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Modulation Strategies for Mutually Commutated Isolated Three-Phase Converter Systems
KTH, School of Electrical Engineering (EES).ORCID iD: 0000-0002-8565-4753
2005 (English)In: 2005 IEEE 36th Power Electronic Specialists Conference (PESC), Vols 1-3, 2005, 2736-2743 p.Conference paper (Refereed)
Abstract [en]

Converter structures consisting of a voltage source converter and a cycloconverter connected by a medium frequency transformer have several attractive properties. Bidirectional AC/DC conversion as well as voltage transformation and isolation by the MF transformer can be achieved. Furthermore, by consistently commutating the two converters in alternation it is possible to achieve soft switching for all semiconductor devices. However the demand for soft switching imposes certain restrictions concerning the modulation of the converter system that may adversely affect the harmonic properties of the AC side voltage. Unlike conventional VSC modulation the modulation of the studied converters is heavily dependent on the load angle.

Various possible modulation schemes are described in detail. In particular a new space vector-based modulation method derived by adapting a method originally developed for quasi-resonant DC link converters is presented.

The studied modulation schemes are compared and evaluated in terms of the output voltage harmonic distortion and the RMS transformer current. The comparison shows that the presented SVM-based method gives superior performance in almost all operating points. An exception is operation at loa angles near +/-pi/2 rad and intermediate modulation depth. Under these circumstances the harmonic properties deteriorate due to an increased production of low frequency harmonics.

Place, publisher, year, edition, pages
2005. 2736-2743 p.
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
URN: urn:nbn:se:kth:diva-5264DOI: 10.1109/PESC.2005.1582020ISI: 000237036502121ScopusID: 2-s2.0-33847749443OAI: diva2:8192
36th IEEE Power Electronics Specialists Conference, 12-16 June 2005, Recife, Brazil
QC 20100615Available from: 2005-06-02 Created: 2005-06-02 Last updated: 2011-10-14Bibliographically 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.
Trita-ETS, ISSN 1650-674X ; 2005:7
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
urn:nbn:se:kth:diva-256 (URN)91-7178-080-7 (ISBN)
Public defence
2005-06-08, H1, ETS, Teknikringen 33, Stockholm, 10:00
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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