologies are bridge arm multiplexing structures,which may result in excessive stress of a single bridge arm;therefore,their advantages over M3C in the number of devices will be weakened,the control degrees of freedom will be reduced,and hence,traditional stress optimization methods are not applicable.To further enhance their advantages in the number of devices,in this paper,hexagonal MMC and nonago-nal MMC are unified in circular MMC topologies,the mathematical model of bridge arm current in circular MMC is established,and the influence of system parameters on current stress of circular MMC bridge arm is explored.It is proved that the phase angle difference between ports of nonagonal MMC is the key parameter affecting current stress of circular MMC bridge arm,and the comprehensive current cycle component stress of circular MMC bridge arm has a periodic re-lationship with the phase angle difference between the ports.By adjusting the phase angle difference between the ports of nonagonal MMC with the same frequency,the comprehensive current cycle component stress of bridge arm can be re-duced by as great as 29.25%according to theoretical analysis.Finally,the correctness of the conclusions of this paper is verified by building a simulation model on the RT-LAB simulation experimental platform.