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Magnetic Leakage Fields and End Region Eddy Current Power Losses in Synchronous Generators
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. (Hydro Power at the Division for Electricity)ORCID iD: 0000-0001-9125-2083
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Description
Abstract [en]

The conversion of mechanical energy to electrical energy is done mainly with synchronous generators. They are used in hydropower generators and nuclear plants that presently account for about 80% of the electric energy production in Sweden. Because of the dominating role of the synchronous generators, it is important to minimize the power losses for efficient use of natural resources and for the economies of the electric power companies and their customers. For a synchronous machine, power loss means undesired heat production. In electric machines, there are power losses due to windage, friction in bearings, resistance in windings, remagnetization of ferromagnetic materials, and induced voltages in windings, shields and parts that are conductive but ideally should be non-conductive.

The subject of this thesis is prediction of end region magnetic leakage fields in synchronous generators and the eddy current power losses they cause. The leakage fields also increase the hysteresis losses in the end regions. Magnetic flux that takes paths such that eddy current power losses increase in end regions of synchronous generators is considered to be leakage flux. Although only a small fraction of the total magnetic flux is end region leakage flux, it can cause hot spots, discoloration and reduce the service life of the insulation on the core laminations. If unattended, damaged insulation could lead to electric contact and eddy currents induced by the main flux between the outermost laminations. That gives further heating and deterioration of the insulation of laminations deeper into the core. In a severe case, the core can melt locally, cause a cavity, buckling and a short circuit of the main conductors. The whole stator may have to be replaced. However, the end region leakage flux primarily causes heating close to the main stator conductors which makes the damage possible to discover by visual inspection before it has become irrepairable.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. , 76 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1575
Keyword [en]
magnetic leakage fields, leakage flux, eddy currents, losses, synchronous generator
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering Mathematical Analysis
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
URN: urn:nbn:se:uu:diva-331182ISBN: 978-91-513-0103-7 (print)OAI: oai:DiVA.org:uu-331182DiVA: diva2:1148649
Public defence
2017-11-30, Room 2001, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:00 (English)
Opponent
Supervisors
Available from: 2017-11-07 Created: 2017-10-11 Last updated: 2017-11-07
List of papers
1. Axial Magnetic Fields at the Ends of a Synchronous Generator at Different Points of Operation
Open this publication in new window or tab >>Axial Magnetic Fields at the Ends of a Synchronous Generator at Different Points of Operation
2015 (English)In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 51, no 2, 8100208Article in journal (Refereed) Published
Abstract [en]

Axial magnetic fields leaking out at the ends of a conventional rotating synchronous machine cause losses. Therefore, it is important to be able to predict the axial magnetic fields. A linear steady-state model for the axial magnetic flux density phasor in the end regions of non-salient synchronous generators has previously been verified experimentally. This paper describes an extension of the model to salient pole synchronous generators and a method for calculating the coefficients. Experiments and 3-D finite element simulations justify a distinction between axial flux density contributions from the d and q components of the stator current. How the coefficients and the axial magnetic fields in the ends of a small synchronous generator change with steady-state operation conditions is here shown with measurements and to some extent with 3-D finite element simulations.

Keyword
Axial magnetic flux, hydropower generator, operation conditions
National Category
Physical Sciences Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-255312 (URN)10.1109/TMAG.2014.2347269 (DOI)000353595800015 ()
Available from: 2015-06-16 Created: 2015-06-15 Last updated: 2017-10-11Bibliographically approved
2. Axial Magnetic Fields, Axial Force, and Losses in the Stator Core and Clamping Structure of a Synchronous Generator with Axially Displaced Stator
Open this publication in new window or tab >>Axial Magnetic Fields, Axial Force, and Losses in the Stator Core and Clamping Structure of a Synchronous Generator with Axially Displaced Stator
2017 (English)In: Electric power components and systems, ISSN 1532-5008, E-ISSN 1532-5016, Vol. 45, no 4, 410-419 p.Article in journal (Refereed) Published
Abstract [en]

Axial displacement of the stator in a synchronous machine gives rise to axial magnetic field both at the ends and deep inside the stator. The axial magnetic field causes losses. This article contains results from two studies with an axially displaced stator. In the first study, axial magnetic leakage fields in the ends of a small synchronous generator at load and no load were measured and simulated. In the second study, axial force and iron losses at no load were calculated with non-linear materials and a three-dimensional, time-stepped finite element method. For some machines with vertical shafts, the sum of iron losses and thrust bearing losses can be reduced if the rotor is lowered or the stator raised, whichever is best.

