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Control Strategies for Mutually Commutated Converter Systems without Cycloconverter Turn-off Capability
KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics (closed 20110930).
KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics (closed 20110930).ORCID iD: 0000-0002-8565-4753
KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics (closed 20110930).ORCID iD: 0000-0002-1755-1365
2008 (English)In: EEE POWER ELECTRONICS SPECIALISTS CONFERENCE RECORDS, 2008, 1344-1350 p.Conference paper, Published paper (Refereed)
Abstract [en]

Mutually commutated converter systems consisting of a voltage source converter and a cycloconverter connected by a medium-frequency transformer allow bidirectional AC/DC conversion as well as voltage transformation and isolation by the transformer. However, such converter systems require an extra power conversion stage compared to conventional converter systems, which may result in low system efficiency despite the soft-switching commutation scheme. In order to reduce the power losses in the cycloconverter, it is desirable to utilize fast thyristors instead of IGBTs. As a consequence, the cycloconverter valves lack turn-off capability because a thyristor can not be turned off by its gate. This paper describes and evaluates the most common control strategies in order to control a thyristor-based cycloconverter in a mutually commutated converter system. The behaviour of the cycloconverter depends strongly on the chosen control strategy when one of the output currents approaches zero, which may inherently affect the harmonic properties of the output voltages and currents. Experimental results from a 20 kVA prototype show that it is possible to successfully operate a mutually commutated converter system without turn-off capability.

Place, publisher, year, edition, pages
2008. 1344-1350 p.
Series
IEEE power electronics specialists conference records, ISSN 0275-9306
Keyword [en]
Electric inverters, Electric rectifiers, Electric switchgear, Power electronics, Thyristors, Ac/dc conversion, Control strategies, Converter system, Cyclo converter, Fast thyristors, Harmonic properties, Output currents, Output voltages, Power conversion stage, Power losses, Soft-switching, System efficiencies, Turn-off, Voltage transformation, Voltage-source converter, Control systems
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:kth:diva-14305DOI: 10.1109/PESC.2008.4592121ISI: 000260398500215Scopus ID: 2-s2.0-52349110730ISBN: 978-1-4244-1667-7 (print)OAI: oai:DiVA.org:kth-14305DiVA: diva2:332109
Conference
39th IEEE Power Electronic Specialists Conference (PESC 08) Rhodes, GREECE, JUN 15-19, 2008
Note

QC 20100802

Available from: 2010-08-02 Created: 2010-08-02 Last updated: 2014-12-16Bibliographically approved
In thesis
1. System Aspects and Modulation Strategies of an HVDC-based Converter System for Wind Farms
Open this publication in new window or tab >>System Aspects and Modulation Strategies of an HVDC-based Converter System for Wind Farms
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

 

In this thesis, a new HVDC-based converter system for wind farms is investigated. It is based on a mutually commutated soft-switching converter system and provides a unique integrated solution for the wind turbine generator drive systems, the wind turbine interconnection, and the power conversion for HVDC transmission.

In a wind farm, the mutually commutated converter system is a distributed system. A medium-frequency collection grid connects the converter station, equipped with a single-phase voltage source converter and a medium-frequency transmission transformer, with the wind turbines, each containing a cycloconverter and a medium-frequency distribution transformer. In this thesis, various system aspects regarding the application of a distributed mutually commutated converter system in a wind farm are investigated. Special attention is paid to the design of a medium-frequency collection grid that has an acceptable level of transient overvoltages, the design of medium-frequency transformers with suitable magnetic, electric and thermal properties, and the development of a strategy to commutate the voltage source converter during low power generation.

In order to adapt the mutually commutated converter system for an application in a wind farm, it had to be further developped. Different carrier-based and space-vector oriented modulation methods have been investigated. It turns out that for any load angle there is a quasi-discontinuous pulse width modulation strategy that can produce the same pulse patterns as space vector modulation. In addition, a modulation strategy has been developed that allows to replace the IGBTs in the cycloconverter with cheap, robust, and reliable fast thyristors, despite their absence of turn-off capability. The feasibility of different modulation strategies for mutually commutated converter systems has been verified on a down-scaled prototype converter system with both IGBT- and thyristor-based cycloconverters.

Finally, a feasible wind farm layout is proposed, which considerably reduces the energy generation costs for large winds farms distant to a strong grid connection point. As a consequence, the proposed solution may facilitate the establishment of remotely located wind farms.

 

Place, publisher, year, edition, pages
Stockholm, Sweden: KTH, 2009. viii, 80 p.
Series
Trita-EE, ISSN 1653-5146 ; 2009:018
Keyword
Isolated AC/DC Converter, Mutual Commutation, Soft Switching, Voltage Source Converter, Cycloconverter, Modulation Strategies, Medium-Frequency Transformer, HVDC Transmission, Wind Power, Wind Farms
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-10267 (URN)978-91-7415-292-0 (ISBN)
Public defence
2009-05-18, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20100802Available from: 2009-05-12 Created: 2009-04-28 Last updated: 2010-08-03Bibliographically approved

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Norrga, StaffanNee, Hans-Peter

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