For diagnosis of the “Chemical degradation of oil” criticality, Sea Marconi uses its own diagnostic metrics, namely:
visual signs on the transformer (and those from any internal inspection) are interpreted;
through analysis of oil the symptoms are identified, i.e. the symptomatic indicators (aspect, colour, TAN, dielectric dissipation factor, particulate matter) and their characteristic values.
The limits given in IEC 60422 are intended as “recommended“; as occurs for dissolved gas limits, it is advisable to determine typical values of warning and alarm on a statistical basis divided by families (of equipment and oil) regarding one’s machine pool.
the database is used to study family or subjective case histories (in the search, for example, for failures in twin machines);
factors of uncertainty, speed and evolution over time (trends) of symptomatic indicators are taken into consideration and monitored during the life cycle phases;
on the basis of assessment of these key factors, the specific criticality is classified according to type and priority, and type and priority of corrective actions are identified at the same time.
Real example
Voltage: 400 kV, Power: 250 MVA
50,000 Kg of non-inhibited paraffin-based mineral oil
Total acidity of 0.25 mg KOH/g (“poor” value compared with Table 5 IEC 60422),
Dielectric dissipation factor = 0.27 (“poor” value compared with Table 5 IEC 60422)
Interfacial tension = 20 mN/m (“poor” value compared with Table 5 IEC 60422)
Dissolved copper = 0.97 mg/kg (“poor” value compared with Table 5 IEC 60422)
Colour = 6 dark (“poor” value compared with Table 5 IEC 60422)
Paper weight = 2,500 kg
Through the test of oxidation stability (IEC 61125:1992) it is possible to measure an amount of sludge equal to 0.2% in mass (of oil) which, on 50.000 kg of oil, means having about 100 kg of sludge resulting from degradation of the oil itself.
Impregnating oil cannot be drained completely; typically, 10-15% remains inside the transformer, absorbed from the papers, and in the interstices and dead spots of the machine. This means that in case of an oil change, the new filling oil would be contaminated by old undrained oil.
