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Circulating current control in modular multilevel converters with fundamental switching frequency
KTH, Skolan för elektro- och systemteknik (EES), Elektrisk energiomvandling.
KTH, Skolan för elektro- och systemteknik (EES), Elektrisk energiomvandling.
KTH, Skolan för elektro- och systemteknik (EES), Elektrisk energiomvandling.
KTH, Skolan för elektro- och systemteknik (EES), Elektrisk energiomvandling.ORCID-id: 0000-0002-8565-4753
Vise andre og tillknytning
2012 (engelsk)Inngår i: Conference Proceedings - 2012 IEEE 7th International Power Electronics and Motion Control Conference - ECCE Asia, IPEMC 2012, IEEE , 2012, s. 249-256Konferansepaper, Publicerat paper (Fagfellevurdert)
Abstract [en]

The modular multilevel converter is a suitable topology for high-voltage applications as it combines very low switching frequency and excellent harmonic performance. In fact, it has been shown that the modular multilevel converter can even be operated at the fundamental switching frequency. If the circulating current is not controlled, a second-order harmonic component will appear. This component increases the resistive losses and the capacitor voltage ripple. Different control methods have been developed for eliminating this component in the circulating current. These are, however, based on continuous representations of the system and no control method suitable for fundamental switching frequency have yet been proposed. This paper presents a control method that combines a fundamental switching frequency scheme with an active control of the circulating current. The controller is verified experimentally on a 10-kVA laboratory prototype with five submodules per arm. The experimental validation is performed in both inverter and rectifier modes.

sted, utgiver, år, opplag, sider
IEEE , 2012. s. 249-256
Emneord [en]
Electric converters, Motion control, Power electronics
HSV kategori
Identifikatorer
URN: urn:nbn:se:kth:diva-100853DOI: 10.1109/IPEMC.2012.6258844Scopus ID: 2-s2.0-84866782674ISBN: 978-145772086-4 (tryckt)OAI: oai:DiVA.org:kth-100853DiVA, id: diva2:545454
Konferanse
2012 IEEE 7th International Power Electronics and Motion Control Conference - ECCE Asia, IPEMC 2012; Harbin; 2 June 2012 through 5 June 2012
Forskningsfinansiär
StandUp
Merknad

QC 20121121

Tilgjengelig fra: 2012-08-20 Laget: 2012-08-20 Sist oppdatert: 2016-04-21bibliografisk kontrollert
Inngår i avhandling
1. Modeling and Design of Modular MultilevelConverters for Grid Applications
Åpne denne publikasjonen i ny fane eller vindu >>Modeling and Design of Modular MultilevelConverters for Grid Applications
2012 (engelsk)Licentiatavhandling, med artikler (Annet vitenskapelig)
Abstract [en]

Grid-connected high-power converters are found in high-voltage direct current transmission (HVDC), static compensators (STATCOMs), and supplies for electric railways. Such power converters should have a high reliability, high efficiency, good harmonic performance, low cost, and a small footprint. Cascaded converters are promising solutions for high-voltage high-power converters since they allow the combination of excellent harmonic performance and low switching frequencies. A high reliability can also be achieved by including redundant submodules in the chain of cascaded converters.

One of the emerging cascaded converter topologies is the modular multilevel converter (M2C). This thesis aims to bring clarity to the dimensioning aspects and limiting factors of M2Cs. The dc-capacitor in each submodule is a driving factor for the size and weight of the converter. It is found that the voltage variations across the submodule capacitors will distort the voltage waveforms and also induce alternating components in the current that is circulating between the phase-legs. It is, however, shown that it is possible to control the alternating voltage by feed-forward control. It is also shown that if the circulating current is controlled, the injection of a second-order harmonic component can extend the operating region of the converter. The reason for this is that when the converter is operating close to the boundary of overmodulation the phase and amplitude of the second-order harmonic is chosen such that the capacitors are charged prior to the time when a high voltage should be inserted by the submodules.

The controller that is used must be able to balance the sbmodule capacitor voltages. Typically, an increased switching frequency will enhance the performance of the balancing control scheme. In this thesis it is shown that the capacitor voltages can be balanced with programmed modulation, even if fundamental switching frequency is used. This will, however, increase the voltage ripple across the aforementioned capacitors. In order to quantify the requirements on the dc-capacitors a general analysis is provided in this thesis which is based on the assumption that the capacitor voltages are well balanced. It is found that for active power transfer, with a 50 Hz sinusoidal voltage reference, the capacitors must be rated for a combined energy storage of 21 kJ/MW if the capacitor voltages are allowed to increase by 10% above their nominal values.

sted, utgiver, år, opplag, sider
Stockholm: KTH Royal Institute of Technology, 2012. s. x, 39
Serie
Trita-EE, ISSN 1653-5146 ; 2012:060
Emneord
Modular multilevel converter, feed-forward control, modulation, switching frequency, energy storage
HSV kategori
Identifikatorer
urn:nbn:se:kth:diva-105779 (URN)978-91-7501-580-4 (ISBN)
Presentation
2012-12-17, H1, Teknikringen 33, KTH, Stockholm, 10:00 (engelsk)
Opponent
Veileder
Merknad

QC 20121127

Tilgjengelig fra: 2012-11-27 Laget: 2012-11-26 Sist oppdatert: 2020-01-22bibliografisk kontrollert
2. 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: 2020-01-22bibliografisk kontrollert

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