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Controlling the ac-side voltage waveform in a modular multilevel converter with low energy-storage capability
KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics (closed 20110930).
KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics (closed 20110930).
KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics (closed 20110930).ORCID iD: 0000-0002-8565-4753
KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics (closed 20110930).
Show others and affiliations
2011 (English)In: Proceedings of the 2011-14th European Conference on Power Electronics and Applications (EPE 2011) / [ed] EPE Association, 2011, 1-8 p.Conference paper, Published paper (Refereed)
Abstract [en]

During nominal operation of a modular multilevel converter the stored energy in the submodule capacitors will vary with time. If the energy storage capability of the capacitors is relatively small compared to the energy variations, this will give large variations in the capacitor voltages. These voltage variations will distort the ac-side voltage waveform and induce harmonic components in the current that is circulating between the dc terminals. The adverse effects on the ac-side voltage can be compensated for by identifying the factors that cause the distortion. It is shown that the compensation can be done by means of feed forward control while maintaining stable operating conditions and thus eliminating the need of additional stabilizing controllers. It is also shown that the voltage controller can be combined with a circulating current controller that removes the harmonics in the current that is circulating between the dc terminals. The functionality of the proposed controller is verified by both simulations and experimental results from a 10 kVA laboratory prototype. The simulations illustrate how the proposed controller successfully removes the distortion from the ac-side voltage waveform. The experimental results demonstrate stable operation during a step transient when the output power is increased by 125%.

Place, publisher, year, edition, pages
2011. 1-8 p.
Keyword [en]
Converter control, Modulation strategy, Multilevel converters
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
SRA - Energy
Identifiers
URN: urn:nbn:se:kth:diva-40866ISI: 000308003502053Scopus ID: 2-s2.0-80053483246ISBN: 978-1-61284-167-0 (print)ISBN: E-ISBN: 978-90-75815-15-3 (print)OAI: oai:DiVA.org:kth-40866DiVA: diva2:442528
Conference
EPE 2011
Funder
StandUp
Note

QC 20110930

Available from: 2011-09-21 Created: 2011-09-21 Last updated: 2014-10-10Bibliographically approved
In thesis
1. Modeling and Design of Modular MultilevelConverters for Grid Applications
Open this publication in new window or tab >>Modeling and Design of Modular MultilevelConverters for Grid Applications
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
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.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. x, 39 p.
Series
Trita-EE, ISSN 1653-5146 ; 2012:060
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-105779 (URN)978-91-7501-580-4 (ISBN)
Presentation
2012-12-17, H1, Teknikringen 33, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20121127

Available from: 2012-11-27 Created: 2012-11-26 Last updated: 2012-11-27Bibliographically approved
2. 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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