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

• Applicant: Professor Dr.-Ing. Axel Mertens
• Subject Area: Electrical Energy Systems, Power Management, Power Electronics, Electrical Machines and Drives
• Term: from 2014 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 254417319

• Final Report Year: 2023

Final Report Abstract

Modular multilevel converters enable power conversion between two systems with different voltage, frequency and number of phases. Target applications are in the higher power range, where the option of connecting many modules in series is advantageous. These inverters are therefore easily scalable and can be configured for medium to very high voltages. The number of modules influences the number of voltage levels, which leads to low filter requirements and low-harmonic output voltages. Well-known representatives of this class of converters are the “Modular Multilevel Converter” (M2C) and the “Modular Multilevel Matrix Converter” (M3C). This project aimed at a general control approach for a class of such converters to regulate the energy content of the individual branches, for which a subordinate control of the branch currents is used. The number of phases of each of the connected systems is a natural number. In the first phase of the project, it was possible to find a general approach based on the topology of the circuit to describe and identify the control degrees of freedom. On this basis, a generally formulated control approach could be found, whereby systematic procedures for finding the corresponding system matrices could be specified. In addition, it was possible to find an optimization approach in the case that more degrees of freedom are available than required for the control. This made it possible to put the methods known from the literature for controlling the M3C into a common framework, and at the same time improve them. Validation took place in a scaled-down laboratory test setup on various topologies, as well as in simulations with up to 5 phases in one system. In the second project phase, additions were made to the control concept in order to include specific operating modes intended for unique operating points in the concept, for instance equal frequencies in both systems for the M3C. The most important result here is that these approaches can also be used for regular operating points in order to simultaneously reduce the losses and voltage fluctuations of the module capacitors. In addition, methods for dealing with asymmetrical voltages of the connected systems, for control in the event of failure of one or more branches (reduced topologies), and for the simultaneous use of currents and voltages as degrees of freedom in control were developed and examined. The goals of the project were fully achieved and partially exceeded in both phases. The corresponding publications are receiving an excellent echo from the community.

Publications

• Current control and branch energy balancing of the Modular Multilevel Matrix Converter. 2015 IEEE Energy Conversion Congress and Exposition (ECCE) (2015, 9), 6360-6367. IEEE.
  Karwatzki, Dennis; Baruschka, Lennart; Kucka, Jakub & Mertens, Axel
  
• Branch energy balancing with a generalised control concept for modular multilevel topologies — Using the example of the modular multilevel converter. 2016 18th European Conference on Power Electronics and Applications (EPE'16 ECCE Europe) (2016, 9), 1-10. IEEE.
  Karwatzki, Dennis; Baruschka, Lennart; Dokus, Marc; Kucka, Jakub & Mertens, Axel
  
• Control approach for a class of modular multilevel converter topologies. 2016 IEEE Energy Conversion Congress and Exposition (ECCE) (2016, 9), 1-9. IEEE.
  Karwatzki, Dennis & Mertens, Axel
  
• Control of a three-phase to five-phase modular multilevel matrix converter based on a generalized control concept. 2017 19th European Conference on Power Electronics and Applications (EPE'17 ECCE Europe), 57(2017, 9),P.1-P.10. IEEE.
  Karwatzki, Dennis; Dokus, Marc & Mertens, Axel
  
• „Analyse und Regelung einer Klasse von modularen Multilevelumrichter-Topologien“, Dissertation, 2017
  Karwatzki, D.
  
• Generalized Control Approach for a Class of Modular Multilevel Converter Topologies. IEEE Transactions on Power Electronics, 33(4), 2888-2900.
  Karwatzki, Dennis & Mertens, Axel
  
• Optimization of the Low-Frequency Mode for Modular Multilevel Converters Based On Frequency Domain Analysis. 2019 21st European Conference on Power Electronics and Applications (EPE '19 ECCE Europe) (2019, 9). IEEE.
  Dierks, Rebecca; Kucka, Jakub & Mertens, Axel
  
• Using Both the Circulating Currents and the Common-Mode Voltage for the Branch Energy Control of Modular Multilevel Converters. 2020 22nd European Conference on Power Electronics and Applications (EPE'20 ECCE Europe), 114(2020, 9),P.1-P.10. IEEE.
  Dierks, Rebecca; Kucka, Jakub & Mertens, Axel
  
• Validation of an Extended Loss-Optimised Branch Energy Control for Modular Multilevel Converters under Unbalanced Grid Conditions. IECON 2021 – 47th Annual Conference of the IEEE Industrial Electronics Society (2021, 10, 13). IEEE.
  Dierks, Rebecca & Mertens, Axel
  
• „Novel Operation Mode of the Modular Multilevel Matrix Converter based on a Dimensioning Algorithm,“ in 2022 24th European Conference on Power Electronics and Applications (EPE'22 ECCE Europe), Hanover, Germany, 2022
  Mertens, R. & Dierks A.
  
• Reduced 8-Branch Modular Multilevel Matrix Converter with an Operating Point Optimised Control Strategy in a Branch Failure Event. 2023 25th European Conference on Power Electronics and Applications (EPE'23 ECCE Europe) (2023, 9, 4), 1-9. IEEE.
  Himker, Rebecca & Mertens, Axel