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Modulation of Modular Multilevel Converters for HVDC Transmission
KTH, School of Electrical Engineering (EES), Electric power and energy systems. KTH Royal Institute of Technology.ORCID iD: 0000-0001-8788-0415
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The outstanding features of modular multilevel converters (MMC) have recently gained much attention in the high-voltage direct-current (HVDC) transmission field. Power quality, converter cost and system performance are three crucial aspects of HVDC MMCs which are directly linked to the converter modulation and switching schemes. High power quality and performance require high switching frequency and large cell capacitor whereas low switching frequency and small cell capacitor are needed to reduce the converter cost.

The main objective of this thesis is to propose a practical switching method for HVDC MMCs which balances the aforementioned contradictory requirements. A mathematical analysis of the converter switching pattern, against the power quality and converter cost, has been conducted to formulate an optimization problem for MMCs. Different objective functions are studied for the formulated optimization problem such as converter loss minimization, voltage imbalance minimization and computational burden minimization. This thesis proposes three methods to address different objective functions. Ultimately, a real-time simulator has been built to practically verify and investigate the performance of the proposed methods in a realistic point-to-point HVDC link.

The most significant outcome of this thesis is the tolerance band-based switching scheme which offers a direct control of the cell capacitor voltage, low power losses, and robust dynamic performance. As a result, the converter switching frequency can reach frequencies as low as 70 Hz (with the proposed cell tolerance band (CTB) method). A modified optimized CTB method is proposed to minimize the converter switching losses and it could reduce the converter switching losses by 60% in comparison to the conventional phase shifted carrier modulation method.

It is concluded intelligent utilization of sorting algorithm can enable efficient HVDC station operation by reducing the converter cost.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. , 57 p.
Series
TRITA-EE, ISSN 1653-5146 ; 2016:139
Keyword [en]
HVDC, switching schemes, modulation, high power converters, power grids
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-192607ISBN: 978-91-7729-098-8OAI: oai:DiVA.org:kth-192607DiVA: diva2:971302
Public defence
2016-10-07, Sal D3, Kungl Tekniska Högskolan, Lindstedtsvägen 5, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20160916

Available from: 2016-09-16 Created: 2016-09-15 Last updated: 2016-09-16Bibliographically approved
List of papers
1. Evaluation of different carrier-based PWM methods for modular multilevel converters for HVDC application
Open this publication in new window or tab >>Evaluation of different carrier-based PWM methods for modular multilevel converters for HVDC application
2012 (English)In: IECON 2012 - 38th Annual Conference on IEEE Industrial Electronics Society, IEEE , 2012, 388-393 p.Conference paper (Refereed)
Abstract [en]

The outstanding features of modular multilevel converters (M2C) make it attractive for high voltage direct current (HVDC) systems. In order to achieve high efficiency in HVDC converter stations, the switching frequency and the capacitor voltage ripple of the converter should be minimized. A suitable modulation algorithm should achieve an optimal tradeoff between these two requirements. This paper evaluates different carrier-based PWM algorithms and discusses the most challenging technical aspects of an efficient M2C. It is observed that decoupling the waveform synthesis from the selection of which cell to switch at each instant has beneficial impact on operation performance. The evaluation is done by time-domain simulation considering a grid connected, three-phase M2C converter and an advanced control system. Results of this study can be used for implementing more economical HVDC converters.

Place, publisher, year, edition, pages
IEEE, 2012
Series
, IEEE Industrial Electronics Society. Annual Conference. Proceedings, ISSN 1553-572X
Keyword
carrier-based PWM, cell number optimization, low switching frequency
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-107765 (URN)10.1109/IECON.2012.6388789 (DOI)000316962900059 ()2-s2.0-84872974834 (ScopusID)978-146732421-2 (ISBN)
Conference
38th Annual Conference on IEEE Industrial Electronics Society, IECON 2012; Montreal, QC; 25 October 2012 through 28 October 2012
Funder
StandUp
Note

QC 20130218

Available from: 2012-12-17 Created: 2012-12-17 Last updated: 2016-09-16Bibliographically approved
2. Tolerance band modulation methods for modular multilevel converters
Open this publication in new window or tab >>Tolerance band modulation methods for modular multilevel converters
Show others...
2015 (English)In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, Vol. 30, no 1, 311-326 p., 6739183Article in journal (Refereed) Published
Abstract [en]

Modular multilevel converters (M2Cs) are increasingly used in high-voltage direct current (HVDC) systems. The efficiency of M2Cs is influenced by the modulation and cell selecting methods, which determines the switching frequency and capacitor voltage ripple in the converter station. A new approach to modulation of the M2C is presented in this paper. Tolerance band methods are employed to obtain the switching instants, and also cell selection. The proposed methods overcome the modulation problem for converters with few cells on one hand and also reduce the sorting efforts for cell balancing purposes of many cells converter on the other hand. Three different algorithms are also proposed to balance the cell capacitor voltages. The evaluation is done in time-domain simulation by which the performance of each method is studied in both the steady-state and transient cases. It is observed that using tolerance band methods not only reduces the switching frequency but also allows for handling severe fault cases in a grid-connected system. Eventually, the most promising tolerance band method has been implemented and verified in a real-time digital simulator, RTDS®. The average switching frequency of 70 Hz has been achieved for the system under study, while the capacitor voltage ripple is limited to 10% of the nominal cell voltage.

