Newsletter from SINTEF Energy Research

SINTEF Energy Research NO-7465 Trondheim, Norway, Phone: 73 59 72 00, www.sintef.no/energy

5-2006Transformer technology: life assessment, diagnostics, materials protection25 years experienceSINTEF Energy Research, together with its twin parther The Norwe-gian University of Science and Technology have more than 25 years of experience working on transformers. Often projects are carried out in collaboration with both industry and utilities.

Research is centred around well equipped high voltage and material laboratories. For transient studies mo-dern computer programs are available. Also in-service diagnostics are offered.SINTEF is very active in CIGRE work.

Experience covers the following areas: ac breakdown in models and impact of moisture and particles Insulation coordination Breakdown from switching impulse superimposed on ac Streamer propagation in long gaps Streamers from particles in barrier designs PD under ac; needle-plane gap to evaluate liquids PD pattern recognition from transformer defect models Transformer maintenance handbook Survey of transformer diagnostics Cellulose ageing Neutralisation value and moisture balance Dielectric response of cellulose FRA of transformers Electromagnetic and transient field modeling

Life expectancy for oil-impregnated celluloseA large part of the transformer population has reached 30 years and many of these have water content in the 3% range. A new understan-ding of ageing of pressboard and cellulose is established where the roles of oxygen, water and acids are identified. For transformers with a high water content and high operating temperatures life may be severely shortened. Ageing of transformers is mainly governed by acid catalyzed hydrolysis via H+-ions from dissociated organic acids. This may reduce life with a factor of more than 10 (see figure). Water concentration governs dissociation. Oxygen may speed up ageing by a factor of 2-3

Improved methods for diagnostics of acids are developed.

Keeping the transformer dry will provide a long life, while water will reduce it, particularly together with high temperature.

Maintenance may increase lifeRemoving water together with the particularly harmful low molecu-lar acids may extend the life of a transformer. Using ageing models one may estimate cost benefit.

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References:

L.Lundgaard : Partial discharge Part XIV : Acoustic partial discharge detection - Fundamental considerations. IEEE Insulation Magazine, Sept/Oct. 1992, Vol 8, No.5

L.E. Lundgaard, G. Berg, A.P. Brede, S.L.Kyrkjeeide: Acoustic location of discharges in power transformers, ISH, Delft, 2003

Lars E. Lundgaard, Walter Hansen, Dag Linhjell, Terence J. Painter: Ageing of oil-impregnated paper in power transformers, IEEE TDEI Vol. 19, No.1, Jan 2004, pp230-239.

S.Ingebrigtsen , M.Dahlund; W.Hansen, D.Linhjell, L.E.Lundgaard: Solubility of carboxylic acids in Paper (Kraft)-Oil Systems, CEIDP, Boulder, Colorado, 2004

L.Lundgaard, K.Herstad, M.U.Anker, J.Sletbak: Flashover Along Solid Surfaces Parallel to the Electric Field in Liquid Insulation at 50 Hz, CIGRE 86, Paris, paper 15 08.

Lundgaard L.E, Linhjell D., Sund J.B., Jorendal G.: Influence of Simultaneous AC-Stress on Impulse Breakdown in Oil-Paper Insulation System. Conf. rec. of 8. Int Symp. on High Voltage Engineering, Paper no 25.01, Yokohama, 1993, pp 231-234.

L.E.Lundgaard, .Hestad, S.Ingebrigtsen: Review of Test methods for Dielectric Properties of Liquids for HV transformer Application, Nordic Insulation Symposium - Nord-IS 05, Trondheim 2005. pp83-87.

E. Bjerkan, H. K. Hidalen: High Frequency FEM-based Power Transformer Modeling: Investigation of Internal Stresses due to Network-Initia-ted Overvoltages, Proc. Int. Conf. on Power System Transients, Montreal, Canada, June 19-23, 2005. Honoured as the best student paper and to be printed in the EPSR Journal.

