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

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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.