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A Soft-Switched Bi-Directional Isolated AC/DC Converter for AC-Fed Railway Propulsion Applications
KTH, Superseded Departments, Electrical Systems.ORCID iD: 0000-0002-8565-4753
2002 (English)In: International Conference on Power Electronics, Machines and Drives 2002: IEE Conf. Publ. No. 487, 16-18 April 2002, Bath, UK, 2002, 433-438 p.Conference paper, Published paper (Refereed)
Abstract [en]

A novel isolated ac/dc converter topology incorporates avoltage source converter (VSC) with capacitivesnubbers and a cycloconverter, connected by a mediumfrequency transformer. The concept offers thepossibility of bilateral power flow as well as three-levelpulse width modulation on the ac side. It is shown that by alternately commutating the VSC and the cycloconverter it is possible to achieve either zerovoltageor zero-current switching conditions for all ofthe semiconductor elements. At low load the ac loadcurrent becomes insufficient for recharging the snubbercapacitors of the VSC and the commutations may therefore become too slow. However, a quasi-resonantcommutation mode for the voltage source converter is proposed that allows for sufficiently fast commutations down to zero load.

Place, publisher, year, edition, pages
2002. 433-438 p.
Series
IEE CONFERENCE PUBLICATIONS, ISSN 0537-9989 ; 487
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:kth:diva-5260ISI: 000177846400077ISBN: 0-85296-747-0 (print)OAI: oai:DiVA.org:kth-5260DiVA: diva2:8188
Note
QC 20100614Available from: 2005-06-02 Created: 2005-06-02 Last updated: 2010-10-14Bibliographically approved
In thesis
1. On soft-switching isolated AC/DC converters without auxiliary circuit
Open this publication in new window or tab >>On soft-switching isolated AC/DC converters without auxiliary circuit
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The thesis is concerned with a class of power electronic converter systems that permit bidirectional AC/DC conversion with galvanic separation of the AC and DC sides by a transformer operating at elevated frequency. The systems consist of a capacitively snubbered voltage source converter and a cycloconverter, separated by the mentioned transformer. The use of a medium-frequency transformer permits considerable reduction of the transformer weight and size. Both systems with single-phase and three-phase AC terminal are conceivable.

It is shown that the converter concept is compatible with soft switching of the semiconductor valves, which considerably reduces the switching losses and the stress on the valves. This can be achieved in all points of operation without any need for auxiliary valves dedicated to the commutation process. A resonant mode of commutation, which is vital for extending the soft-switching operating region also to low load conditions, is described in detail.

The demand for soft switching imposes certain important constraints on the possibilities for pulse width modulation of the AC side voltage. These tend to result in low-order harmonics in the output voltages that are not negligible. The impact of the constraints on the resulting pulse patterns is analysed by analytical derivation of the harmonic spectra. This way the low-order harmonics were found to originate from sidebands around multiples of the switching frequency. Furthermore, advanced modulation schemes, both for single-phase and three-phase systems are described and analysed. These schemes allow for improved harmonic performance and overall loss reduction. The improvement in harmonic performance is most pronounced at low modulation index.

A down-scaled prototype converter with a rated power of 40 kVA was designed and built up. The prototype can be configured for operation with single-phase or three-phase AC terminal. In the thesis special emphasis is put on the implementation of the commutation and modulation methods in the digital control system. Experimental results from the prototype verify the intended operating principles. Measurements made on the prototype indicate an overall efficiency of up to 92%.

The envisaged applications are found in the high power range, where the advantages of soft switching are most significant. Two examples of possible applications are outlined: line side power conversion in rail vehicles and systems for energy capture from offshore wind power parks.

Place, publisher, year, edition, pages
Stockholm: KTH, 2005. xi, 130 p.
Series
Trita-ETS, ISSN 1650-674X ; 2005:7
Keyword
Electrical power technology, voltage source converter, cycloconverter, isolated converter, medium frequency transformer, soft switching, modulation, commutation, wind power, electric traction, Elkraftteknik
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-256 (URN)91-7178-080-7 (ISBN)
Public defence
2005-06-08, H1, ETS, Teknikringen 33, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100614Available from: 2005-06-02 Created: 2005-06-02 Last updated: 2010-10-14Bibliographically approved

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Norrga, Staffan

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