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On the Internal Dynamics and AC-Motor Drive Application of Modular Multilevel Converters
KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is an effort to investigate the operation and the performanceof modular multilevel converters (M2Cs). Proven to be the most promisingtopology in high-voltage high-power applications, it is necessary to put aneffort in understanding the physical laws that govern the internal dynamicsof such converters, in order to design appropriate control methods. AlthoughM2Cs belong to the well-studied family of voltage-source converters (VSCs),and claim a modular structure, their control is significantly more complicatedcompared to two- or three-level VSCs, due to the fact that a much highernumber of switches and capacitors are needed in such a topology. This thesishighlights the important parameters that should be considered when designingthe control for an M2C, through analyzing its internal dynamics, and alsosuggests ways to control such converters ensuring stable operation withoutcompromising the performance of the converter.Special focus is given on ac motor-drive applications as they are very demandingand challenging for the converter performance. Interactions betweenthe internal dynamics and the dynamics of the driven motor are experimentallyinvestigated. The problem of operating the converter when connectedto a motor standing still is visited, even under the condition that a greatamount of torque and current are requested, in order to provide an idea forthe converter requirements under such conditions. Finally, an optimization ofthe converter operation is suggested in order to avoid overrating the convertercomponents in certain operation areas that this is possible.All analytical investigations presented in this thesis are confirmed by experimentalresults on a laboratory prototype converter, which was developedfor the purposes of this project. Experimental verification proves the validityof the theoretical investigations, as well as the correct performance of thecontrol methods developed during this project on a real, physical converter,hoping that the results of this thesis will be useful for large-scale implementations,in the mega- or even giga-watt power range.

Abstract [sv]

Denna avhandling är ett försök att undersöka drift och egenskaper avmodulära multinivåomvandlare (M2C:er). Eftersom denna topologi anses varaden mest lovande inom högspänings-högeffekt-tillämpningar är, och somett underlag för att kunna formulera lämpliga styrmetoder, är det nödvändigtatt lägga kraft i att försöka förståde fysikaliska lagar som styr den inredynamiken i sådana omvandlare. Även om M2C:erna tillhör den välstuderadefamiljen av spänningsstyva omvandlare (VSC:er), och har en modulärstruktur, är deras reglering avsevärt mer komplicerad jämfört med två- ellertre-nivåomvandlare, eftersom ett mycket större antal switchar och kondensatorerär nödvändiga i en sådan topologi. Denna avhandling sätter fingretpå de parametrar som måste beaktas när man konstruerar regleringen för enM2C, genom att analysera den interna dynamiken, samt att föreslå sätt attstyra sådana omvandlare såatt stabil drift kan säkerställas utan att negativtpåverka prestanda.Ett speciellt fokus läggs på växelströmsmotordrifter eftersom de är särskiltutmanande vad gäller prestanda. Växelverkan mellan den interna dynamikenoch motorns dynamik undersöks experimentellt. Problemet att driva motornvid stillestånd behandlas även i fallet med hög ström och högt moment för atterhålla kunskap om kraven påomvandlaren i sådana fall. Slutligen föreslås enoptimering av omvandlarens drifttillstånd för att undvika överdimensioneringav omvandlarens komponenter i de fall detta är möjligt.Alla analytiska undersökningar som läggs fram i denna avhandling är bekräftadegenom experimentella resultat från en laboratorieomvandlare, somutvecklats inom ramen för detta arbete. Den experimentella verifieringen bevisargiltigheten av alla teoretiska undersökningar. Den visar också på demycket goda prestanda som de utvecklade styrmetoderna har vid drift aven verklig fysisk omvandlare. Förhoppningen är att resultaten från detta arbetekan komma till använding i storskaliga implementerinar i mega- ellergiga-wattklassen.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. , xii, 74 p.
Series
TRITA-EE, ISSN 1653-5146 ; 2014:063
Keyword [en]
Capacitor-Voltage Control, Controller Interaction, Converter Control, Energy Balance, Lyapunov Stability, Medium-Voltage Drives, Modular Multilevel Converter, Modulation, Open-Loop Control, Optimization, Variable-Speed Drives, Voltage-Source Converter.
Keyword [sv]
Energibalans, Kondensatorspänningsstyrning, Lyapunov-stabilitet, Mellan-spänning drivsystem, Modulation, Modulär multinivåomvandlare, Omvandlarstyrning, Optimering, Regulatorväxelverkan, Spänningsstyva omvandlare, Varvtalsstyrda motordrifter, Öppen styrning.
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-156200ISBN: 978-91-7595-382-3 (print)OAI: oai:DiVA.org:kth-156200DiVA: diva2:765743
Public defence
2014-12-15, Sal F3, Lindstedsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20141201

