New operation modes of modular multilevel converters with output voltage modulation significantly reducing the module capacitor requirements

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Modular multilevel converters offer a simple construction based on a large number of small identical modules, a good scalability of voltage, and a low harmonic distortion of the output voltage, so that loads like electric machines can be directly connected to the converter output without the need of additional filters. It is of great importance that the output voltage slopes and the voltage steps are limited to the values caused by a single IGBT, so that conventional insulation systems can be used for the connected equipment, and overvoltages due to reflection at long cables or similar effects are very limited and do not lead to unacceptable voltage ratings. A major drawback is that very large capacitors have to be installed in the modules, leading to large volumes and high cost. In preliminary work we could demonstrate by simulations that it is possible to drastically reduce this module capacitance by means of a modulated output voltage with moderate frequency. While allowing all other advantages of modular converters to persist, the multilevel property is largely lost and must be compensated by a sufficient filtering of the output voltage (at least using an L-filter). This proposal adresses the properties and limitations of this new class of operation modes for modular power converters. The proposed operation mode has to be investigated thoroughly in order to understand the trade-offs between the parameters of the operation mode, the dimensioning of the converter components and the output filter requirements. In modular multilevel converters with their quasi-analog output voltage, a generalized concept of modulation can be defined, in contrast to conventional switching power electronic converters. This extended concept of modulation shall be substantiated by an appropriate theory. Based on a systematic search, promising variations of this new generalized modulation shall be identified.