The power transformer basic components,
which are organic in nature, are the insulating oil and cellulose based
insulation (paper, insulation boards and blocks) [Heathcote1]. These components form the basis for the transformer
health or life, as these are usually the fastest degrading materials making up
the transformer [Emsley2].
Solid Insulation
Solid Insulation
The solid insulation is
composed of Kraft paper, pressboard, transformer board and cellulose made up of
electrical grade paper insulation manufactured from unbleached sulphate
cellulose [IEEE 60641-3-1].
Acid-hydrolysis, pyrolysis and oxidation are processes, which causes the depolymerization of paper [McNutt1, Lundgaard1, Unsworth1]. Oxidation is a process that is considered a form of combustion where the products of the reaction are water and carbon dioxide. Pyrolysis and thermal heating of the paper insulation produces significant amounts of CO and CO2 [Emsley2, Griffin1].
Studies have found that Insuldur upgraded paper does not produce as much 2FAL as Kraft paper concluding that 2FAL cannot be used as an indicator of ageing for all paper types [Lundgaard1, Prevost1]. Although both water and oxygen play an important role in the paper degradation process, water is the most significant contributor because the catalytic efficiency of dry acids is low [Lundgaard1].
For thermal conditions such as pyrolysis in oil significant levels of ethylene are produced supported by hydrogen and methane [Wang1]. Arcing in oil causes significant levels of both hydrogen and acetylene to be produced [Kelly1].
Acid-hydrolysis, pyrolysis and oxidation are processes, which causes the depolymerization of paper [McNutt1, Lundgaard1, Unsworth1]. Oxidation is a process that is considered a form of combustion where the products of the reaction are water and carbon dioxide. Pyrolysis and thermal heating of the paper insulation produces significant amounts of CO and CO2 [Emsley2, Griffin1].
Studies have found that Insuldur upgraded paper does not produce as much 2FAL as Kraft paper concluding that 2FAL cannot be used as an indicator of ageing for all paper types [Lundgaard1, Prevost1]. Although both water and oxygen play an important role in the paper degradation process, water is the most significant contributor because the catalytic efficiency of dry acids is low [Lundgaard1].
For thermal conditions such as pyrolysis in oil significant levels of ethylene are produced supported by hydrogen and methane [Wang1]. Arcing in oil causes significant levels of both hydrogen and acetylene to be produced [Kelly1].
Kraft paper has a cellulose base
that is composed of linear, polymeric chains of cyclic b-D-glucopyranosyl
units [Lundgaard1]. These chains consist of linear condensation polymer, which is
composed of a hydrocarbon glucose molecule that is formed by D-
anhydroglucopyranose units joined by ß 1.4 – glucosidic bonds. Paper insulation
has a general molecular formula of [C12H14O4(OH)6]n
with n in the range of 300 to 750 [DiGiorgio1]. The cellulose breaks down causing
the lengths of the chains to become smaller with the process generating CO, CO2
and H2O [Wilkinson1]. Other products produced during this breakdown
process like –OH and –OH2OH groups further promote the cellulose to become both
hygroscopic and vulnerable to oxidative degradation [Oommen3, Unsworth1]. Water
is found in the paper insulation as a vapour, absorbed to surfaces, as free
water in the capillaries and as imbibed free water [Du1]. The presence of
moisture plays a critical role in the life of the transformer insulation
[Lundgaard1].
Mineral Insulating Oil
Mineral Insulating Oil
Transformer mineral insulating
oil is composed from naphthenic crude oils which is a mixture of hydrocarbon
compounds of alkanes, naphthenes and aromatic hydrocarbons. Mineral oil has a
general molecular formula of CnH2n+2 with n in the range
of 20 to 40 [DiGiorgio1].
The transformer mineral oil
has numerous particles and compounds dissolved within it. These compounds are
as a result of byproducts in the degradation process or being introduced from
the external environment. The most common are dissolved gases, acids, water,
corrosive sulphur, silicon and furans. Insulating oil by nature has a low
affinity for water but the solubility increases significantly with an increase
in temperature. It is further highlighted that water can exist in transformers
in a dissolved state in oil, tightly bound to the oil molecules or as free
water [Du1].
