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Jahn, I., Bessegato, L., Björk, J., Hohn, F., Norrga, S., Svensson, N., . . . Despouys, O. (2019). A Proposal for Open-Source HVDC Control. In: : . Paper presented at 2019 IEEE PES Innovative Smart Grid Technologies Europe (ISGT-Europe).
Open this publication in new window or tab >>A Proposal for Open-Source HVDC Control
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2019 (English)Conference paper, Published paper (Refereed)
Abstract [en]

Future multiterminal high-voltage direct-current (HVDC) grids are considered an enabling technology to efficiently integrate large amounts of renewable energy into the existing grid. However, already in today’s existing point-to-point HVDC links, harmonic interaction issues and instabilities related to the controland protection system of the converters have been reported. The converter control software is usually black-boxed and problems are therefore solved in close cooperation with the HVDC vendor. This paper aims to provide a starting point for a discussion onan open-source HVDC control system. In particular, it covers the control design including technical and non-technical aspects. The open-source approach can be useful to solve current as wellas future control-related problems, both in point-to-point links as well as in multiterminal and multivendor HVDC grids.

Keywords
HVDC transmission, Open source software
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-257736 (URN)
Conference
2019 IEEE PES Innovative Smart Grid Technologies Europe (ISGT-Europe)
Note

QC 20190917

Available from: 2019-09-03 Created: 2019-09-03 Last updated: 2019-10-14Bibliographically approved
Bessegato, L., Ilves, K., Harnefors, L., Norrga, S. & Östlund, S. (2019). Control and Admittance Modeling of an AC/AC Modular Multilevel Converter for Railway Supplies. IEEE transactions on power electronics
Open this publication in new window or tab >>Control and Admittance Modeling of an AC/AC Modular Multilevel Converter for Railway Supplies
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2019 (English)In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107Article in journal (Refereed) In press
Abstract [en]

Modular multilevel converters (MMCs) can be configured to perform ac/ac conversion, which makes them suitable as railway power supplies. In this paper, a hierarchical control scheme for ac/ac MMCs for railway power supplies is devised and evaluated, considering the requirements and the operating conditions specific to this application. Furthermore, admittance models of the ac/ac MMC are developed, showing how the suggested hierarchical control scheme affects the three-phase and the single-phase side admittances of the converter. These models allow for analyzing the stability of the interconnected system using the impedance-based stability criterion and the passivity-based stability assessment. Finally, the findings presented in this paper are validated experimentally, using a down-scaled MMC. 

Keywords
Modular multilevel converters, ac/ac converters, current control, voltage control, admittance, frequency-domain analysis, linearization techniques, stability, railway engineering.
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
Identifiers
urn:nbn:se:kth:diva-248130 (URN)
Note

QC 20190404

Available from: 2019-04-04 Created: 2019-04-04 Last updated: 2019-08-19Bibliographically approved
Heinig, S., Jacobs, K., Ilves, K., Bessegato, L., Bakas, P., Norrga, S. & Nee, H.-P. (2019). Implications of Capacitor Voltage Imbalance on the Operation of the Semi-Full-Bridge Submodule. IEEE transactions on power electronics, 34(10), 9520-9535, Article ID 8598807.
Open this publication in new window or tab >>Implications of Capacitor Voltage Imbalance on the Operation of the Semi-Full-Bridge Submodule
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2019 (English)In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, Vol. 34, no 10, p. 9520-9535, article id 8598807Article in journal (Refereed) Published
Abstract [en]

Future meshed high-voltage direct current grids require modular multilevel converters with extended functionality. One of the most interesting new submodule topologies is the semi-full-bridge because it enables efficient handling of DC-side short circuits while having reduced power losses compared to an implementation with full-bridge submodules. However, the semi-full-bridge submodule requires the parallel connection of capacitors during normal operation which can cause a high redistribution current in case the voltages of the two submodule capacitors are not equal. The maximum voltage difference and resulting redistribution current have been studied analytically, by means of simulations and in a full-scale standalone submodule laboratory setup. The most critical parameter is the capacitance mismatch between the two capacitors. The experimental results from the full-scale prototype show that the redistribution current peaks at 500A if the voltage difference is 10V before paralleling and increases to 2500A if the difference is 40V. However, neglecting very unlikely cases, the maximum voltage difference predicted by simulations is not higher than 20-30V for the considered case. Among other measures, a balancing controller is proposed which reduces the voltage difference safely if a certain maximum value is surpassed. The operating principle of the controller is described in detail and verified experimentally on a down-scaled submodule within a modular multilevel converter prototype. It can be concluded that excessively high redistribution currents can be prevented. Consequently, they are no obstacle for using the semi-full-bridge submodule in future HVDC converters.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2019
Keywords
AC-DC power conversion, HVDC converters, HVDC transmission, Power transmission, Fault tolerance, Power system faults
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
Identifiers
urn:nbn:se:kth:diva-240824 (URN)10.1109/TPEL.2018.2890622 (DOI)000474581900016 ()2-s2.0-85068640873 (Scopus ID)
Funder
SweGRIDS - Swedish Centre for Smart Grids and Energy Storage, CPC4
Note

