In electrical transformers, insulation is mainly ensured by the oil and paper system.
The construction of transformers provides for operational practices such as to guarantee a concentration of water in paper typically less than 0.5% -1% in mass, in relation to the contractual requirements defined between the manufacturer and the purchaser (ref. 5.4.3 IEC 60422). The limit on water in oil depends on the category of transformer; for new transformers (> 170 KV) prior to energisation, the maximum permissible concentration of water in oil is 10 mg/Kg (ref. tab 3 IEC 60422).
After the initial drying of the new transformer, the water concentration increases in relation to the transformer type, environmental conditions, operating conditions, and oil and paper degradation processes.The increase in water in the transformer has external causes (e.g. entry from the atmosphere) and internal causes, due to complex mechanisms of degradation of the oil-paper system. The latter include processes of chemical-physical degradation of the paper known as “hydrolysis processes“. During the life cycle of the transformer, there is a condition of continuous modification of the balance between the water absorbed into the paper and that dissolved in oil.
The migration of water between paper and oil depends mainly on the temperature and the “operating profile” of the transformer.
Relationship with temperature
The migration of water between paper and oil depends mainly on the temperature and the “operating profile” of the transformer.The higher the temperature rises, the more we observe the migration of water from papers to oil; the lower the temperature drops, the more water passes from oil to the papers.
The thermal profile of the transformer is not homogeneous in all its internal parts; in particular it changes according to the single winding, in the high, medium and low parts, the circulation and cooling ducts and the geometry of the refrigeration system, and the deposits of sludge that can change over time.A major study of these conditions is contained in the CIGRE WG 230 document of 2007 titled “Moisture equilibrium and moisture migration within transformer insulation systems”, whose convenor was Victor Sokolov (deceased in 2008), one of the top experts in the field.
Water in the transformer causes a progressive reduction of the paper-oil insulating properties (discharge voltage, dissipation factor), generating electrical defects (e.g. partial discharges) that can evolve into electrical discharges and power arcs with electrical faults in the transformer.
99% of the water contained in power transformers is found in cellulose materials.
Click here to access Sea Marconi's major publications on the topic:
V. Tumiatti, S. Levchik, G. Camino “Paper ageing esperiments” CIGRE Task Force 15.01.03 – Lubliana november 1995.
S. Levchik, J. Scheirs, G. Camino, V. Tumiatti – “Study of the origin of furanic compounds in the thermal degradation of cellulosic insulating in electrical transformers” – Polymer degradation and stability 61 (1998) 507-511.
J. Diana, V. Tumiatti, G. Camino – “Diagnostic testing of oil samples and interpretation of results” – Proceeding of the Conference – Power Transformer Maintenance – Faculty of Engineering – University of Pretoria – R.S. Africa, 26-27 may 1998.
V. Tumiatti – “L’analisi dei fluidi tecnici come strumento di diagnosi del degrado per l?efficace prevenzione dei guasti” – Seminario dall?Istituto di Ricerca Internazionale sulla manutenzione produttiva ai sistemi oleodinamici ed alla lubrificazione degli impianti, Milano 25-26 novembre 1998.
J. Scheirs, G. Camino, V. Tumiatti, M. Avidano: “Study of mechanism of thermic degradation of cellulosic paper insulation in electrical transformers oil”. Die Angewandte Makromulekalare chemie (1998) 19-24 (Mr 4504).
V. Tumiatti, R. Actis, A. Armandi, G. Di Iorio, G. Camino – “Diagnostic testing of oil samples on electric transformers” – (to be presented for SMI?99 ? 3° Convegno internazionale sulla manutenzione di impianti industriali, Bologna 17-20 febbraio 1999).
J. Scheirs, G. Camino, V. Tumiatti, E. Serena, D. Allan, C. Jones, A. Emsley, M. Avidano – “Review of some developments in cellulose insulation condition monitoring” (to be submitted).
S. Kapila, P. Nam, V. Tumiatti, A. Armandi – “Evaluation of Analysis Techniques for Finger printing Mineral Transformer Oil” – CIGREWG15.01.TF06 – Leatherhead (UK) 13.01.1999.
J. Scheirs, G. Camino, M. Avidano, V. Tumiatti – “Origin of Furanic Compounds in Thermal Degradation of Cellulosic Insulating Paper” – Journal of Applied Polymer Science, Vol. 69 2541-2547 (1998).
M. Pompili, F. Scatiggio, V. Tumiatti. (2009). “Liquidi isolanti: nuove prospettive ed evoluzione normativa”. U & C. Unificazione e Certificazione, vol. LIV.; p. 41-44, ISSN: 0394-9605
R. Actis, S. Flet. Water in insulating system of oil-filled electrical equipment De-hydration techniques of power transformers in service on-load treatment – Proceedings of My Transfo 2008, Turin (IT) December 17-18, 2008 pag. 53