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许多模块组成，每个模块都包含功率半导体和电容器，但通常没有电源输入。几个模块的输出串联起来，形成一个支路。这使得分支电压波形的合成具有相当好的分辨率和低失真。因此，可以设计用于无变压器连接到中压电网的变流器。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控制在一定范围内。虽然一个支路模块之间的电压共享很容易实现，但支路总能量含量的控制往往需要额外的循环电流或共模电压。分支能量的控制是最重要和最具挑战性的任务。在“减少臂数的多相系统的模块化直交-交流多电平变换器——一类模块化多电平变换器的系统方法”项目中，对模块化多电平变换器进行了系统和统一的研究。本研究包括完全填充的nxm矩阵拓扑及其对称约简变体。这些转换器将n端系统与m端系统连接起来。在完全填充nxm矩阵拓扑的情况下，一个系统的每个相位通过模块化多电平转换器的分支连接到另一个系统的每个相位。对于对称简化的nxm矩阵拓扑，将删除其中一些转换器分支，从而得到对称配置。该项目的结果是一种新的通用控制方法，可应用于上述所有模块化多层拓扑。本项目申请是关于相应的后续项目，旨在对特殊工作点、非对称系统和非对称约简拓扑的广义控制方法进行扩展。具有挑战性的特殊操作点包括例如两个相同频率的系统的连接。采用广义控制方法对不平衡系统进行控制，可以在电网故障和不完善时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转换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的变体进行操作。这一后续项目将完成已进行的调查和制定的控制方法。因此，广义描述和控制将对所有拓扑和相关操作点有效。

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.