Modular Direct AC-AC Multilevel Converters for Multiphase Systems with reduced number of arms - a systematic approach for a class of modular mutlilevel converters

模块化多电平转换器由多个模块组成，每个模块包括功率半导体和一个电容器，但通常没有功率馈送。多个模块的输出串联在一起形成一个支路。这允许合成具有相当高分辨率和低失真的分支电压波形。因此，可以设计出无变压器接入中压电网的变流器。挑战在于模块电容电压的控制；必须通过适当的操作策略将其保持在限制范围内。虽然一个支路的模块之间的电压共享很容易实现，但控制一个支路的总能量含量通常需要额外的环流或共模电压。支路能量的控制是一项最重要也是最具挑战性的任务。在项目《用于臂数减少的多相系统的模块化直接交-交多电平变流器-一类模块化多电平变流器的系统方法》中，对模块化多电平变流器进行了系统而统一的研究。这项研究包括完全填充的nxm-矩阵拓扑及其对称约化变体。这些转换器连接n端子系统和m端子系统。在完全填充nxm矩阵拓扑的情况下，一个系统的每一相通过模块化多电平转换器的分支连接到另一系统的每一相。对于对称简化的nxm矩阵拓扑，这些转换器分支中的一些被移除，从而产生对称配置。该项目的成果是一种新的广义控制方法，可以应用于上述所有模块化多电平拓扑学；该项目应用是关于相应的后续项目，旨在将广义控制方法扩展到特殊工作点、不对称系统和不对称简化的拓扑结构。例如，具有挑战性的特殊操作点包括两个相同频率的系统的连接。广义控制方法对不平衡系统的自适应使得在电网故障和不完善时能够实现稳定的支路能量平衡控制。对非对称简化拓扑的扩展允许在变流器分支发生故障的情况下将完全填充的拓扑作为非对称简化变体运行。该后续项目将完成所进行的调查和开发的控制方法。因此，广义描述和控制将对所有拓扑和相关工作点有效。

Modular Multilevel Converters consist of a number of modules, each compring power semiconductors and a capacitor, but generally no power infeed. The outputs of several modules are connected in series to form a branch. This allows synthesis of branch voltage waveforms with rather fine resolution and low distortion. Thus, converters for transformerless connection to the medium voltage grid can be designed. The challenge is in the control of the module capacitor voltages; they must be kept within limits by appropriate operating strategies. While voltage sharing between the modules of one branch is easy to achieve, the control of the total energy content of a branch often requires additional circulating currents or common mode voltages. The control of the branch energies is the most important and challenging task.Within the project “Modular Direct AC-AC Multilevel Converters for Multiphase Systems with Reduced Number of Arms – a Systematic Approach for a Class of Modular Multilevel Converters”, a systematic and unified investigation of modular multilevel converter has been conducted. This investigation includes fully populated nxm-matrix topologies and their symmetrically reduced variants. These converters connect an n-terminal system with an m-terminal system. In the case of fully populated nxm-matrix topologies, each phase of one system is connected to each phase of the other system by a branch of the modular multilevel converter. For symmetrically reduced nxm-matrix topologies, some of these converter branches are removed, resulting in a symmetric configuration. The outcome of this project is a novel generalized control approach, which can be applied to all of the aforementioned modular multilevel topologies.This project application is about the corresponding follow-up project, which aims at extending the generalized control approach for special operating points, asymmetric systems and asymmetrically reduced topologies. The challenging special operating points include for example the connection of two systems of the same frequency. The adaption of the generalized control approach for unbalanced systems enables a stable branch energy balancing control during grid faults and imperfections. An extension to asymmetrically reduced topologies allows the operation of a fully populated topology as an asymmetrically reduced variant in case of failure of a converter branch.This follow-up project will complete the conducted investigations and the developed control approach. As a result, the generalized description and control will be valid for all topologies and relevant operating points.