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Tolerance band modulation methods for modular multilevel converters
KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.ORCID iD: 0000-0001-8788-0415
KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.ORCID iD: 0000-0002-8565-4753
KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
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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. Vol. 30, no 1, 311-326 p., 6739183
Keyword [en]
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: urn:nbn:se:kth:diva-166885DOI: 10.1109/TPEL.2014.2305114ISI: 000341624200026Scopus ID: 2-s2.0-84906748780OAI: oai:DiVA.org:kth-166885DiVA: diva2:818556
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StandUp
Note

QC 20150609

Available from: 2015-06-09 Created: 2015-05-21 Last updated: 2017-12-04Bibliographically approved
In thesis
1. Modulation of Modular Multilevel Converters for HVDC Transmission
Open this publication in new window or tab >>Modulation of Modular Multilevel Converters for HVDC Transmission
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
HVDC, switching schemes, modulation, high power converters, power grids
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
Identifiers
urn:nbn:se:kth:diva-192607 (URN)978-91-7729-098-8 (ISBN)
Public defence
2016-10-07, Sal D3, Kungl Tekniska Högskolan, Lindstedtsvägen 5, Stockholm, 10:00 (English)
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Supervisors
Note

QC 20160916

Available from: 2016-09-16 Created: 2016-09-15 Last updated: 2016-09-16Bibliographically approved

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Hassanpoor, ArmanNorrga, StaffanNee, Hans-Peter

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