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Modeling and Design of Modular MultilevelConverters for Grid Applications
KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
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 [en]
Modular multilevel converter, feed-forward control, modulation, switching frequency, energy storage
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:kth:diva-105779ISBN: 978-91-7501-580-4 (print)OAI: oai:DiVA.org:kth-105779DiVA: diva2:572136
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
List of papers
1. Steady-State Analysis of Interaction Between Harmonic Components of Arm and Line Quantities of Modular Multilevel Converters
Open this publication in new window or tab >>Steady-State Analysis of Interaction Between Harmonic Components of Arm and Line Quantities of Modular Multilevel Converters
2012 (English)In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, Vol. 27, no 1, 57-68 p.Article in journal (Refereed) Published
Abstract [en]

The fundamental frequency component in the arm currents of a modular multilevel converter is a necessity for the operation of the converter, as is the connection and bypassing of the submodules. Inevitably, this will cause alternating components in the capacitor voltages. This paper investigates how the arm currents and capacitor voltages interact when the submodules are connected and bypassed in a sinusoidal manner. Equations that describe the circulating current that is caused by the variations in the total inserted voltage are derived. Resonant frequencies are identified and the resonant behaviour is verified by experimental results. It is also found that the effective values of the arm resistance and submodule capacitances can be extracted from the measurements by least square fitting of the analytical expressions to the measured values. Finally, the analytical expression for the arm currents is verified by experimental results.

Place, publisher, year, edition, pages
IEEE: , 2012
Keyword
Analytical steady-state model, arm currents, experimental results, modular multilevel converter, phase-leg resonance
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-55863 (URN)10.1109/TPEL.2011.2159809 (DOI)000298048500004 ()2-s2.0-83755187987 (Scopus ID)
Funder
StandUp
Note

QC 20150626

Available from: 2012-01-03 Created: 2012-01-03 Last updated: 2017-12-08Bibliographically approved
2. Analysis of arm current harmonics in modular multilevel converters with main-circuit filters
Open this publication in new window or tab >>Analysis of arm current harmonics in modular multilevel converters with main-circuit filters
2012 (English)In: International Multi-Conference on Systems, Signals and Devices, SSD 2012 - Summary Proceedings, IEEE , 2012Conference paper, Published paper (Refereed)
Abstract [en]

In a modular multilevel converter the circulating current that flows through each phase leg can affect the performance and efficiency of the converter. If measures are not taken to control the circulating current, it will inevitably contain a second-order harmonic. There are various solutions for eliminating this second-order harmonic. One of the proposed solutions includes a main-circuit filter that is tuned to block the second-order harmonic in the circulating current. This paper presents an analytical relation between the ac-side current and the higher-order harmonics in the circulating current when such a filter is used. It is found that when third-order harmonic injection is used, a large fourth-order harmonic component may appear in the circulating current. This is verified by simulating a 32-MVA converter designed for grid connected applications. The simulation results support the conclusion that it is essential to take this effect into consideration when designing the main-circuit filter.

Place, publisher, year, edition, pages
IEEE, 2012
Keyword
AC-side, Analytical relations, Circulating current, Current harmonics, Fourth-order, Grid-connected, Harmonic components, Harmonic injection, Higher order harmonics, Modular multilevel converters, Second orders, Third-order
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-92265 (URN)10.1109/SSD.2012.6197915 (DOI)2-s2.0-84861630463 (Scopus ID)978-146731590-6 (ISBN)
Conference
9th International Multi-Conference on Systems, Signals and Devices, SSD 2012;Chemnitz;20 March 2012 through 23 March 2012
Funder
StandUp
Note

QC 20120802

Available from: 2012-03-30 Created: 2012-03-30 Last updated: 2016-04-21Bibliographically approved
3. Controlling the ac-side voltage waveform in a modular multilevel converter with low energy-storage capability
Open this publication in new window or tab >>Controlling the ac-side voltage waveform in a modular multilevel converter with low energy-storage capability
Show others...
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%.

Keyword
Converter control, Modulation strategy, Multilevel converters
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-40866 (URN)000308003502053 ()2-s2.0-80053483246 (Scopus ID)978-1-61284-167-0 (ISBN)E-ISBN: 978-90-75815-15-3 (ISBN)
Conference
EPE 2011
Funder
StandUp
Note

QC 20110930

Available from: 2011-09-21 Created: 2011-09-21 Last updated: 2014-10-10Bibliographically approved
4. On Energy Storage Requirements in Modular Multilevel Converters
Open this publication in new window or tab >>On Energy Storage Requirements in Modular Multilevel Converters
2014 (English)In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, Vol. 29, no 1, 77-88 p.Article in journal (Refereed) 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.

