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Optimization-Based Cell Selection Method for Grid-Connected Modular Multilevel Converters
KTH, School of Electrical Engineering (EES), Electric power and energy systems.ORCID iD: 0000-0001-8788-0415
KTH, School of Electrical Engineering (EES), Electric power and energy systems.ORCID iD: 0000-0002-8565-4753
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) PublishedText
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. Vol. 31, no 4, 2780-2790 p.
Keyword [en]
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
URN: urn:nbn:se:kth:diva-180195DOI: 10.1109/TPEL.2015.2448573ISI: 000365953100009ScopusID: 2-s2.0-84960095195OAI: diva2:895949

QC 20160120

Available from: 2016-01-20 Created: 2016-01-08 Last updated: 2016-09-16Bibliographically 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.
TRITA-EE, ISSN 1653-5146 ; 2016:139
HVDC, switching schemes, modulation, high power converters, power grids
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
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)

QC 20160916

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

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