E. Bjerkan, H. K. Hidalen, O. Moreau: Importance of a proper Iron core Representation in high Frequency Power Transformer Models, Proc. Int. Symp. In high Voltage Engineering (ISH05), Beijing, China, Aug. 25-29, 2005.

E. Bjerkan, H. K. Hidalen, O. Moreau: FRA Sensitivity Analysis using High Frequency Modeling of Power Transformers based on the Finite Element Method, Proc. Int. Symp. In high Voltage Engineering (ISH05), Beijing, China, Aug. 25-29, 2005.

H. K. Hidalen, B. Mork, F. Gonzalez-Molina, D. Ishchenko, N. Chiesa: Implementation of the Hybrid Transformer Model in ATPDraw, Proc. EMTP Users Group meeting, Warsaw, Poland, Sept. 12-13, 2005.

Contact:Senior Research Scientist Lars Lundgaard, Phone: + 47 73 59 42 55, Email: Lars.Lundgaard@sintef.noAssociate Professor Hans Kristian Hidalen, Phone: + 47 73 59 42 25, Email: Hans.Hoidalen@elkraft.ntnu.no

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DiagnosticsSeveral studies on partial discharge detection, both electrical and acoustic have been made:

Energetic discharges was produced from a bad connection of a field grading ring.

Combined acoustic and electric PD- detection located the fault with decimenter accuracy.

High frequency modeling of a power transformer

Power system transientsSINTEF is together with NTNU and MTU in Michigan-USA involved in development of improved simulation models of power transformers for transient analysis. This covers topological correct core models, saturation effects, frequency and voltage dependent los-ses, zero sequence behavior, and remanence. An ongoing PhD project focuses on inrush currents. The figure below shows a simulation of inrush currents with the newly develop Hybrid Transformer model (XFMR) compared to the classical BCTRAN approach with satura-tion based on open circuit test report values.

Transformer core modeling and inrush calculation

Diagnostics for ageing assessment and quality control of maintenace Furanic compound analysis does not tell the age of thermally upgra-ded Insuldur paper

The low molecular acids that carry information about the status of the ageing can give misinterpretation of moisture of cellulose as they increase water solubility of oil.

Low molecular acids change the partitioning of water between oil and paper

Breakdown of transformer insulationThe dielectric properties of oil/paper insulation has been studied ex-tensively both in practical level design models and more fundamental studies of liquid properties.

Insulation model

The role of particulate contaminants on the voltage withstand level of insulation models has been studied, revealing that for switching impulse wet particles may reduce the withstand by 30%.

Breakdown in long oil gaps has been studied, and a very fast break-down mode documented.

Standardized methods for study-ing liquid dielectric performance are found inferior and a large re-search program aimed at studying fundamental properties of liquid insulation is initiated.

Electromagnetic modelingThe transformer is a vital component is most power systems, but the models used to predict its transient behaviors are simple. A transfor-mer is a nonlinear component both in frequency and flux dependency.

SINTEF has since year 2000 worked extensively to explore these behaviors. Initially by studying diagnostics of transformer windings using the frequency response analysis (FRA) method and sponsoring a PhD on high frequency modeling of power transformers. Presently SINTEF is involved in power transformer modeling for power system transient studies with a focus on inrush currents and sponsoring a PhD on transformer behavior under switching operations.

FRA measurements and modelingFRA is a high frequency diagnostic method used to reveal transformer winding deformations/changes. SINTEF has been involved in field measurements, theoretical method development, and Cigre activities. The figure below shows measurements on two supposedly equal windings; one old with copper sulphur contamination and one new without.

Comparison of windings with/without Cu2S

The next figure shows high frequency transformer modeling based on finite element approaches. The key point here is to utilize symmetry and establish equivalent insulation and core materials to obtain a manageable model size. The SINTEF sponsored PhD study concluded that it is important to consider the core also for frequencies above 10 kHz.

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