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A Submodule Implementation for Parallel Connection of Capacitors in Modular Multilevel Converters
KTH, Skolan för elektro- och systemteknik (EES), Elektrisk energiomvandling.
KTH, Skolan för elektro- och systemteknik (EES), Elektrisk energiomvandling.ORCID-id: 0000-0002-8565-4753
KTH, Skolan för elektro- och systemteknik (EES), Elektrisk energiomvandling.
Vise andre og tillknytning
2015 (engelsk)Inngår i: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, Vol. 30, nr 7, s. 3518-3527Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

In modular multilevel converters there is a trade-off between the switching frequency and the voltage ripple in the submodule capacitors. The reason for this is that it becomes increasingly difficult to balance the capacitor voltages when the switching frequency is reduced. This paper presents a new submodule circuit which improves the balancing of the capacitor voltages at low switching frequencies. The proposed submodule circuit consists of two capacitors and eight switches, forming a three-level submodule. Ideally, the voltage and current ratings of the switches can be chosen such that the combined power rating of the semiconductors is the same as for the equivalent solution with conventional half-bridge submodules. The proposed submodule circuit provides the possibility of connecting the two capacitors in parallel when the intermediate voltage level is used. This will reduce the capacitor voltage ripple, especially at low switching frequencies and thus allow for a reduction of the size, weight, and cost of the submodule capacitors. The proposed submodule circuit is validated by both simulations and experimental results. It is found that the parallel connection of the submodule capacitors will, in fact, significantly improve the balancing of the capacitor voltages.

sted, utgiver, år, opplag, sider
2015. Vol. 30, nr 7, s. 3518-3527
Emneord [en]
Capacitor voltages, modular multilevel converter (MMC), switching losses
HSV kategori
Identifikatorer
URN: urn:nbn:se:kth:diva-153757DOI: 10.1109/TPEL.2014.2345460ISI: 000353126300007Scopus ID: 2-s2.0-84923933547OAI: oai:DiVA.org:kth-153757DiVA, id: diva2:753567
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QC 20150622

Tilgjengelig fra: 2014-10-08 Laget: 2014-10-08 Sist oppdatert: 2017-12-05bibliografisk kontrollert
Inngår i avhandling
1. Modeling and Design of Modular Multilevel Converters for Grid Applications
Åpne denne publikasjonen i ny fane eller vindu >>Modeling and Design of Modular Multilevel Converters for Grid Applications
2014 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
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.

sted, utgiver, år, opplag, sider
Stockholm: KTH Royal Institute of Technology, 2014. s. x, 55
Serie
TRITA-EE, ISSN 1653-5146 ; 2014:045
Emneord
Modular multilevel converter, feed-forward control, modulation, switching frequency, energy storage
HSV kategori
Identifikatorer
urn:nbn:se:kth:diva-153762 (URN)978-91-7595-293-2 (ISBN)
Disputas
2014-11-03, Sal F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (engelsk)
Opponent
Veileder
Merknad

QC 20141010

Tilgjengelig fra: 2014-10-10 Laget: 2014-10-08 Sist oppdatert: 2016-02-26bibliografisk kontrollert

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