In electrical transformers, insulation is mainly ensured by the joint use of solid material (kraft paper) and insulating liquids (especially mineral oils) in intimate contact with each other.
Mineral oil degrades in service due to conditions of use. In many applications, the insulating oil is in contact with air and is subject to oxidation. High temperatures accelerate degradation.The presence of metals, organo-metallic compounds or both can act as oxidation catalysts. Oil colour changes, formation of acidic compounds and, at an advanced oxidation stage, the occurrence of sludge may occur. Dielectric properties and, in extreme cases, thermal properties, can be compromised.
Sea Marconi of Chap. 4 (Properties and deterioration/degradation of oil) of IEC 60422 Ed.4-2013.
Like oil, paper is also subject to degradation; its degradation products are solid, liquid and volatile (gas) compounds, some soluble in oil (furans, methanol, ethanol), others insoluble in oil (particles, sludge). Sludge derived from oil and that derived from paper add up to form the total sludge.
Click here to access Sea Marconi's major publications on the topic:
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
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. 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.
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.
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