Keyword
AC-DC power converters, energy storage, HVDC transmission
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-105771 (URN)10.1109/TPEL.2013.2254129 (DOI)000324020500012 ()2-s2.0-84880863213 (Scopus ID)
Funder
StandUp
Note

QC 20131004

Available from: 2012-11-26 Created: 2012-11-26 Last updated: 2017-12-07Bibliographically approved
5. Capacitor Voltage Ripple Shaping in Modular Multilevel Converters Allowing for Operating Region Extension
Open this publication in new window or tab >>Capacitor Voltage Ripple Shaping in Modular Multilevel Converters Allowing for Operating Region Extension
Show others...
2011 (English)In: IECON 2011: 37TH ANNUAL CONFERENCE ON IEEE INDUSTRIAL ELECTRONICS SOCIETY, New York: IEEE , 2011, 4403-4408 p.Conference paper, Published paper (Refereed)
Abstract [en]

The second-order harmonic in the circulating current of a modular multilevel converter (M2C) influences the capacitor voltage ripple. If no measures are taken to control it, it is not possible to operate the converter with unity modulation index. An open-loop method that precalculates the instantaneous values of the circulating current and the capacitor voltages is used, in order to control the circulating current. A desired second-order harmonic is intentionally induced in the circulating current in order to make the peak of the capacitor voltage coincide with the maximum requested voltage, aiming either to extend the limits of the instantaneous available voltage or avoid unnecessarily high capacitor voltages. A method for numerical estimation of the appropriate amplitude and phase of the induced second-order harmonic is described. The method is experimentally evaluated on a three-phase down-scaled laboratory prototype. From the experiments it was found that significantly improved operating conditions could be obtained.

Place, publisher, year, edition, pages
New York: IEEE, 2011
Series
IEEE Industrial Electronics Society. Annual Conference. Proceedings, ISSN 1553-572X
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-61342 (URN)000299032404116 ()2-s2.0-84856515329 (Scopus ID)978-1-61284-972-0 (ISBN)
Conference
37th Annual Conference of the IEEE Industrial Electronics Society, IECON 2011; Melbourne, VIC; 7 November 2011 through 10 November 2011
Funder
StandUp
Note

QC 20120117

Available from: 2012-01-17 Created: 2012-01-17 Last updated: 2016-04-26Bibliographically approved
6. A New Modulation Method for the Modular Multilevel Converter Allowing Fundamental Switching Frequency
Open this publication in new window or tab >>A New Modulation Method for the Modular Multilevel Converter Allowing Fundamental Switching Frequency
2012 (English)In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, Vol. 27, no 8, 3482-3494 p.Article in journal (Refereed) Published
Abstract [en]

This paper presents a new modulation method for the modular multilevel converter. The proposed method is based on a fixed pulse pattern where harmonic elimination methods can be applied. In the proposed modulation method, the pulse pattern is chosen in such a way that the stored energy in each submodule remains stable. It is shown that this can be done at the fundamental switching frequency without measuring the capacitor voltages or using any other form of feedback control. Such a modulation scheme has not been presented before. The theoretical results are verified by both simulations and experimental results. The simulation results show successful operation at the fundamental switching frequency with a larger number of submodules. When a smaller number of submodules are used, harmonic elimination methods may be applied. This is verified experimentally on a converter with eight submodules per phase leg. The experimental results verify that stable operation can be maintained at the fundamental switching frequency while successfully eliminating the fifth harmonic in the ac-side voltage.

Keyword
Fixed pulse pattern, modular multilevel converter (M2C), switching frequency, voltage balancing
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-95223 (URN)10.1109/TPEL.2012.2185832 (DOI)000303204100003 ()2-s2.0-84860283132 (Scopus ID)
Funder
StandUp
Note

QC 20150624

Available from: 2012-05-22 Created: 2012-05-21 Last updated: 2017-12-07Bibliographically approved
7. Circulating current control in modular multilevel converters with fundamental switching frequency
Open this publication in new window or tab >>Circulating current control in modular multilevel converters with fundamental switching frequency
Show others...
2012 (English)In: Conference Proceedings - 2012 IEEE 7th International Power Electronics and Motion Control Conference - ECCE Asia, IPEMC 2012, IEEE , 2012, 249-256 p.Conference paper, Published paper (Refereed)
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.

Place, publisher, year, edition, pages
IEEE, 2012
Keyword
Electric converters, Motion control, Power electronics
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-100853 (URN)10.1109/IPEMC.2012.6258844 (DOI)2-s2.0-84866782674 (Scopus ID)978-145772086-4 (ISBN)
Conference
2012 IEEE 7th International Power Electronics and Motion Control Conference - ECCE Asia, IPEMC 2012; Harbin; 2 June 2012 through 5 June 2012
Funder
StandUp
Note

QC 20121121

Available from: 2012-08-20 Created: 2012-08-20 Last updated: 2016-04-21Bibliographically approved

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