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Efficient Modeling of Modular Multilevel Converters for HVDC Transmission Systems
KTH, School of Electrical Engineering and Computer Science (EECS). (Elkraftteknik, Electric Power and Energy Systems)
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The drive towards getting more and more electrical energy from renewable sources, requires more efficient electric transmission systems. A stronger grid, with more controllability and higher capacity, that can handle power fluctuations due to a mismatch between generation and load is also needed. High-voltage dc (HVDC) provides efficient and economical power transmission over very long distances, and will be a key player in shaping-up the future electric grid. Due to its outstanding features, the modular multilevel converter (MMC) has already been widely accepted as a key converter topology in voltage-source converter (VSC)-based HVDC transmission systems.

In order to study the feasibility of future MMC-based HVDC grids, adequate simulation models are necessary. The main objective of the thesis is to propose MMC reduced-order simulation models capable of accurately replicating the response of an MMC during all relevant operating conditions. Such models are the basic building blocks in developing efficient simulation models for HVDC grids. This thesis presents two MMC equivalent simulation models, the continuous model (CM) and the detailed equivalent model (DEM). Compared to the CM, the DEM is also capable of demonstrating the individual sumodule behavior of an MMC. These models are validated by comparing with the detailed MMC model as well as with experimental results obtained from an MMC prototype in the laboratory. The most significant feature of the models is the representation of the blocking capability of the MMC, presented for the first time in the literature for an MMC equivalent simulation model. This feature is very important in replicating the accurate transient behavior of an MMC during energization and fault conditions. This thesis also investigates the performance of the MMC with redundant submodules in the arms. Two different control strategies are used and compared for integrating redundant submodules.

The proposed MMC models are used in developing point-to-point and multiterminal HVDC (MTDC) systems. A reduced-order model of a hybrid HVDC breaker is also developed and employed in the MTDC system, making the test system capable of accurately replicating the behavior of the MMCbased MTDC system employing hybrid HVDC breakers. The conclusion of the analysis of dc-side faults in a MTDC system is that fast-acting HVDC breakers are necessary to isolate only the faulted part in the MTDC system to ensure the power flow in rest of the system is not interrupted.

A generic four-terminal HVDC grid test system using the CM model is also developed. The simulated system can serve as a standard HVDC grid test system. It is well-suited to electromagnetic transient (EMT) studies in a limited version of commercially available EMT-type software. The dynamic performance of the HVDC grid is studied under different fault conditions.

Abstract [sv]

Utvecklingen mot att utvinna alltmer elektrisk energi från förnybara källor kräver mer effektiva elektriska transmissionssystem. Det behövs också ett starkare nät, med högre styrbarhet och högre kapacitet, som kan hantera effektfluktuationer pga obalans mellan generering och last. Högspänd likströmsöverföring (HVDC) erbjuder energieffektiv och kostnadseffektiv effektöverföring på långa avstånd. På grund av sina överlägsna egenskaper har modulära multinivå-omvandlare (MMC) accepterats som den rådande tekniken för HVDC med spänningsstyva effektomvandlare (VSC).

För att kunna studera framtida MMC-baserade HVDC-nät är lämpliga simuleringsmodeller nödvändiga. Huvudmålet med denna avhandling är att ta fram kompakta simuleringsmodeller för MMC. Dessa kompakta modeller ska kunna efterlikna responsen för en MMC i alla relevanta fall, och ska kunna användas som beräkningseffektiva byggblock vid simulering av HVDC-nät. Denna avhandling presenterar två ekvivalenta simuleringsmodeller för MMC, den kontinuerliga modellen (CM) och den detaljerade ekvivalenta modellen (DEM). I jämförelse med CM kan DEM också representera egenskaper av enstaka submoduler i en MMC. Modellerna valideras genom inbördes jämförelse och jämförelse med experimentella resultat från en MMC prototyp. Den mest betydelsefulla egenskapen hos modellerna är representationen av blockeringsfunktionen för MMC:n, vilket presenterades för första gången för simuleringsmodeller genom detta arbete. Denna funktion är mycket väsentlig för att beskriva transienta egenskaper hos MMC:n vid uppstart och felfall. Avhandlingen undersöker även MMC:ns egenskaper med redundanta submoduler i omvandlar-armarna. Två olika styrmetoder används och jämförs.

