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On Energy Storage Requirements 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.ORCID-id: 0000-0002-1755-1365
2014 (Engelska)Ingår i: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, Vol. 29, nr 1, s. 77-88Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

The modular multilevel converter is a promising topology for high-voltage and high-power applications. By using submodules equipped with dc-capacitors excellent output voltage waveforms can be obtained at low switching frequencies. The rated energy storage of the submodule capacitors is a driving factor of the size, cost, and weight of the submodules. Although the modular multilevel converter has been thoroughly investigated in the literature, a more detailed analysis of the energy-storage requirements will provide an important contribution for dimensioning and analysis of modular multilevel converters. Such an analysis is presented in this paper. The analysis relates the power transfer capability to the stored energy in the converter and the findings are validated by both simulations and experimental results. The required size of the submodule capacitors in a 4.5 MW grid-connected converter is first calculated and the calculated operating range is then compared with simulation results. The experimental results show that if the average capacitor voltage is allowed to increase 10% above the nominal value an energy storage to power transfer ratio of 21 J/kW can be achieved. It is concluded that the presented theory can relate the power transfer capability to the energy storage in the converter and is thus a valuable tool in the design and analysis of modular multilevel converters.

Ort, förlag, år, upplaga, sidor
2014. Vol. 29, nr 1, s. 77-88
Nyckelord [en]
AC-DC power converters, energy storage, HVDC transmission
Nationell ämneskategori
Annan elektroteknik och elektronik
Identifikatorer
URN: urn:nbn:se:kth:diva-105771DOI: 10.1109/TPEL.2013.2254129ISI: 000324020500012Scopus ID: 2-s2.0-84880863213OAI: oai:DiVA.org:kth-105771DiVA, id: diva2:572046
Forskningsfinansiär
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Anmärkning

QC 20131004

Tillgänglig från: 2012-11-26 Skapad: 2012-11-26 Senast uppdaterad: 2017-12-07Bibliografiskt granskad
Ingår i avhandling
1. Modeling and Design of Modular MultilevelConverters for Grid Applications
Öppna denna publikation i ny flik eller fönster >>Modeling and Design of Modular MultilevelConverters for Grid Applications
2012 (Engelska)Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Stockholm: KTH Royal Institute of Technology, 2012. s. x, 39
Serie
Trita-EE, ISSN 1653-5146 ; 2012:060
Nyckelord
Modular multilevel converter, feed-forward control, modulation, switching frequency, energy storage
Nationell ämneskategori
Annan elektroteknik och elektronik
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 (Engelska)
Opponent
Handledare
Anmärkning

QC 20121127

Tillgänglig från: 2012-11-27 Skapad: 2012-11-26 Senast uppdaterad: 2012-11-27Bibliografiskt granskad
2. Modeling and Design of Modular Multilevel Converters for Grid Applications
Öppna denna publikation i ny flik eller fönster >>Modeling and Design of Modular Multilevel Converters for Grid Applications
2014 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Stockholm: KTH Royal Institute of Technology, 2014. s. x, 55
Serie
TRITA-EE, ISSN 1653-5146 ; 2014:045
Nyckelord
Modular multilevel converter, feed-forward control, modulation, switching frequency, energy storage
Nationell ämneskategori
Annan elektroteknik och elektronik
Identifikatorer
urn:nbn:se:kth:diva-153762 (URN)978-91-7595-293-2 (ISBN)
Disputation
2014-11-03, Sal F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (Engelska)
Opponent
Handledare
Anmärkning

QC 20141010

Tillgänglig från: 2014-10-10 Skapad: 2014-10-08 Senast uppdaterad: 2016-02-26Bibliografiskt granskad

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