Properties and deterioration/degradation of oil
The reliable performance of mineral insulating oil in an insulation system depends upon certain basic oil characteristics that can affect the overall performance of the electrical equipment.
In order to accomplish its multiple roles of dielectric, coolant and arc-quencher, the oil needs to possess certain properties, in particular:
- high dielectric strength to withstand the electric stresses imposed in service
- sufficiently low viscosity so that its ability to circulate and transfer heat is not impaired
- adequate low-temperature properties down to the lowest temperature expected at the installation site
- resistance to oxidation to maximize service life
In service, mineral oil degrades due to the conditions of use. In many applications, insulating oil is in contact with air and is therefore subject to oxidation. Elevated temperatures accelerate degradation. The presence of metals, organo-metallic compounds or both may act as a catalyst for oxidation. Changes in colour, the formation of acidic compounds and, at an advanced stage of oxidation, precipitation of sludge may occur. Dielectric and, in extreme cases, thermal properties may be impaired.
In addition to oxidation products, many other undesirable contaminants, such as water, solid particles and oil-soluble polar compounds can accumulate in the oil during service and affect its electrical properties. The presence of such contaminants and any oil degradation products are indicated by a change of one or more properties as described in Table 1.
Mineral insulating oil is a mixture of predominantly paraffinic, naphthenic or aromatic hydrocarbons, mainly resulting from the distillation and refining of crude oil. The obtained oil can then be added and/or blended (mixed) for specific applications.
In several countries, it is a common operating practice for oil recovery and recycling. In order to meet this regulatory requirement, IEC formalised the IEC 62701 standard, cancelled in February 2015 and subsequently reconsidered (January 2016) in terms of merging with standard 60296 ed.4 by decision of the IEC Standardisation Management Board (SMB).According to these guidelines, new mineral-based insulating oils will not only be those classified as “virgin” but also those classified as “recycled”.
In this scenario, the management of the life cycle of oils with different characteristics creates much more complex situations (e.g. in the phases of formalisation of purchase and acceptance requirements of supplies).
Degradation processes occur during the various phases of the life cycle of the transformer and are distinguished by “degraded electrical insulation of oil” and “chemical degradation of oil” (see differences below):
Degraded electrical insulation of oil
Chemical degradation of oil
|Causes||Physical contaminants such as water, particles, dissolved gases resulting from external sources (atmospheric) or internal sources (degradation of materials through thermal or electrical stress)||Oxidation polar compounds (sludge), and consumption of additives (if present) resulting from degradation of internal materials through thermal or electrical stress|
|Effects||Reduction of the discharge voltage||Reduction of chemical-physical properties such as total acidity, dielectric dissipation factor, interfacial tension, resistivity, concentration of additives and oxidation stability|
|Countermeasures||Physical treatment (microfiltration, dehumidification and vacuum degassing) or selective adsorption in the case of water in oil||Selective reclamation or depolarisation treatments by Sea Marconi (also effective against corrosive and acidic compounds)|
The mechanisms of chemical degradation of oil are the result of complex reactions (e.g. catalytic oxidation) between the organic substances of the oil and the materials contained within the transformer.The magnitude of the phenomenon depends on some peculiar characteristics such as type of insulating liquid, type of equipment (power, voltage, etc.), its load profile (percentage of nominal load, duration in hours), conditions of environmental severity in which it operates and supervision and maintenance policies.
Degradation is mainly characterised by variation in time (pejoratively) of some specific properties (symptomatic indicators) compared with the values of the same indicators for a new (unused) insulating liquid.The evolution of oil degradation processes acts to accelerate the ageing process of solid insulators (papers).
List of major Sea Marconi publications on the subject:
V. Tumiatti – “L’analisi dei fluidi tecnici come strumento di diagnosi del degrado per l’efficace prevenzione dei guasti” – Seminar of the International Research Institute on Production Maintenance for Hydraulic Systems and Lubrication Systems, Milan, 25-26 November 1998.
V. Tumiatti, R. Actis, A. Armandi, G. Di Iorio, G. Camino – “Diagnostic testing of oil samples on electrical transformers” – (to be presented for SMI’99 – 3rd International Conference on Industrial Plant Maintenance , Bologna, 17-20 February 1999).
S. Kapila, P. Nam, V. Tumiatti, A. Armandi “Evaluation of Analysis Techniques for Fingerprinting 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. Unification and Certification, Vol. LIV.; p. 41-44, ISSN:0394-9605