References
[Cigre WG12.18]
|
Cigre Working Group 12.18, “Guidelines for Life Management
Techniques for Power Transformers,” Cigre, 22 June 2002
|
[Digiorgio1]
|
DiGiorgio, J. B., “Dissolved Gas Analysis of Mineral Oil
Insulating Fluids,” DGA Expert System: A Leader in Quality, Value and
Experience 1, Northern Technology and Testing, pages 1-17, http://www.nttworldwide.com/tech2102.htm,
2005
|
[Du1]
|
Du, Y., Zahn, M., Lesieutre, B. C., Mamishev, A. V., Lindgren,
S. R., “Moisture Equilibrium in Transformer Paper-oil Systems,” IEEE
Electrical Insulation Magazine, 15(1), pages 11-20, 1999
|
[Emsley2]
|
Emsley, A. M., Stevens, G. C., “Review of Chemical Indicators of
Degradation of Cellulosic Electric Paper Insulation in Oil-filled
Transformers,” IEE Proceedings - Science, Measurement and Technology, Vol.
141, No. 5, pages 324-334, 1994
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[Ferguson1]
|
Ferguson, R., Lobeiras, A., Sabou, J., “Suspended Particles in
the Liquid Insulation of Aging Power Transformers,” IEEE Electrical
Insulation Magazine, Vol. 18, No. 4, pages 17-23, 2002
|
[Griffin1]
|
Griffin, P. J., “Criteria for the Interpretation of Data for
Dissolved Gases in Oil from Transformers (A Review),” Electrical Insulating
Oils, STP 998, American Society for Testing and Materials, Philadelphia,
pages 89-106, 1988
|
[Heathcote1]
|
Heathcote, M. J., “The J & P Transformer Book,” Thirteenth
Edition, Newnes, 2007
|
[IEEE 60641-3-1]
|
IEEE 60641-3-1, “Pressboard and press paper for electrical
purposes – Part 3: Specifications for individual materials – Sheet 1:
Requirements for pressboard,” IEC Publication, 2008
|
[Kachler1]
|
Kachler, A. J., Hohlein, I., “Ageing of Cellulose at Transformer
Service Temperatures. Part1: Influence of Type of Oil and Air on the Degree
of Polymerisation of Pressboard, Dissolved Gases and Furanic Compounds in
oil,” IEEE Electrical Insulation Magazine, Vol. 21, No. 2, pages 15-21, 2005
|
[Kelly1]
|
Kelly, J. J., “Transformer Fault Diagnosis by Dissolved-gas
Analysis,” IEEE Transactions on Industry Applications, Vol. IA-16, No. 6,
pages 777-782 , November / December 1980
|
[Lundgaard1]
|
Lundgaard,
L. E., Hansen, W., Linhjell, D., Painter, T. J., “Ageing of Oil-impregnated
Paper in Power Transformers,” IEEE Transactions on Power Delivery, Vol.
19, No. 1, pages 230-239,
January
2004
|
[McNutt1]
|
McNutt,
W. J., “Insulation Thermal Life Considerations for Transformer Loading
Guides,” IEEE Transactions on Power Delivery, Vol. 7, No. 1, pages 392-401,
January 1992
|
[Oommen3]
|
Oommen, T. V., Prevost, T. A., “Cellulose Insulation in
Oil-filled Power Transformers: Part II Maintaining Insulation Integrity and
Life,” IEEE Electrical Insulation Magazine, Vol. 22, No. 2, pages 5-14, 2006
|
[Phadungthin1]
|
Phadungthin, R., Chaidee, E., Haema, J., Suwanasri, T.,
“Analysis of Insulating Oil to Evaluate the Condition of Power Transformer,”
Electrical Engineering Electronics Computer Telecommunications and
Information Technology (ECTI-CON), pages 108-111, 2010
|
[Prevost1]
|
Prevost, T. A., Oommen, T. V., “Cellulose Insulation in
Oil-filled Power Transformers: Part 1- History and Development,” IEEE
Electrical Insulation Magazine, Vol. 22, No. 1, pages 28-35, 2006
|
[Unsworth1]
|
Unsworth, J., Mitchell, F., “Degradation of Electrical
Insulating Paper Monitored with High Performance Liquid Chromatography,” IEEE
Transactions on Electrical Insulation, Vol. 25, No. 4, Pages 737-746, August
1990
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[Wang1]
|
Wang, H., Butler, K. L., “Modeling Transformers with Internal
Incipient Faults,” IEEE Transactions on Power Delivery, Vol. 17,
No. 2, pages 500-509, April 2002
|
[Wilkinson1]
|
Wilkinson, M. D., Dyer, P., “Continuous Moisture Management:
Extending Transformer Service Life,” IEE Conference Publication No. 482,
Vol. 1, CIRED 2001, 18-21 June 2001
|
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