De framtagna MMC-modellerna används för att utveckla punkt-till-punkt- och multiterminal-HVDC (MTDC). En kompakt modell för en hybdrid-HVDC-brytare tas också fram och används i MTDC-systemet. Därigenom kan ett MMC-baserat MTDC-system med hybrid-HVDC-brytare beskrivas noggrant. Analysen av fel på likströmssidan av MTDC-systemet fastslår att snabba HVDC-brytare är nödvändiga för att isolera den felbehäftade delen av nätet utan att stoppa effektflödet i resten av systemet.

Ett generiskt fyrterminal-HVDC-system med CM-modellen utvecklas också. Det simulerade systemet kan tjäna som ett standard-testsystem för elektromagnetiskt transienta (EMT) studier vid användning av den begränsade versionen av den kommersiellt tillgängliga EMT-programvaran. De dynamiska egenskaperna av HVDCnätet studeras också för olika felfall.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. , p. 49
Series
TRITA-EECS-AVL ; 2018:39
Keywords [en]
High-Voltage dc, Modular Multilevel Converter, Voltage-Source Converter, HVDC Grids, Multiterminal DC Systems, Hybrid HVDC Breaker, Modeling
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-228310ISBN: 978-91-7729-798-7 (print)OAI: oai:DiVA.org:kth-228310DiVA, id: diva2:1209004
Public defence
2018-06-14, Q2, Osquldas väg 10, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20180522

Available from: 2018-05-22 Created: 2018-05-21 Last updated: 2018-05-22Bibliographically approved
List of papers
1. Prospects and challenges of future HVDC SuperGrids with modular multilevel converters
Open this publication in new window or tab >>Prospects and challenges of future HVDC SuperGrids with modular multilevel converters
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2011 (English)In: Proceedings of the 2011-14th European Conference on Power Electronics and Applications (EPE 2011) / [ed] EPE Association, 2011Conference paper, Published paper (Refereed)
Abstract [en]

In order to transmit massive amounts of power generated by remotely located power plants, especially offshore wind farms, and to balance the intermittent nature of renewable energy sources, the need for a stronger high voltage transmission grid is anticipated. Due to limitations in AC power transmission the most likable choice for such a grid is a high voltage DC (HVDC) grid. However, the concept of the HVDC grid is still under active development as different technical challenges exist, and it is not yet possible to construct such a DC grid. This paper deals with prospects and technical challenges for the future HVDC SuperGrids. Different topologies for a SuperGrid and the possibility to use modular multilevel converters (M2Cs) are presented. A comprehensive overview of different sub-module implementations of M2C is given. An overview of short circuit behaviour of the M2C is also given, as well as a discussion on the choice between cables or overhead lines and DC-side resonance issues.

Keywords
Multilevel converters, Multiterminal HVDC, Power transmission, Voltage Source converter (VSC)
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-40862 (URN)000308003504014 ()2-s2.0-80053497133 (Scopus ID)978-1-61284-167-0 (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: 2018-05-22Bibliographically approved
2. HVDC SuperGrids with modular multilevel converters - The power transmission backbone of the future
Open this publication in new window or tab >>HVDC SuperGrids with modular multilevel converters - The power transmission backbone of the future
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2012 (English)In: International Multi-Conference on Systems, Signals and Devices, SSD 2012, IEEE , 2012, p. 6198119-Conference paper, Published paper (Refereed)
Abstract [en]

In order to transmit massive amounts of power generated by remotely located power plants, especially offshore wind farms, and to balance the intermittent nature of renewable energy sources, the need for a stronger high voltage transmission grid is anticipated. Due to limitations in ac power transmission the most likable choice for such a grid is a high-voltage dc (HVDC) grid. However, the concept of the HVDC grid is still under active development as different technical challenges exist, and it is not yet possible to construct such a dc grid. This paper deals with prospects and technical challenges for future HVDC SuperGrids. Different topologies for a SuperGrid and the possibility to use modular multilevel converters (M2Cs) are presented. A comprehensive overview of different submodule implementations of M2C is given as well as a discussion on the choice between cables or overhead lines, the protection system for the dc grid and dc-side resonance issues.

