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Perturbation approach to reconstructions of boundary deformations in waveguide structures
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis we develop inverse scattering algorithms towards the ultimate goal of online diagnostic methods. The aim is to detect structural changes inside power transformers and other major power grid components, like generators, shunt reactors etc. Power grid components, such as large power transformers, are not readily available from the manufacturers as standard designs. They are generally optimized for specific functions at a specific position in the power grid. Their replacement is very costly and takes a long time.

Online methods for the diagnostics of adverse changes of the mechanical structure and the integrity of the dielectric insulation in power transformers and other power grid components, are therefore essential for the continuous operation of a power grid. Efficient online diagnostic methods can provide a real-time monitoring of mechanical structures and dielectric insulation in the active parts of power grid components. Microwave scattering is a candidate that may detect these early adverse changes of the mechanical structure or the dielectric insulation. Upon early detection, proper actions to avoid failure or, if necessary, to prepare for the timely replacement of the damaged component can be taken. The existing diagnostic methods lack the ability to provide online reliable information about adverse changes inside the active parts. More details about the existing diagnostic methods, both online and offline, and their limitations can be found in the licentiate thesis preceding the present PhD thesis.

We use microwave scattering together with the inverse scattering algorithms, developed in the present work, to reconstruct the shapes of adverse mechanical structure changes. We model the propagation environment as a waveguide, in which measurement data can be obtained only at two ends (ports). Since we want to detect the onset of some deformation, that only slightly alters the scattering situation (weak scattering), we have linearized the inverse problem with good results. We have calculated the scattering parameters of the waveguide in the first-order perturbation, where they have linear dependencies on the continuous deformation function. A linearized inverse problem with a weak scattering assumption typically results in an ill-conditioned linear equation system. This is handled using Tikhonov regularization, with the L-curve method for tuning regularization parameters.

We show that localized one-dimensional and two-dimensional shape deformations, for rectangular and coaxial waveguide models, are efficiently reconstructed using the inverse scattering algorithms developed from the first principles, i.e. Maxwell’s theory of electromagnetism. An excellent agreement is obtained between the reconstructed and actual deformation shapes for a number of studied cases. These results show that it is possible to use the inverse algorithms, developed in the present thesis, as a theoretical basis for the design of a future diagnostic device. As a part of the future work, it remains to experimentally verify the results obtained so far, as well as to further study the theoretical limitations posed by linearization (first-order perturbation theory) and by the assumption of the continuity of the metallic waveguide boundaries and their deformations.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology: KTH Royal Institute of Technology, 2016. , xii, 53 p.
Series
TRITA-EE, ISSN 1653-5146 ; 2015:109
Keyword [en]
inverse problems, waveguides, microwaves, perturbation theory
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-180429ISBN: 978-91-7595-801-9 (print)OAI: oai:DiVA.org:kth-180429DiVA: diva2:893763
Public defence
2016-02-12, F3, Lindstedtsvägen 26, KTH, Stockholm, 14:00 (English)
Opponent
Supervisors
Note

QC 20160119

Available from: 2016-01-19 Created: 2016-01-13 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 transformer winding deformations
Open this publication in new window or tab >>First-order perturbation approach to transformer winding deformations
2013 (English)In: Progress In Electromagnetics Research Letters, ISSN 1937-6480, Vol. 43, 1-14 p.Article in journal (Refereed) Published
Abstract [en]

An on-line method to detect radial mechanical deformations of power transformer winding turns is presented. First-order perturbation theory is applied to a transformer winding surrounded by the transformer tank wall and the iron core. The transformer wind- ing is modeled as thin conducting cylindrical rings (winding segments or turns) situated within a coaxial waveguide, where the outer con- ducting cylinder represents the transformer tank wall while the inner conducting cylinder represents the iron core. Antennas which radi- ate and measure microwave fields are proposed inside the transformer tank in order to identify and quantify the mechanical deformations of winding turns. The direct propagation problem is solved using conventional waveguide theory with mode-matching and cascading techniques. An optimization algorithm is then used to solve the inverse problem whereby a good agreement between the reconstructed and true deformations of the winding segments is obtained.

Keyword
Cascading technique, Coaxial waveguides, Conducting cylinder, First-order perturbation theory, Mechanical deformation, Optimization algorithms, Perturbation approach, Transformer winding deformations
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-133247 (URN)2-s2.0-84884567532 (Scopus ID)
Note

QC 20131029

Available from: 2013-10-29 Created: 2013-10-29 Last updated: 2017-12-06Bibliographically approved
4. Perturbation Approach to Reconstructing Deformations in a Coaxial Cylindrical Waveguide
Open this publication in new window or tab >>Perturbation Approach to Reconstructing Deformations in a Coaxial Cylindrical Waveguide
2015 (English)In: Mathematical problems in engineering (Print), ISSN 1024-123X, E-ISSN 1563-5147, 915497Article in journal (Refereed) Published
Abstract [en]

We study a detection method for continuous mechanical deformations of coaxial cylindrical waveguide boundaries, using perturbation theory. The inner boundary of the waveguide is described as a continuous PEC structure with deformations modeled by suitable continuous functions. In the present approach, the computation complexity is significantly reduced compared to discrete conductor models studied in our previous work. If the mechanically deformed metallic structure is irradiated by the microwave fields of appropriate frequencies, then, by means of measurements of the scattered fields at both ends, we can reconstruct the continuous deformation function. We apply the first-order perturbation method to the inverse problem of reconstruction of boundary deformations, using the dominant TEM-mode of the microwave radiation. Different orders of Tikhonov regularization, using the L-curve criterion, are investigated. Using reflection data, we obtain reconstruction results that indicate an agreement between the reconstructed and true continuous deformations of waveguide boundaries.

National Category
Other Mathematics
Identifiers
urn:nbn:se:kth:diva-177434 (URN)10.1155/2015/915497 (DOI)000364064300001 ()2-s2.0-84946924717 (Scopus ID)
Funder
Swedish Energy Agency, 34146-1
Note

QC 20151120

Available from: 2015-11-20 Created: 2015-11-20 Last updated: 2017-12-01Bibliographically approved
5. Two-dimensional boundary shape reconstructions in rectangular and coaxial waveguides
Open this publication in new window or tab >>Two-dimensional boundary shape reconstructions in rectangular and coaxial waveguides
(English)Manuscript (preprint) (Other academic)
National Category
Other Mathematics
Identifiers
urn:nbn:se:kth:diva-180567 (URN)
Note

QS 2016

Available from: 2016-01-19 Created: 2016-01-19 Last updated: 2016-01-19Bibliographically approved

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