For diagnosis of the “Degradation of insulating papers” 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 the papers (if available as a result of internal inspection) and oil, the symptoms and their values are identified. In particular, for some indicators such as 2FAL and CO2, the standard (IEC 62874:2014) shows typical values based on the age of the equipment, the type of electrical equipment used and type of oil. As regards the other symptomatic indicators not “covered” by the standard, it is recommended that typical values be obtained of warning and alarm by means of statistical analysis on 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.
Changing the insulating fluid changes the diagnostic assessments of degradation processes. For example, natural ester fluids have a much higher natural ability to dissolve water than a mineral oil.
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)
The indicators that identify degradation of the papers are added to those of the aforementioned degradation of oil
CO2 => 16,500 ?l/l (higher than the typical value, “high ageing rate” 98 percentile)
2FAL = > 6.5 mg/kg (higher than the typical value, “high ageing rate” 98 percentile)
Methanol = 1200 ?g/kg (higher than the typical value of the transformer family)
Ethanol = 300 ?g/kg (higher than the typical value of the transformer family)
The DP of this transformer has decreased in 35 years from 1000 to 200, understood as a mean value, which conventionally corresponds to the end of thermal life.At the same time, a loss of paper mass of 25% is estimated; in fact, its weight decreases from the initial 2,500 kg to 1,875 kg.
The insulating papers are impregnated with non-inhibited paraffinic oil. After the impregnation cycle (typically under vacuum, 60-80 °C, and at least 72 hours), up to 150-180% of the initial mass of kraft paper becomes impregnated with oil, with a weight range between 2,812 kg and 3,375 kg (compared with 1,875 kg dry).
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.