Place, publisher, year, edition, pages
IEEE, 2012
Keywords
HVDC grid, Multi-terminal, Multilevel Converters, Power Transmission, Voltage Source Converter (VSC)
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-92262 (URN)10.1109/SSD.2012.6198119 (DOI)2-s2.0-84861621116 (Scopus ID)9781467315906 (ISBN)
Conference
9th International Multi-Conference on Systems, Signals and Devices, SSD 2012;Chemnitz; Germany 20 March 2012 through 23 March 2012
Funder
StandUp
Note

QC 20150708

Available from: 2012-03-30 Created: 2012-03-30 Last updated: 2018-05-22Bibliographically approved
3. Validation of the continuous model of the modular multilevel converter with blocking/deblocking capability
Open this publication in new window or tab >>Validation of the continuous model of the modular multilevel converter with blocking/deblocking capability
2012 (English)In: AC and DC Power Transmission (ACDC 2012), 10th IET International Conference on, 2012Conference paper, Published paper (Refereed)
Abstract [en]

This paper presents the continuous model for the Modular Multilevel Converter (M2C). The model operates in two modes, either operating as a voltage source in deblocked mode or as a rectifying diode bridge in blocked mode. The model is validated by comparison with a detailed M2C model having 36 submodules per arm, using different control strategies. The comparison is based on time-domain simulations in PSCAD/EMTDC. The continuous model shows a very good agreement with the detailed model.

Series
IET Conference Publications ; Volume 2012, Issue 610
Keywords
Blocking, Continuous model, Converter modeling, Modular multilevel converter, Simulation
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-107723 (URN)10.1049/cp.2012.1979 (DOI)2-s2.0-84879647770 (Scopus ID)9781849197007 (ISBN)
Conference
10th IET International Conference on AC and DC Power Transmission, ACDC 2012; Birmingham; United Kingdom; 4 December 2012 through 5 December 2012
Funder
StandUp
Note

QC 20130617

Available from: 2012-12-17 Created: 2012-12-17 Last updated: 2018-05-22Bibliographically approved
4. Continuous modeling of open-loop control based negative sequence current control of modular multilevel converters for HVDC transmission
Open this publication in new window or tab >>Continuous modeling of open-loop control based negative sequence current control of modular multilevel converters for HVDC transmission
2013 (English)In: Eur. Conf. Power Electron. Appl., EPE, 2013Conference paper, Published paper (Refereed)
Abstract [en]

Negative sequence currents are obtained during ac-side asymmetrical faults of converters in highvoltage direct current (HVDC) transmission systems. Consequently, second order harmonics in the dc-side voltage and current, unbalanced ac-side currents, and power oscillations can be observed. This paper presents a negative sequence current control (NSCC) scheme that eliminates second order harmonic ripples in the voltage and current of the dc-side during unbalanced grid conditions. Controllers for this purpose are investigated using a continuous model of the modular multilevel converter (M2C). The proposed scheme utilizes an open-loop controller for lower level control of the M2C. The continuous model used also has the capability to model blocking and deblocking events which may be used during protective actions. Simulation results reveal that the proposed NSCC scheme is effective in suppressing dc-side voltage and current ripples. Moreover, it keeps the ac-side phase currents balanced during asymmetrical fault conditions.

Series
2013 15th European Conference on Power Electronics and Applications, EPE 2013
Keywords
Fault handling strategy, HVDC, Modeling, Multilevel converters, Power transmission
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-139412 (URN)10.1109/EPE.2013.6634635 (DOI)2-s2.0-84890226294 (Scopus ID)9781479901166 (ISBN)
Conference
2013 15th European Conference on Power Electronics and Applications, EPE 2013, 2 September 2013 through 6 September 2013, Lille
Note

QC 20140116

Available from: 2014-01-16 Created: 2014-01-13 Last updated: 2018-05-22Bibliographically approved
5. Efficient Modeling of Modular Multilevel Converters in HVDC-Grids Under Fault Conditions
Open this publication in new window or tab >>Efficient Modeling of Modular Multilevel Converters in HVDC-Grids Under Fault Conditions
2014 (English)In: 2014 IEEE PES General Meeting | Conference & Exposition, IEEE Computer Society, 2014, p. 6939166-, article id 6939166Conference paper, Published paper (Refereed)
Abstract [en]

