Ageing Components and By-products: Organic

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

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]. 

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

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 C_nH_2n+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]. 

Aging of oil insulation is markedly different for open (free breathing) and closed systems [Kachler1]. Aging of the oil insulation is accelerated by the high ingress of oxygen, which after reaching equilibrium of approximately 20000 ppm produces a source of additional energy for the ageing of the oil [Ferguson1]. Transformer oil degradation is primarily caused by decomposition, contamination and oxidation [Phadungthin1]. Mineral oil may break down under elevated temperatures due to abnormal loading or fault conditions such localized hotspots and electrical faults. Moisture, dielectric, acidity and oxygen have a major effect on the ageing and break down of the oil [Cigre WG12.18].

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
[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
[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