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Semi-full-bridge submodule for modular multilevel converters
KTH, School of Electrical Engineering (EES), Electric power and energy systems.
KTH, School of Electrical Engineering (EES), Electric power and energy systems.ORCID iD: 0000-0002-8565-4753
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2015 (English)In: Power Electronics and ECCE Asia (ICPE-ECCE Asia), 2015 9th International Conference on, IEEE conference proceedings, 2015, 1067-1074 p.Conference paper (Refereed)Text
Abstract [en]

The energy variations in each arm of the modular multilevel converter comprises two components. The first component relates to the difference between the instantaneous input and output power of each phase-leg, and the second component relates to the energy which is moved back and forth between the two arms of the phase-leg. The latter component can be reduced or even eliminated if the peak-to-peak amplitude of the alternating voltage is greater than the pole-to-pole voltage of the dc link. This will, however, require submodules which can insert negative voltages. Therefore, a semi-full-bridge submodule which uses less semiconductors than the conventional full-bridge is proposed. Simulation results shows that by using the negative voltage-levels the capacitor voltage ripple can be reduced by up to 59%. Experimental results also shows that a 7-level voltage waveform can be generated using only one semi-full-bridge submodule with two capacitors per arm.

Place, publisher, year, edition, pages
IEEE conference proceedings, 2015. 1067-1074 p.
Keyword [en]
power convertors;7-level voltage waveform;capacitor voltage ripple;modular multilevel converters;negative voltage-levels;peak-to-peak amplitude;pole-to-pole voltage;semifull-bridge submodule;Capacitance;Capacitors;Converters;Energy storage;Modulation;Semiconductor diodes;Switches;energy ripple;full-bridge submodules;modular multilevel converter;modulation index
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:kth:diva-182750DOI: 10.1109/ICPE.2015.7167913ScopusID: 2-s2.0-84961933336OAI: oai:DiVA.org:kth-182750DiVA: diva2:905660
Conference
Power Electronics and ECCE Asia (ICPE-ECCE Asia), 2015 9th International Conference on
Funder
StandUp
Note

QC 20160226

Available from: 2016-02-23 Created: 2016-02-23 Last updated: 2016-02-26Bibliographically 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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