High-voltage direct current (HVDC) grids using modular multilevel converters (M2Cs) have strongly been considered for the integration of distant renewable energy sources and also as a backbone to the existing ac-grids. The dynamic performance of the M2C is of particular interest in these grids. For electromagnetic transient (EMT) programs, modeling of HVDC-grids using detailed M2C models is unrealistic, as it requires extremely high computational effort and simulation time. In this paper an HVDC-grid test system is developed using a continuous simulation model of the M2C. The model is also capable of describing the blocking events of the M2C. Using time-domain simulations in PSCAD/EMTDC, the dynamic performance of the M2C in HVDC-grids under fault conditions is investigated. Simulation results reveal that the continuous M2C model can efficiently be used to study the dynamic performance of the M2C in HVDC-grids with high computational speed, under different fault conditions.

Place, publisher, year, edition, pages
IEEE Computer Society, 2014
Series
IEEE Power and Energy Society General Meeting PESGM, ISSN 1944-9925
Keywords
Converters, HVDC transmission, modeling, modular multilevel converter (M2C), power conversion
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-164495 (URN)10.1109/PESGM.2014.6939166 (DOI)000349551502019 ()2-s2.0-84930986993 (Scopus ID)978-1-4799-6415-4 (ISBN)
Conference
2014 IEEE Power and Energy Society General Meeting, National Harbor, United States, 27 July 2014 through 31 July 2014
Funder
StandUp
Note

QC 20150420

Available from: 2015-04-20 Created: 2015-04-17 Last updated: 2018-05-22Bibliographically approved
6. A computationally efficient continuous model for the modular multilevel converter
Open this publication in new window or tab >>A computationally efficient continuous model for the modular multilevel converter
Show others...
2014 (English)In: IEEE Journal of Emerging and Selected Topics in Power Electronics, ISSN 2168-6777, Vol. 2, no 4, p. 1139-1148, article id 6840290Article in journal (Refereed) Published
Abstract [en]

Simulation models of the modular multilevel converter (MMC) play a very important role for studying the dynamic performance. Detailed modeling of the MMC in electromagnetic transient simulation programs is cumbersome, as it requires high computational effort and simulation time. Several averaged or continuous models proposed in the literature lack the capability to describe the blocked state. This paper presents a continuous model, which is capable of accurately simulating the blocked state. This feature is very important for accurate simulation of faults. The model is generally applicable, although it is particularly useful in high-voltage dc applications.

Place, publisher, year, edition, pages
IEEE, 2014
Keywords
Blocking, continuous model, electromagnetic transient (EMT) programs, high-voltage dc (HVDC) transmission, modular multilevel converter (MMC), voltage-source converter (VSC)
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-166458 (URN)10.1109/JESTPE.2014.2332041 (DOI)000358608400042 ()2-s2.0-84908549452 (Scopus ID)
Funder
StandUp
Note

QC 20150512

Available from: 2015-05-11 Created: 2015-05-11 Last updated: 2018-05-22Bibliographically approved
7. A new HVDC grid test system for HVDC grid dynamics and protection studies in EMT-type software
Open this publication in new window or tab >>A new HVDC grid test system for HVDC grid dynamics and protection studies in EMT-type software
Show others...
2015 (English)In: IET Seminar Digest, Institution of Engineering and Technology, 2015, no CP654Conference paper, Published paper (Refereed)
Abstract [en]

This paper proposes a new HVDC grid test system for electro-magnetic transient analysis, suitable for HVDC power system studies ranging from protection to dynamic studies investigating converter behaviour and interactions. In the recent past research interest in HVDC grids has increased, leading to a multitude of studies concerning dc power flow and optimal power flow, dynamics and HVDC grid protection. However, each of these studies makes use of different grid topologies, configurations and transmission line parameters. In this paper, a standard HVDC grid test system is proposed and an implementation in EMT-type software is provided. The implementation in EMT-type software makes use of a frequency dependent cable model, continuous converter model and a reduced dc breaker model. By means of a protection study, the effectiveness and computational efficiency of the proposed HVDC grid test system is demonstrated. The model with its parameters will be made publicly available.

