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

• Applicant: Professor Dr.-Ing. Axel Mertens
• Subject Area: Electrical Energy Systems, Power Management, Power Electronics, Electrical Machines and Drives
• Term: since 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 324166923

In the expiring project period, a quasi-2-level (Q2L) PWM Operating mode for Modular Multilevel Converters (MMC) with controlled branch currents was validated. It enables a drastic reduction of the module capacitors to a few percent of their usual size. This is a decisive advantage for cost, volume and weight. At the same time, many advantages of MMC are retained, such as scalability, redundancy, use of low-cost low-voltage semiconductors in the modules, etc. In particular, the steps in the output voltage and the associated dv/dt are also limited to the values of a single module, which is important in electric drives with long motor cables for insulation design and the risk of bearing currents. An important disadvantage of the Q2L-MMC is the increased distortion of the output voltages and currents compared to MMC, similar to the conditions in conventional two-point inverters. This is where the proposed follow-up project starts. The concept shall be extended to quasi-n-level MMC with a low number of active levels, thus significantly reducing the output waveform distortion. In the current project, the approach with controlled branch currents could already be transferred to a quasi-3-level (Q3L) MMC which was validated in simulations. In addition, a hybrid Q3L-MMC topology with less effort, as well as the transfer of Q3L to a modular matrix multilevel converter (MMMC) have been simulated. Based on this, the aim of the proposed follow-up project is as follows. The variants mentioned need to be validated experimentally before an evaluation and comparison with existing solutions takes place. Also, the concept shall be extended and generalized to n-level converters, including validation of a Q5L operation in the laboratory. In the case of hybrid variants, in addition to the topology concepts already mentioned, another approach is attractive that has not yet been studied in detail. This new approach offers many potential advantages, even over conventional multilevel converters. As part of the proposed project, this innovative concept will also be researched and evaluated.

• DFG Programme: Research Grants