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Analysis and Operation of Modular Multilevel Converters With Phase-Shifted Carrier PWM
KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
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, 268-283 p.Article in journal (Refereed) Published
Abstract [en]

Many publications have been presented on the modulation and control of the modular multilevel converter, some of which are based on phase-shifted carrier modulation. This paper presents an analysis of how the switching frequency affects the capacitor voltages, circulating currents, and alternating voltages using phase-shifted carrier modulation. It is found that switching frequencies that are integer multiples of the fundamental frequency should be avoided as they can cause the capacitor voltages to diverge. Suitable switching frequencies are derived for which the arm and line quantities will be periodic with symmetric operating conditions in the upper and lower arms. Thus, the practical outcome of this paper is a detailed description of how the switching frequency should be chosen in order to achieve advantageous operating conditions. The theoretical results from the analysis are validated by both simulations and experimental results.

Place, publisher, year, edition, pages
2015. Vol. 30, no 1, 268-283 p.
Keyword [en]
Capacitor voltages, circulating current, modular multilevel converter (M2C), phase-shifted PWM, switching harmonics
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:kth:diva-153755DOI: 10.1109/TPEL.2014.2321049ISI: 000341624200023Scopus ID: 2-s2.0-84906810276OAI: oai:DiVA.org:kth-153755DiVA: diva2:753564
Funder
StandUp
Note

QC 20150622

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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