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On energy variations in modular multilevel converters with full-bridge submodules for Ac-Dc and Ac-Ac applications
KTH, School of Electrical Engineering (EES), Electrical Energy Conversion. (High Power Electronics)
KTH, School of Electrical Engineering (EES), Electrical Energy Conversion. (High Power Electronics)ORCID iD: 0000-0002-8565-4753
KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.ORCID iD: 0000-0002-1755-1365
2013 (English)In: Power Electronics and Applications (EPE), 2013 15th European Conference on, 2013Conference paper, Published paper (Refereed)
Abstract [en]

The modular multilevel converter is a promising topology for high-voltage high-power applications. By the series-connection of submodules it can generate high-quality voltage waveforms with low harmonic distortion at low switching frequencies. The submodules are low-voltage converters with capacitive energy storages. These capacitive energy storages are a driving factor of the size, weight, and cost of the converter. For this reason it is important to ensure that the stored energy in the converter is distributed as evenly among the submodules as possible. In this paper it is found that during nominal operating conditions, a large amount of energy is moved back and forth between the upper and lower arms in the converter. These energy oscillations can, however, be reduced or even eliminated if an appropriate modulation index or voltage ratio is used. It is also found that an appropriate modulation index or voltage ratio can reduce the power rating of the semiconductors as well. The theoretical findings are validated by simulating two systems with different voltage ratios for ac-ac conversion.

Place, publisher, year, edition, pages
2013.
Keyword [en]
AC-AC power convertors, AC-DC power convertors, energy storage, power capacitors, AC-AC power converter, AC-DC power converter, capacitive energy storages, energy oscillations, energy variations, full-bridge submodules, modular multilevel converters, modulation index, nominal operating conditions, semiconductor power rating reduction, voltage ratio, Bidirectional control, Capacitors, Energy storage, Inductors, Integrated circuits, Modulation, Topology, AC/AC converter, Energy storage, Multilevel converters, Traction application
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
SRA - Energy
Identifiers
URN: urn:nbn:se:kth:diva-134876DOI: 10.1109/EPE.2013.6634324Scopus ID: 2-s2.0-84890158232OAI: oai:DiVA.org:kth-134876DiVA: diva2:668805
Conference
15th European Conference on Power Electronics and Applications (EPE) 2013
Funder
StandUp
Note

QC 20140225

Available from: 2013-12-02 Created: 2013-12-01 Last updated: 2014-10-10Bibliographically approved
In thesis
1. Modeling and Design of Modular Multilevel Converters for Grid Applications
Open this publication in new window or tab >>Modeling and Design of Modular Multilevel Converters for Grid Applications
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis aims to bring clarity to the dimensioning aspects and limiting factors of the modular multilevel converter (MMC). Special consideration is given to the dc capacitors in the submodules as they are a driving factor for the size and weight of the converter. It is found that if the capacitor voltages are allowed to increase by 10% the stored energy must be 21 kJ/MW in order to compensate the capacitor voltage ripple. The maximum possible output power can, however, be increased by injecting a second-order harmonic in the circulating current.

A great advantage of cascaded converters is the possibility to achieve excellent harmonic performance at low switching frequencies. Therefore, this thesis also considers the relation between switching harmonics, capacitor voltage ripple, and arm quantities. It is shown that despite subharmonics in the capacitor voltages, it is still possible to achieve periodic arm quantities. The balancing of the capacitor voltages is also considered in further detail. It is found that it is possible to balance the capacitor voltages even at fundamental switching frequency although this will lead to a comparably large capacitor voltage ripple. Therefore, in order to limit the peak-to-peak voltage ripple, it is shown that a predictive algorithm can be used in which the resulting switching frequency is approximately 2–3 times the fundamental frequency.

This thesis also presents two new submodule concepts. The first submodule simply improves the trade-off between the switching frequency and capacitor voltage balancing. The second submodule includes the possibility to insert negative voltages which allows higher modulation indices compared to half-bridge submodules.

A brief comparison of cascaded converters for ac-ac applications is also presented. It is concluded that the MMC appears to be well suited for ac-ac applications where input and output frequencies are close or equal, such as in interconnection of ac grids. In low-frequency applications such as low-speed drives, however, the difficulties with handling the energy variations in the converter arms are much more severe in the MMC compared to the other considered topologies.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. x, 55 p.
Series
TRITA-EE, ISSN 1653-5146 ; 2014:045
Keyword
Modular multilevel converter, feed-forward control, modulation, switching frequency, energy storage
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-153762 (URN)978-91-7595-293-2 (ISBN)
Public defence
2014-11-03, Sal F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20141010

Available from: 2014-10-10 Created: 2014-10-08 Last updated: 2016-02-26Bibliographically approved

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Norrga, StaffanNee, Hans-Peter

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