Available from: 2014-12-01 Created: 2014-11-24 Last updated: 2014-12-01Bibliographically approved
List of papers
1. On Dynamics and Voltage Control of the Modular Multilevel Converter
Open this publication in new window or tab >>On Dynamics and Voltage Control of the Modular Multilevel Converter
2009 (English)In: 2009 13th European Conference on Power Electronics and Applications, EPE '09, IEEE , 2009, 3353-3362 p.Conference paper, Published paper (Refereed)
Abstract [en]

This paper discusses the impact of modulation on stability issues of the Modular Multilevel Converter (M2C). The main idea is to describe the operation of this converter system mathematically, and suggest a control method that offers stable operation in the whole operation range. A possible approach is to assume a continuous model, where all the modules in each arm are represented by variable voltage sources. and as a result, all pulse width modulation effects are disregarded. After simulating this model and testing different control methods, useful conclusions on the operation of the M2C have been extracted. The control methods are then implemented on a model with discrete half-bridge modules, in order to compare the results and to validate continuous model approach. When assuring that this model functions as expected, the goal of this paper is to conclude into a self-stabilizing voltage controller. A controller is proposed, which eliminates circulating currents between the phase legs and balances the arm voltages regardless of the imposed alteranting current.

Place, publisher, year, edition, pages
IEEE, 2009
Keyword
Converter control, Multilevel converters, Modulation strategy, Emerging topology, VSC
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-30152 (URN)000275384101138 ()2-s2.0-72949122524 (Scopus ID)978-1-4244-4432-8 (ISBN)
Conference
2009 13th European Conference on Power Electronics and Applications, EPE '09; Barcelona; Spain; 8 September 2009 through 10 September 2009
Note

QC 20110304

Available from: 2011-03-04 Created: 2011-02-21 Last updated: 2014-12-01Bibliographically approved
2. Open-Loop Control of Modular Multilevel Converters Using Estimation of Stored Energy
Open this publication in new window or tab >>Open-Loop Control of Modular Multilevel Converters Using Estimation of Stored Energy
Show others...
2011 (English)In: IEEE transactions on industry applications, ISSN 0093-9994, E-ISSN 1939-9367, Vol. 47, no 6, 2516-2524 p.Article in journal (Refereed) Published
Abstract [en]

The internal control of a modular multilevel converter aims to equalize and stabilize the submodule capacitor voltages independent of the loading conditions. It has been shown that a submodule selection mechanism, included in the modulator, can provide voltage sharing inside the converter arm. Several procedures for controlling the total stored energy in each converter arm exist. A new approach is described in this paper. It is based on estimation of the stored energy in the arms by combining the converter electromotive force reference, the measured alternating output current, and the known direct voltage. No feedback controllers are used. Experimental verification on a three-phase 10 kVA prototype is presented along with the description of the new procedure.

Keyword
Terms—Energy balance, modular multilevel converter (M2C), modulation, open-loop control, prototype.
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-48801 (URN)10.1109/TIA.2011.2168593 (DOI)000297343800024 ()2-s2.0-82055172549 (Scopus ID)
Funder
StandUp
Note
QC 20111128Available from: 2011-11-23 Created: 2011-11-23 Last updated: 2017-12-08Bibliographically approved
3. Global Asymptotic Stability of Modular Multilevel Converters
Open this publication in new window or tab >>Global Asymptotic Stability of Modular Multilevel Converters
Show others...
2014 (English)In: IEEE transactions on industrial electronics (1982. Print), ISSN 0278-0046, E-ISSN 1557-9948, Vol. 61, no 2, 603-612 p.Article in journal (Refereed) Published
Abstract [en]

Modular multilevel converters require that the controller is designed so that the submodule capacitor voltages are equalized and stable, independent of the loading conditions. Assuming that the individual capacitor-voltage sharing is managed effectively, an open-loop strategy has been designed to ensure that the total amount of energy stored inside the converter always will be controlled. This strategy, using the steady-state solutions of the dynamic equations for controlling the total stored energy in each converter arm, has proven to be effective. The intention of this paper is to explain in a rigorous way the mechanism behind the suggested strategy and to prove that, when this open-loop strategy is used, the system becomes globally asymptotically stable. Experimental verification on a three-phase 10-kVA prototype is presented.

