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On soft-switching isolated AC/DC converters without auxiliary circuit
KTH, School of Electrical Engineering (EES), Electrical Machines and Power Electronics.ORCID iD: 0000-0002-8565-4753
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 [en]
Electrical power technology, voltage source converter, cycloconverter, isolated converter, medium frequency transformer, soft switching, modulation, commutation, wind power, electric traction
Keyword [sv]
Elkraftteknik
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:kth:diva-256ISBN: 91-7178-080-7 (print)OAI: oai:DiVA.org:kth-256DiVA: diva2:8193
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
List of papers
1. A Soft-Switched Bi-Directional Isolated AC/DC Converter for AC-Fed Railway Propulsion Applications
Open this publication in new window or tab >>A Soft-Switched Bi-Directional Isolated AC/DC Converter for AC-Fed Railway Propulsion Applications
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.

Series
IEE CONFERENCE PUBLICATIONS, ISSN 0537-9989 ; 487
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-5260 (URN)000177846400077 ()0-85296-747-0 (ISBN)
Note
QC 20100614Available from: 2005-06-02 Created: 2005-06-02 Last updated: 2010-10-14Bibliographically approved
2. A Novel Soft-switched Bidirectional Isolated AC/DC Converter
Open this publication in new window or tab >>A Novel Soft-switched Bidirectional Isolated AC/DC Converter
2002 (English)In: PCIM Europe 2002 - Power Electronics, 14-16 May 2002, Nuremberg, Germany, 2002, 81-87 p.Conference paper, Published paper (Other academic)
Abstract [en]

A novel ac/dc converter topologyincorporates a voltage source converter withcapacitive snubbers and a cycloconverter,connected by a transformer. By alternatelycommutating the two converters it is possible toachieve soft switching conditions for all of thesemiconductor elements. A control method isstudied by which it is possible to reduce thetransformer core losses by only magnetising thetransformer when a PWM pulse should beapplied on the ac side of the system.Furthermore, it is shown that resonantcommutation of the voltage source converter ispossible by utilizing the cycloconverter for shortcircuiting the transformer terminals.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-5261 (URN)
Note
QC 20100614Available from: 2005-06-02 Created: 2005-06-02 Last updated: 2010-07-14Bibliographically approved
3. An Experimental Study of a Soft-switched Isolated AC/DC Converter without Auxiliary Circuit
Open this publication in new window or tab >>An Experimental Study of a Soft-switched Isolated AC/DC Converter without Auxiliary Circuit
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.

Series
IEEE POWER ELECTRONICS SPECIALISTS CONFERENCE RECORDS, ISSN 0275-9306
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-5262 (URN)10.1109/PESC.2004.1355498 (DOI)000224587600395 ()2-s2.0-8744282490 (Scopus ID)0-7803-8399-0 (ISBN)
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
4. A three-phase soft-switched isolated AC/DC converter without auxiliary circuit
Open this publication in new window or tab >>A three-phase soft-switched isolated AC/DC converter without auxiliary circuit
2004 (English)In: IEEE IND APPLICAT SOC ANN M, 2004, 1768-1775 p.Conference paper, Published paper (Refereed)
Abstract [en]

An ac/dc converter topology comprises a voltage source converter (VSC) with capacitive snubbers and a 2-phase by 3-phase cycloconverter, connected by an MF transformer. By alternately commutating the two converters it is possible to achieve soft switching conditions for all of the semiconductor elements. Furthermore, it is shown that resonant commutation of the voltage source converter is possible by utilizing the cycloconverter in order to momentarily short-circuit the transformer terminals. This allows for soft commutation of the VSC down to zero load. Furthermore, the design and operation of a 40 kVA prototype converter system based on the studied concept is described.

Series
IEEE INDUSTRY APPLICATIONS SOCIETY ANNUAL MEETING, ISSN 0197-2618
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-25268 (URN)10.1109/IAS.2004.1348710 (DOI)000225148600261 ()2-s2.0-10944265776 (Scopus ID)
Conference
39th Annual Meeting of the IEEE-Industry-Applications-Society Seattle, WA, OCT 03-07, 2004
Note

QC 20101014. 39th Annual Meeting of the IEEE-Industry-Applications-Society. Seattle, WA. OCT 03-07, 2004

Available from: 2010-10-14 Created: 2010-10-14 Last updated: 2016-06-20Bibliographically approved
5. Modulation Strategies for Mutually Commutated Isolated Three-Phase Converter Systems
Open this publication in new window or tab >>Modulation Strategies for Mutually Commutated Isolated Three-Phase Converter Systems
2005 (English)In: 2005 IEEE 36th Power Electronic Specialists Conference (PESC), Vols 1-3, 2005, 2736-2743 p.Conference paper, Published 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.

Series
IEEE POWER ELECTRONICS SPECIALISTS CONFERENCE RECORDS, ISSN 0275-9306
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-5264 (URN)10.1109/PESC.2005.1582020 (DOI)000237036502121 ()2-s2.0-33847749443 (Scopus ID)
Conference
36th IEEE Power Electronics Specialists Conference, 12-16 June 2005, Recife, Brazil
Note
QC 20100615Available from: 2005-06-02 Created: 2005-06-02 Last updated: 2011-10-14Bibliographically approved

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