Place, publisher, year, edition, pages
TAYLOR & FRANCIS INC, 2017
Keyword
axial magnetic field, axial force, losses, synchronous generator, finite element analysis
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-319683 (URN)10.1080/15325008.2016.1266062 (DOI)000397048400005 ()
Available from: 2017-04-07 Created: 2017-04-07 Last updated: 2017-10-11Bibliographically approved
3. Harmonically Time Varying, Traveling Electromagnetic Fields along a Plate and a Laminate with a Rectangular Cross Section, Isotropic Materials and Infinite Length
Open this publication in new window or tab >>Harmonically Time Varying, Traveling Electromagnetic Fields along a Plate and a Laminate with a Rectangular Cross Section, Isotropic Materials and Infinite Length
2017 (English)In: Progress in Electromagnetics Research B, ISSN 1937-6472, E-ISSN 1937-6472, Vol. 77, 117-136 p.Article in journal (Refereed) Published
Abstract [en]

This article contains derivation of propagation factors and Fourier series for harmonically time varying, traveling electromagnetic fields in a plate and a laminate with rectangular cross sections, isotropic materials and infinite length. Different and quite general fields are taken into account on all boundaries. Choices of boundary conditions and continuity conditions are discussed. Certain combinations of types of boundary conditions make the derivation possible for a laminate. Comparisons are made between results of Fourier series and finite element calculations.

National Category
Mathematics Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-331171 (URN)10.2528/PIERB17061909 (DOI)
Available from: 2017-10-11 Created: 2017-10-11 Last updated: 2017-10-18Bibliographically approved
4. Harmonically Time Varying, Traveling Electromagnetic Fields along a Laminate Approximated by a Homogeneous, Anisotropic Block with Infinite Length
Open this publication in new window or tab >>Harmonically Time Varying, Traveling Electromagnetic Fields along a Laminate Approximated by a Homogeneous, Anisotropic Block with Infinite Length
2017 (English)In: Progress in Electromagnetics Research B, ISSN 1937-6472, E-ISSN 1937-6472Article in journal (Refereed) Submitted
Abstract [en]

Analytical expressions that include arbitrarily directed fields on all laminate boundaries can be used for calculation of the fields inside the laminate when the boundary fields are known from, e.g., measurements. A linear laminate block could be used in non-destructive testing for comparisons between different laminates. This article contains derivation of Fourier series of harmonically time varying, traveling electromagnetic fields in homogeneous, anisotropic approximations of laminates. The component of the magnetic field strength in the stacking direction is used as a source term in two-dimensional Poisson equations for the magnetic field strength in other directions. This approximation is here used in three dimensions under the precondition that the conductivity is much smaller in the laminate stacking direction than in the other directions. Sine interpolation and different choices of types of boundary conditions are discussed. Different alternative subdivisions of the Poisson boundary value problems are treated. Shorted derivations of simple analytical expressions are given for both traveling and standing waves in two dimensions. Results from Fourier series in the three-dimensional case are compared with results from finite element calculations.

National Category
Mathematical Analysis Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-331175 (URN)
Available from: 2017-10-11 Created: 2017-10-11 Last updated: 2017-10-17
5. A Loss Model and Finite Element Analyses of the Influence of Load Angle Oscillation on Stator Eddy Current Losses in a Synchronous Generator
Open this publication in new window or tab >>A Loss Model and Finite Element Analyses of the Influence of Load Angle Oscillation on Stator Eddy Current Losses in a Synchronous Generator
2017 (English)In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069Article in journal (Refereed) Submitted
Abstract [en]

The load angle of a synchronous generator connected to a power grid has an eigenfrequency that depends on the operating conditions. The existence of an eigenfrequency can make the generator sensitive to electrical and mechanical disturbances and motivates the use of damper windings and power stabilizing systems. The eddy current losses in the stator core and clamping structure increase as a consequence of the load angle oscillations. This is shown both with transient finite element analyses and analytically via a loss model derived from a steady state phasor model of the eddy current loss density. The model is also applicable to the quasi-steady states occurring during load angle oscillations.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-331176 (URN)
Available from: 2017-10-11 Created: 2017-10-11 Last updated: 2017-10-17

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