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Online power transformer diagnostics using multiple modes of microwave radiation
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

In the present thesis, we propose and investigate a new approach to diagnose the effects of the various degradation mechanisms, including thermal degradation at hot spots, winding deformations due to the mechanical forces from short circuit currents, partial discharges due to local electric field surges, and increased moisture levels in the cellulose insulation due to decomposition, that affect electric power transformers during their normal operation in an electric power grid.

Although the proposed diagnostics method can in principle be used to detect various degradation mechanisms mentioned above, we focus in the present thesis on mechanical deformations of transformer winding structures. Such mechanical deformations are most often caused by mechanical forces from short circuit currents, but they may also be caused by initial manufacturing errors and inconsistencies not detected by the power transformers’ suppliers quality assurance processes.

We model a transformer winding surrounded by the transformer-tank wall and the magnetic core as a two-dimensional parallel plate waveguide or as a three-dimensional coaxial waveguide, where one metallic boundary (plate or cylinder) represents the wall of the transformer tank and the other metallic boundary (plate or cylinder) represents the iron core that conducts the magnetic flux. In between there is a set of parallel or coaxial conductors representing the winding segments.

The new principle proposed in the present thesis is to insert a number of antennas into a transformer tank to radiate and measure microwave fields interacting with metallic structures and insulation. The responses from the emitted microwave radiation are expected to be sensitive to material properties that reflect the changes caused by any harmful deterioration processes mentioned above. Specifically, we investigate the mechanical deformations of transformer winding structures by determining the locations of the individual winding segments or turns, using measurements of the scattered fields at both ends of the winding structure. We solve the propagation problem using conventional waveguide theory, including mode-matching and cascading techniques.

The inverse problem is solved using modified steepest-descent optimization methods. The optimization model is tested by comparing our calculated scattering data with synthetic measurement data generated by the commercial program HFSS.

A good agreement is obtained between the calculated and measured positions of winding segments for a number of studied cases, which indicates that the diagnostics method proposed in the present thesis couldbe potentially useful as a basis for the design of a future commercial on-line winding monitoring device. However, further development of the theoretical analysis of a number of typical winding deformations, improvements of the optimization algorithms and a practical study with measurements on an actual power transformer structure are all needed to make an attempt to design a commercial winding monitoring device feasible.

 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , x, 93 p.
Series
TRITA-EE, ISSN 1653-5146 ; 2013:033
Keyword [en]
inverse problem; multiple modes; power transformers
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:kth:diva-129766ISBN: 978-91-7501-832-4 (print)OAI: oai:DiVA.org:kth-129766DiVA: diva2:653608
Presentation
2013-10-25, H1, Teknikringen 33, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20131007

Available from: 2013-10-07 Created: 2013-10-04 Last updated: 2016-02-24Bibliographically approved
List of papers
1. Online power transformer diagnostics using multiple modes of microwave radiation to reconstruct winding conductor locations
Open this publication in new window or tab >>Online power transformer diagnostics using multiple modes of microwave radiation to reconstruct winding conductor locations
2014 (English)In: Inverse Problems in Science and Engineering, ISSN 1741-5977, E-ISSN 1741-5985, Vol. 22, no 1, 84-95 p.Article in journal (Refereed) Published
Abstract [en]

A novel approach to diagnostics of transformer winding deformations, caused by mechanical forces from short-circuit currents, is presented. We employ a simple model of a transformer as a two-dimensional parallel plate waveguide. The upper plate represents the transformer tank wall and the lower plate represents the iron core which carries the magnetic flux. Between the two plates, we model the transformer winding by a set of parallel conductors. We utilize commercial simulation software to simulate the generation and measurement of microwave radiation at both ends of the winding structure. The radiation interacts with the metallic structures in the model waveguide. The measured responses from the model waveguide are expected to be sensitive to mechanical deformations of the transformer winding. We use conventional waveguide theory to solve the direct propagation problem, and an optimization method to solve the inverse problem. In particular, we determine the locations of winding segments, and obtain a good agreement between reconstructed and true conductor positions.