Place, publisher, year, edition, pages
IEEE, 2015
Keyword
High-voltage direct current transmission, power conversion, real-time systems, switching frequency, switching loss
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-166885 (URN)10.1109/TPEL.2014.2305114 (DOI)000341624200026 ()2-s2.0-84906748780 (ScopusID)
Funder
StandUp
Note

QC 20150609

Available from: 2015-06-09 Created: 2015-05-21 Last updated: 2016-09-16Bibliographically approved
3. Optimization-Based Cell Selection Method for Grid-Connected Modular Multilevel Converters
Open this publication in new window or tab >>Optimization-Based Cell Selection Method for Grid-Connected Modular Multilevel Converters
2016 (English)In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, Vol. 31, no 4, 2780-2790 p.Article in journal (Refereed) Published
Abstract [en]

Modular multilevel converters (MMCs) are widely used in different applications. Due to low-loss operation, compactness, and high modularity, MMC is extremely attractive for high-voltage direct-current (HVDC) transmission systems. The HVDC station loss is highly related to the converter switching pulse pattern, which is generated by modulation algorithm and cell selection methods. This paper formulates the switching pulse pattern generation, as a versatile optimization problem. The problem constraints and objectives are formulated for HVDC applications and compared with similar problems in the field of computer science. To overcome the computational complexity in solving the introduced optimization problem, a heuristic method is proposed for cell selection algorithm. The method utilizes the current level in order to obtain lossless switching at zero-current crossings. The study of the proposed method, in a time-domain simulation platform, shows that the method can reduce the switching converter losses by 60% compared to carrier-based modulation, maintaining the same capacitor voltage ripple. Eventually, the practical functionality of the proposed method is verified in a real-time digital simulator, RTDS, for a 512-level converter in a point to point HVDC link. Although this paper focuses on HVDC, the mathematical model is applicable for any MMC application.

Place, publisher, year, edition, pages
IEEE, 2016
Keyword
High-voltage direct-current (HVDC) transmission, modular multilevel converter (MMC), power conversion, switching frequency, switching loss, zero-current switching
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-180195 (URN)10.1109/TPEL.2015.2448573 (DOI)000365953100009 ()2-s2.0-84960095195 (ScopusID)
Funder
StandUp
Note

QC 20160120

Available from: 2016-01-20 Created: 2016-01-08 Last updated: 2016-09-16Bibliographically approved
4. Loss evaluation for modular multilevel converters with different switching strategies
Open this publication in new window or tab >>Loss evaluation for modular multilevel converters with different switching strategies
2015 (English)In: Power Electronics and ECCE Asia (ICPE-ECCE Asia), 2015 9th International Conference on, IEEE , 2015, 1558-1563 p.Conference paper (Refereed)
Abstract [en]

Apparently, modular multilevel converter (MMC) has been extensively used in high voltage direct current (HVDC) transmission links in recent years. The efficiency of MMC stations are highly related to the switching methods and semiconductor devices. So, various switching methods and semiconductor devices have been investigated and introduced in the field. This paper settles a benchmark for an HVDC link, based on a real project, and investigates the impact of six different switching methods on the converter loss, utilizing a commercial semiconductor device. The evaluation indicates that switching methods which consider the current level at switching instants are more efficient in comparison with the other methods which only consider the number of switching events. The result of this study is essential for more efficient converter stations.

Place, publisher, year, edition, pages
IEEE, 2015
Keyword
HVDC power convertors, HVDC power transmission, switching convertors, HVDC transmission link, high voltage direct current transmission link, modular multilevel converter station, semiconductor device, Benchmark testing, Capacitors, HVDC transmission, Insulated gate bipolar transistors, Sorting, Switches, Switching loss, HVDC transmission, Power conversion, Switching frequency, Switching loss
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-182747 (URN)10.1109/ICPE.2015.7167984 (DOI)2-s2.0-84961932810 (ScopusID)978-895708254-6 (ISBN)
Conference
Power Electronics and ECCE Asia (ICPE-ECCE Asia), 2015 9th International Conference on
Funder
StandUp
Note

QC 20160418

Available from: 2016-02-23 Created: 2016-02-23 Last updated: 2016-09-16Bibliographically approved
5. Tolerance Band Adaptation Method for Dynamic Operation of Grid-Connected Modular Multilevel Converters
Open this publication in new window or tab >>Tolerance Band Adaptation Method for Dynamic Operation of Grid-Connected Modular Multilevel Converters
2016 (English)In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, no 99, 1-1 p.Article in journal (Refereed) Published
Abstract [en]

The use of modular multilevel converters (MMC) in high-voltage direct current (HVDC) transmission systems has grown significantly in the past decade. The efficiency, cell capacitor voltage ripple and dynamic performance are three contradictory aspects of the MMC which are related to the converter switching scheme. Previously introduced tolerance band-based schemes enable efficient and simple control for grid-connected MMCs. This paper addresses the dynamic operation of tolerance band switching schemes by proposing a dynamic boundary setting technique for steady-state operation and a switching scheme scheduling controller for transient fault handling. The performance of proposed methods are validated in a realistic point-to-point HVDC link, modeled in real-time digital simulator (RTDS) where two converters with 512 cells per arm are implemented. Utilizing the proposed methods will enable efficient implementation of tolerance band-based schemes for different operating points, and also a robust transient fault handling.

Place, publisher, year, edition, pages
IEEE, 2016
Keyword
Capacitors, Converters, HVDC transmission, Harmonic analysis, Integrated circuits, Steady-state, Switche, HVDC transmission, Power conversion, Switching frequency, Switching loss
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-182748 (URN)10.1109/TPEL.2016.2521480 (DOI)000380116600020 ()2-s2.0-84978646167 (ScopusID)
Funder
StandUp
Note

QC 20160311

Available from: 2016-02-23 Created: 2016-02-23 Last updated: 2016-09-16Bibliographically approved

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