For the diagnosis of the “Insoluble deposits (sludge)” criticality, Sea Marconi employs its own diagnostic metrics, namely:
visual signs on the transformer (and those from any internal inspection) are interpreted;
through analysis of oil, symptomatic indicators and their characteristic values are identified;
The limits of the “sediments and sludge” properties indicated in IEC 60422, i. e. 0. 02%, are to be understood as “recommended”
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. In natural esters, for example, additives can be up to 5% in mass(0. 3% in mineral oils); therefore their degradation by-products are decisive indicators.
Real example
Year of construction: 1978
Voltage:400 kV, Power:250 MVA
Total transformer mass: 220,000 kg
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 µl/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 = 1.200 µg/Kg (higher than the typical value of the transformer family)
Ethanol = 300 µg/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.
625 kg (paper weight loss) – 103,125 kg – estimated volatile part equivalent to that of water = approximately 418 kg, which is the sludge resulting from paper.
Through the “sediment and sludge” test on oil it is possible to measure a quantity of sludge equal to 0. 2% by mass (of oil) which means that 50,000 kg of oil would give about 100 kg of sludge resulting from degradation of oil.
The total sludge is therefore given by the sum of sludge resulting from paper (418 kg) and that resulting from oil (100 kg), that is to say approximately 518 kg, which settles unevenly on the bottom of the casing, on solid insulating parts and on oil circulation ducts. It is experimentally demonstrated that sludge deposits reduce the cooling capacity of the windings, increasing local temperature by up to 6-8° C and thus doubling the speed of deterioration of the papers according to the well-known Arrhenius equation. This criticality has a significant impact on reduction of the residual thermal life of the transformer and requires timely and effective prevention and risk mitigation actions.
By analogy, this is what happens to people: the accumulation of fat in certain parts in the circulatory system leads to a reduced quality of life of the subject and a reduction in life expectation.
