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Predictive Sorting Algorithm for Modular Multilevel Converters Minimizing the Spread in the Submodule Capacitor Voltages
KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
ABB Corporate Research, Västerås, Sweden .
KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.ORCID iD: 0000-0002-8565-4753
KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.ORCID iD: 0000-0002-1755-1365
2015 (English)In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, Vol. 30, no 1, 440-449 p.Article in journal (Refereed) Published
Abstract [en]

The balancing of the capacitor voltages in modular multilevel converters becomes increasingly difficult when the switching frequency is reduced. Typically, a reduced switching frequency will increase the spread in the capacitor voltages and, thus, the voltage ripple in the individual submodules. This paper presents a capacitor voltage balancing strategy which aims to combine a low switching frequency with a low capacitor-voltage ripple. This is done by a predictive algorithm that controls the converter in such a way that the stored charge in the submodule capacitors is evenly distributed among all the submodules when the capacitor voltages reach their maximum values. In this way, it is possible to limit the peak voltages in the submodule capacitors at switching frequencies as low as 2-3 times the fundamental switching frequency. The proposed capacitor voltage balancing strategy is validated by both simulations and experimental results with 130-Hz and 140-Hz switching frequency. In the simulations, the capacitor voltage ripple was reduced by 24% compared to the case when a conventional sorting algorithm is used, and the experimental results show that it is possible to combine the proposed voltage balancing strategy with a circulating-current controller.

Place, publisher, year, edition, pages
2015. Vol. 30, no 1, 440-449 p.
Keyword [en]
Capacitor voltage balancing, modular multilevel converter (MMC), switching frequency
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:kth:diva-153752DOI: 10.1109/TPEL.2014.2308018ISI: 000341624200037Scopus ID: 2-s2.0-84906810329OAI: oai:DiVA.org:kth-153752DiVA: diva2:753562
Funder
StandUp
Note

QC 20150623

Available from: 2014-10-08 Created: 2014-10-08 Last updated: 2017-12-05Bibliographically 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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