QC 20190107

Available from: 2019-01-03 Created: 2019-01-03 Last updated: 2019-07-31Bibliographically approved
Bessegato, L. (2019). Modeling of Modular Multilevel Converters for Stability Analysis. (Doctoral dissertation). KTH Royal Institute of Technology
Open this publication in new window or tab >>Modeling of Modular Multilevel Converters for Stability Analysis
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Modular multilevel converters (MMCs) have recently become the state-of-the-art solution for various grid-connected applications, such as high-voltage direct current (HVDC) systems and flexible alternating current transmission systems (FACTS). Modularity, scalability, low power losses, and low harmonic distortion are the outstanding properties that make MMCs a key technology for a sustainable future.

 

The main objective of this thesis is the modeling of grid-connected MMCs for stability analysis. The stability of the interconnected system, formed by the converter and the ac grid, can be assessed by analyzing the converter ac-side admittance in relation to the grid impedance. Therefore, a method for the calculation of the ac-side admittance of MMCs is developed. This method overcomes the nonlinearities of the converter dynamics and it can be easily adapted to different applications. Moreover, the effects of different control schemes on the MMC ac-side admittance are studied, showing how the converter admittance can be reshaped. This is a useful tool for system design, because it shows how control parameters can be selected to avoid undesired grid-converter interactions.

 

This thesis also studies ac/ac MMCs for railway power supplies, which are used in countries with a low-frequency railway grid, such as Germany (16.7 Hz) and Sweden (16 2/3 Hz). A hierarchical control scheme for these converters is devised and evaluated, considering the requirements and the operating conditions specific to this application. Furthermore, admittance models of the ac/ac MMC are developed, showing how the suggested hierarchical control scheme affects the three-phase and the single-phase side admittances of the converter. For computing the insertion indices, an open-loop scheme with sum capacitor voltage estimation is applied to the ac/ac MMC. Lyapunov stability theory is used to prove the asymptotic stability of the converter operated with the proposed control method. This specific open-loop scheme is also adapted to a modular multilevel matrix converter, which performs three-to-three phase direct conversion.

 

Finally, this thesis presents the design of a down-scaled MMC prototype for experimental verification, rated at 10 kW with 30 full-bridge submodules. The hardware and the software are designed to be easily reconfigurable, which makes the converter suitable for different research projects focused on MMCs. Experiments on this down-scaled MMC are used to support and validate the key results presented throughout the thesis.

Abstract [sv]

Modulära multinivåomvandlare (MMC) har under senare år utvecklats till den mest relevanta lösningen för olika tillämpningar där kraftelektroniska omriktare är anslutna till växelströmsnät, såsom system för högspänd likströmsöverföring (HVDC) och flexibla system för överföring av växelström (FACTS). Den modulära uppbyggnaden, skalbarhet, låga förluster och låga övertoner är egenskaperna som gör MMC omriktare till en central komponent för framtida hållbara elenergisystem.

 

Huvudsyftet med denna avhandling är modellering av nätanslutna omvandlare av typ MMC för stabilitetsanalys. Stabiliteten för systemet omvandlare och nät, kan bedömas genom att analysera omvandlarens växelströmssidiga admittans i förhållande till nätimpedansen. En metod har därför utvecklats för att beräkna den modulära multinivåomvandlarens admittans. Metoden tar hänsyn till olinjäriteter i omvandlarens dynamik och kan enkelt anpassas till olika tillämpningar. Därutöver studeras effekterna av hur olika reglersystem påverkar omvandlarens admittans och hur omvandlarens admittans kan omformas. Denna möjlighet är användbar vid utformning av en systemlösning, eftersom reglerparametrarna kan väljas för att undvika oönskade störningar mellan nät och omriktare.

 

I avhandlingen undersöks även modulära ac/ac-omvandlare för järnvägsbanmatning. Dessa används i länder med lågfrekvensbanmatning så som Tysk-land med 16,7 Hz och Sverige med 16 2/3 Hz. Ett hierarkiskt reglersystem har utvecklats och utvärderats med avseende på järnvägstillämpningens specifika krav och dess driftsförhållanden. Admittansmodeller har utvecklats, för dessa modulära ac/ac-omvandlare, som visar hur det föreslagna hierarkiska reglersystemet påverkar omvandlarens admittans på både trefas- och enfassidan. För att beräkna ac/ac-omvandlarens inkopplingsförhållande appliceras en öppen styrning som estimerar summan av submodulernas kondensatorspänningar. Lyapunovs stabilitetsteori har använts för att bevisa den asymptotiska stabiliteten hos omvandlaren. Den föreslagna öppna styrningen kan också anpassas till en modulär multinivåomvandlare för direkt trefas till trefas omformning.