Keyword
cylindrical windings, inverse problem, multiple modes, power transformers
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-129762 (URN)10.1080/17415977.2013.827182 (DOI)000328245900007 ()2-s2.0-84890570369 (Scopus ID)
Funder
Swedish Energy Agency, 34146-1
Note

QC 20140122

Available from: 2013-10-04 Created: 2013-10-04 Last updated: 2017-12-06Bibliographically approved
2. Using multiple modes to reconstruct conductor locations in a cylindrical model of a power transformer winding
Open this publication in new window or tab >>Using multiple modes to reconstruct conductor locations in a cylindrical model of a power transformer winding
2013 (English)In: International journal of applied electromagnetics and mechanics, ISSN 1383-5416, E-ISSN 1875-8800, Vol. 41, no 3, 279-291 p.Article in journal (Refereed) Published
Abstract [en]

A study of a novel method to detect mechanical deformations of windings in a power transformer, while in operation, is presented. We employ an approximate model of a transformer winding surrounded by the transformer-tank wall and the magnetic core. The transformer winding is viewed as a structure consisting of thin conducting cylindrical rings (winding segments or turns) situated within a coaxial cylindrical waveguide, where the inner conducting cylinder represents the iron core that conducts the magnetic flux and the outer conducting cylinder represents the wall of the transformer tank. The basic principle is to insert antennas inside the transformer tank above and below the winding to radiate and measure microwave fields that interact with the metallic structure and the insulation. The responses from the radiated waves are assumed to be sensitive to any mechanical deformations that could be caused by electromagnetic forces due to short-circuit currents and possible manufacturing weaknesses. The goal is to be able to determine the radial locations of the individual winding segments or individual turns from measurements of the scattered fields at both ends. The propagation problem is solved by conventional waveguide theory, including mode-matching and cascading techniques. We utilize optimization as a suitable method to solve the inverse problem and obtain a good agreement between the reconstructed and true positions of the winding segments.

Keyword
Inverse problem, multiple modes, power transformers, cylindrical windings
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-121501 (URN)10.3233/JAE-121612 (DOI)000316719400007 ()2-s2.0-84877131308 (Scopus ID)
Funder
The Swedish Energy Agency, 34146-1
Note

QC 20130506

Available from: 2013-05-06 Created: 2013-04-29 Last updated: 2017-12-06Bibliographically approved
3. First-order perturbation approach to elliptic winding deformations
Open this publication in new window or tab >>First-order perturbation approach to elliptic winding deformations
2013 (English)In: 2013 International Symposium on Electromagnetic Theory, EMTS 2013 - Proceedings, IEEE , 2013, 13-16 p.Conference paper, Published paper (Refereed)
Abstract [en]

We apply a novel on-line method to detect elliptic deformations of winding turns in a power transformer. We employ the first-order perturbation theory to a transformer winding surrounded by the transformer tank wall and the magnetic core. The transformer winding is modeled as a structure consisting of thin conducting cylindrical rings (winding segments or turns) situated within a coaxial cylindrical waveguide, where the inner conducting cylinder represents the magnetic core and the outer conducting cylinder represents the wall of the transformer tank. We simulate antennas inside the transformer tank to radiate and measure microwave fields, in order to identify and quantify elliptic deformations of the individual winding segments or individual turns. The propagation problem is solved by conventional waveguide theory, including mode-matching and cascading techniques. We utilize optimization to solve the inverse problem and obtain a good agreement between the reconstructed and true deformations of the winding segments.

Place, publisher, year, edition, pages
IEEE, 2013
Series
URSI International Symposium on Electromagnetic Theory, ISSN 2163-405X
Keyword
Cascading technique, Conducting cylinder, First-order perturbation theory, Microwave field, Perturbation approach, Transformer tanks, Waveguide theory, Winding deformations
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-129763 (URN)000327180500005 ()2-s2.0-84883220077 (Scopus ID)978-1-4673-4939-0 (ISBN)
Conference
2013 21st International Symposium on Electromagnetic Theory, EMTS 2013; Hiroshima; Japan; 20 May 2013 through 24 May 2013
Note

QC 20131004

Available from: 2013-10-04 Created: 2013-10-04 Last updated: 2013-12-20Bibliographically approved
4. Conductor Locations Reconstruction in a Cylindrical Winding Model
Open this publication in new window or tab >>Conductor Locations Reconstruction in a Cylindrical Winding Model
2013 (English)In: Proceedings of PIERS 2013 in Stockholm, Sweden, August 12-15, 2013, 2013Conference paper, Published paper (Refereed)
Series
Progress in Electromagnetics Research Symposium, ISSN 1559-9450
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-129765 (URN)2-s2.0-84884758185 (Scopus ID)
Conference
Progress in Electromagnetics Research Symposium, PIERS 2013 Stockholm, 12 August through 15 August 2013, Sweden
Note

QC 20131004

Available from: 2013-10-04 Created: 2013-10-04 Last updated: 2013-10-07Bibliographically approved

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