Place, publisher, year, edition, pages
Institution of Engineering and Technology, 2015
Keywords
Electromagnetic transient simulation, HVDC grid, Power system fault, Power system protection, Test system, Computational efficiency, DC power transmission, Electric load flow, Electric power transmission, Electric power transmission networks, HVDC power transmission, Software testing, Testing, Transient analysis, Test systems, Electric power system protection
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-175093 (URN)10.1049/cp.2015.0068 (DOI)2-s2.0-84939229610 (Scopus ID)
Conference
11th IET International Conference on AC and DC Power Transmission, 10 February 2015 through 12 February 2015
Funder
StandUp
Note

QC 20151211

Available from: 2015-12-11 Created: 2015-10-09 Last updated: 2018-05-22Bibliographically approved
8. Efficient Modeling of an MMC-Based Multiterminal DC System Employing Hybrid HVDC Breakers
Open this publication in new window or tab >>Efficient Modeling of an MMC-Based Multiterminal DC System Employing Hybrid HVDC Breakers
Show others...
2015 (English)In: IEEE Transactions on Power Delivery, ISSN 0885-8977, E-ISSN 1937-4208, Vol. 30, no 4, p. 1792-1801Article in journal (Refereed) Published
Abstract [en]

The feasibility of future multiterminal dc (MTDC) systems depends largely on the capability to withstand dc-side faults. Simulation models of MTDC systems play a very important role in investigating these faults. For such studies, the test system needs to be accurate and computationally efficient. This paper proposes a detailed equivalent model of the modular multilevel converter (MMC), which is used to develop the MTDC test system. The proposed model is capable of representing the blocked-mode operation of the MMC, and can be used to study the balancing control of the capacitor voltages. In addition, the operation of the MMC when redundant submodules are included in the arms can also be studied. A simplified model of a hybrid high-voltage dc breaker is also developed. Hence, the developed test system is capable of accurately describing the behavior of the MMC-based MTDC system employing hybrid HVDC breakers, during fault conditions. Using time-domain simulations, permanent dc-side faults are studied in the MTDC system. In addition, a scheme to control the fault current through the MMC using thyristors on the ac side of the converter is proposed.

Place, publisher, year, edition, pages
IEEE, 2015
Keywords
DC-side faults, HVDC breaker, HVDC transmission, modeling, modular multilevel converter (MMC)
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-172681 (URN)10.1109/TPWRD.2015.2398825 (DOI)000358503900017 ()2-s2.0-84937948815 (Scopus ID)
Funder
StandUp
Note

QC 20150901

Available from: 2015-09-01 Created: 2015-08-27 Last updated: 2018-05-22Bibliographically approved
9. Performance of the modular multilevel converter with redundant submodules
Open this publication in new window or tab >>Performance of the modular multilevel converter with redundant submodules
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2015 (English)In: IECON 2015 - 41st Annual Conference of the IEEE Industrial Electronics Society, Institute of Electrical and Electronics Engineers (IEEE), 2015, p. 3922-3927, article id 7392712Conference paper, Published paper (Refereed)
Abstract [en]

The modular multilevel converter (MMC) is the state-of-the-art voltage-source converter (VSC) topology used for various power-conversion applications. In the MMC, submodule failures can occur due to various reasons. Therefore, additional submodules called the redundant submodules are included in the arms of the MMC to fulfill the fault-safe operation requirement. The performance of the MMC with redundant submodules has not been widely covered in the published literature. This paper investigates the performance of the MMC with redundant submodules in the arms. Two different control strategies are used and compared for integrating redundant submodules. The response of the MMC to a submodule failure for the two strategies is also studied. Moreover, the operation of the MMC with redundant submodules is validated experimentally using the converter prototype in the laboratory.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2015
Keywords
Modular multilevel converter, redundancy
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-185746 (URN)10.1109/IECON.2015.7392712 (DOI)000382950703162 ()2-s2.0-84973163763 (Scopus ID)9781479917624 (ISBN)
Conference
41st Annual Conference of the IEEE Industrial Electronics Society, IECON 2015, Pacifico YokohamaYokohama, Japan, 9 November 2015 through 12 November 2015
Funder
StandUp
Note

QC 20161104

Available from: 2016-04-26 Created: 2016-04-26 Last updated: 2018-05-22Bibliographically approved

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