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An Experimental Study of a Soft-switched Isolated AC/DC Converter without Auxiliary Circuit
KTH, Superseded Departments, Electrical Systems.ORCID iD: 0000-0002-8565-4753
2004 (English)In: PESC 04: 2004 IEEE 35TH ANNUAL POWER ELECTRONICS SPECIALISTS CONFERENCE, VOLS 1-6, CONFERENCE PROCEEDINGS, 2004, 2406-2412 p.Conference paper, Published paper (Refereed)
Abstract [en]

An isolated AC/DC converter topology incorporates a capacitively snubbered voltage source converter (VSC) and a cycloconverter, coupled by a medium frequency transformer. The topology offers the possibility of bilateral power flow as well as three-level pulse width modulation on the AC side. It is shown that by alternately commutating the VSC and the cycloconverter it is possible to achieve either zero-voltage or zero-current switching conditions for all of the semiconductor devices in all points of operation. This is the case without any need for auxiliary semiconductor devices. At low load the transformer current may be insufficient for recharging the VSC snubber capacitors. In this case however it is possible to utilize the cycloconverter for providing a current path by which a quasi-resonant commutation can be effected.

The design and operation of a 40 kVA prototype converter system is described. It is shown how the rather complex switching logic required to implement the chosen algorithm for commutation and modulation can be realized by using modern programmable logic devices (FPGA). Measurement results from the prototype converter are presented and analysed. The measurements indicate that the studied commutation algorithm works well in practice.

Place, publisher, year, edition, pages
2004. 2406-2412 p.
Series
IEEE POWER ELECTRONICS SPECIALISTS CONFERENCE RECORDS, ISSN 0275-9306
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:kth:diva-5262DOI: 10.1109/PESC.2004.1355498ISI: 000224587600395Scopus ID: 2-s2.0-8744282490ISBN: 0-7803-8399-0 (print)OAI: oai:DiVA.org:kth-5262DiVA: diva2:8190
Conference
35th IEEE Power Electronics Specialists Conference, 21-24 June 2004, Aachen, Germany
Note
QC 20100614Available from: 2005-06-02 Created: 2005-06-02 Last updated: 2011-11-02Bibliographically 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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