Place, publisher, year, edition, pages
IEEE, 2014
Keyword
Lyapunov stability, modular multilevel converters (M2Cs), open-loop control, prototype
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-129429 (URN)10.1109/TIE.2013.2254100 (DOI)000323492000001 ()2-s2.0-84882971184 (Scopus ID)
Funder
StandUp
Note

QC 20131001

Available from: 2013-10-01 Created: 2013-09-30 Last updated: 2017-12-06Bibliographically approved
4. On Interaction between Internal Converter Dynamics and Current Control of High-Performance High-Power AC Motor Drives with Modular Multilevel Converters
Open this publication in new window or tab >>On Interaction between Internal Converter Dynamics and Current Control of High-Performance High-Power AC Motor Drives with Modular Multilevel Converters
2010 (English)In: Proc. IEEE Energy Conversion Congress and Exposition (ECCE), 2010, 4293-4298 p.Conference paper, Published paper (Refereed)
Abstract [en]

The modular multilevel converter (M2C) is a promising converter technology for various high-voltage highpower applications. The reason to this is that low-distortion output quantities can be achieved with low average switching frequencies per switch and without output filters. With the M2C the output voltage has such a low harmonic content that highpower motors can be operated without any derating. However, the apparent large number of devices, requires more complex converter control techniques than a two-level counterpart. Even though there have been several ways suggested to control the converter itself, it is still a challenge to investigate the interaction of these controllers with an external motor current controller. It is shown in the paper that the anticipated interaction will not result in any problems neither for the converter nor for the motor control itself.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-34181 (URN)10.1109/ECCE.2010.5618474 (DOI)2-s2.0-78650088280 (Scopus ID)978-142445286-6 (ISBN)
Conference
Energy Conversion Conference and Exposition (ECCE)
Note
QC 20110628Available from: 2011-06-28 Created: 2011-05-27 Last updated: 2014-12-01Bibliographically approved
5. Modular multilevel converter AC motor drives with constant torque from zero to nominal speed
Open this publication in new window or tab >>Modular multilevel converter AC motor drives with constant torque from zero to nominal speed
Show others...
2014 (English)In: IEEE transactions on industry applications, ISSN 0093-9994, E-ISSN 1939-9367, Vol. 50, no 3, 1982-1993 p.Article in journal (Refereed) Published
Abstract [en]

Modular multilevel converters are shown to have a great potential in the area of medium-voltage drives. Low-distortion output quantities combined with low average switching frequencies for the semiconductor devices create an ideal combination for very high-efficiency drives. However, the large number of devices and capacitors that have to conduct the fundamental-frequency current require more complex converter control techniques than its two-level counterpart. Special care needs to be taken for starting and operation at low speeds, where the low-frequency current may cause significant unbalance between the submodule capacitor voltages and disturb the output waveforms. In this paper, principles for converter operation with high torque in the whole speed range are investigated. Experimental results from a down-scaled 12-kVA prototype converter running a loaded motor at various speeds between standstill and the rated speed are also provided.

Keyword
constant torque, medium-power variable-speed drives, Modular multilevel converters
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-134880 (URN)10.1109/TIA.2013.2286217 (DOI)000338984300040 ()2-s2.0-84901298816 (Scopus ID)
Funder
StandUp
Note

QC 20140613

Available from: 2013-12-02 Created: 2013-12-01 Last updated: 2017-12-06Bibliographically approved
6. Optimal Selection of the Average Capacitor Voltage for Variable-Speed Drives With Modular Multilevel Converters
Open this publication in new window or tab >>Optimal Selection of the Average Capacitor Voltage for Variable-Speed Drives With Modular Multilevel Converters
2015 (English)In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, Vol. 30, no 1, 227-234 p.Article in journal (Refereed) Published
Abstract [en]

Variable-speed drives have reduced voltage requirementswhen operating below the base speed. In a modularmultilevel-converter-based (M2C-based) motor drive it is thenpossible to operate with reduced voltage in the submodulecapacitors, than at the base speed. In this sense, a greatercapacitor-voltage ripple can be accommodated, without exceedingthe maximum peak-capacitor voltage. This paper presents ananalytical investigation for the optimal selection of the averagecapacitor voltage for M2Cs, when the motor is operating withrated torque, below the base speed. This method does not requireany power exchange between the converter arms, so it keepsthe conduction losses at the minimum level. Additionally, themethod decreases the switching losses, due to the decreasedcapacitor-voltage level. The overall ratings of the converterremain the same as in the base-speed operation. It is shownthat this method can be applied at a speed range betweenthe base speed and down to approximately one third of it,i.e, an operating range that covers the requirements for typicalpump- and fan-type applications. The results obtained from theanalytical investigation are experimentally verified on a downscaledlaboratory prototype M2C.

Keyword
Capacitor-voltage control, medium-power variable-speed drives, modular multilevel converters, optimization.
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
Identifiers
urn:nbn:se:kth:diva-156198 (URN)10.1109/TPEL.2014.2316273 (DOI)000341624200019 ()2-s2.0-84906809639 (Scopus ID)
Note

QC 20141201

Available from: 2014-11-24 Created: 2014-11-24 Last updated: 2017-12-05Bibliographically approved

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