 

För att kunna verifiera resultaten experimentellt har en nedskalad prototyp utvecklats. Prototypens märkeffekt är 10 kW och den är uppbyggd av 30 submoduler med helbryggor. Hårdvaran och mjukvaran är utformade så att omvandlaren på ett enkelt sätt kan konfigureras för olika tillämpningar vilket gör den lämplig för olika forskningsprojekt som inkluderar modulära multinivåomriktare. Experiment på den nedskalade MMC:n har genomförts för att validera de resultat och slutsatser som presenteras i avhandlingen.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2019
Keywords
Modular multilevel converters (MMCs), stability, admittance, frequency-domain analysis, linearization techniques, current control, voltage control, ac/ac converters, railway engineering., Modulära multinivåomvandlare (MMC), stabilitet, admittans, frekvensanalys, linjäriseringsmetoder, strömreglering, spänningsreglering, ac/ac omvandlare, järnvägsteknik.
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
Identifiers
urn:nbn:se:kth:diva-248132 (URN)978-91-7873-144-2 (ISBN)
Public defence
2019-04-26, F3, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20190405

Available from: 2019-04-05 Created: 2019-04-04 Last updated: 2019-04-05Bibliographically approved
Bessegato, L., Harnefors, L., Ilves, K. & Norrga, S. (2018). A Method for the Calculation of the AC-Side Admittance of a Modular Multilevel Converter. IEEE transactions on power electronics
Open this publication in new window or tab >>A Method for the Calculation of the AC-Side Admittance of a Modular Multilevel Converter
2018 (English)In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107Article in journal (Refereed) Published
Abstract [en]

Connecting a modular multilevel converter to anac grid may cause stability issues, which can be assessed byanalyzing the converter ac-side admittance in relation to the gridimpedance. This paper presents a method for calculating theac-side admittance of modular multilevel converters, analyzingthe main frequency components of the converter variables individually.Starting from a time-averaged model of the converter,the proposed method performs a linearization in the frequencydomain, which overcomes the inherent nonlinearities of theconverter internal dynamics and the phase-locked loop usedin the control. The ac-side admittance obtained analytically isfirstly validated by simulations against a nonlinear time-averagedmodel of the modular multilevel converter. The tradeoff posedby complexity of the method and the accuracy of the result isdiscussed and the magnitude of the individual frequency componentsis shown. Finally, experiments on a down-scaled prototypeare performed to validate this study and the simplification onwhich it is based.

Keywords
Modular multilevel converters, admittance, linearization techniques, frequency-domain analysis, stability.
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
Identifiers
urn:nbn:se:kth:diva-238815 (URN)10.1109/TPEL.2018.2862254 (DOI)000464911900018 ()2-s2.0-85050974319 (Scopus ID)
Note

QC 20181113

Available from: 2018-11-12 Created: 2018-11-12 Last updated: 2019-05-14Bibliographically approved
Bessegato, L., Norrga, S., Ilves, K. & Harnefors, L. (2017). Ac-side admittance calculation for modular multilevel converters. In: 2017 IEEE 3rd International Future Energy Electronics Conference and ECCE Asia, IFEEC - ECCE Asia 2017: . Paper presented at 3rd IEEE International Future Energy Electronics Conference and ECCE Asia, IFEEC - ECCE Asia 2017, Kaohsiung, Taiwan, 3 June 2017 through 7 June 2017 (pp. 308-312). Institute of Electrical and Electronics Engineers (IEEE)
Open this publication in new window or tab >>Ac-side admittance calculation for modular multilevel converters
2017 (English)In: 2017 IEEE 3rd International Future Energy Electronics Conference and ECCE Asia, IFEEC - ECCE Asia 2017, Institute of Electrical and Electronics Engineers (IEEE), 2017, p. 308-312Conference paper, Published paper (Refereed)
Abstract [en]

Power electronic converters may interact with the grid, thereby influencing dynamic behavior and resonances. Impedance and passivity based stability criteria are two useful methods that allow for studying the grid-converter system as a feedback system, whose behavior is determined by the ratio of grid and converter impedances. In this paper, the ac-side admittance of the modular multilevel converter is calculated using harmonic linearization and considering five specific frequency components of the converter variables. The proposed model features remarkable accuracy, verified through simulations, and insight into the influence of converter and control parameters on the admittance frequency characteristics, which is useful for understanding grid-converter interaction and designing the system.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2017
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-223036 (URN)10.1109/IFEEC.2017.7992056 (DOI)000426696300055 ()2-s2.0-85034016542 (Scopus ID)9781509051571 (ISBN)
Conference
3rd IEEE International Future Energy Electronics Conference and ECCE Asia, IFEEC - ECCE Asia 2017, Kaohsiung, Taiwan, 3 June 2017 through 7 June 2017
Note

QC 20180220

Available from: 2018-02-20 Created: 2018-02-20 Last updated: 2019-04-04Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-8891-5659

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