Useful elements to define the scenario and priorities
– installation of electrical equipment;
– financial value of the electrical equipment and cost of decontamination/disposal;
– type and amount of insulating liquid;
– concentration of PCBs in electrical equipment;
– state of degradation and effects on the functionality of the electric equipment;
– possible coincidence between decontamination activities and other maintenance activities;
– environmental impact associated with possible failures and consequent losses of contaminated oil.
The following are countermeasures for the “Chemical degradation of oil” criticality, a result of the recommendations of IEC 60422 (Table 5, page 31) improved according to state of the art and the use of BAT and BEP:
- Monitoring symptomatic indicators (see symptoms above).
- If the first symptoms of criticality (such as a high rate of paper ageing on a transformer with less than 10 years of life) occur, it can be scientifically predicted that the machine will have a cycle of life that is much lower than that expected, and therefore it is appropriate to plan a profound revision of the transformer or, more likely, its replacement in the next 3/5 years.In this condition it is recommended that the frequency of symptomatic analysis be increased in order to monitor trends.
- Apply appropriate oil treatments
in order to reduce critical factors and in particular to keep the moisture in solid insulators (as well as acidity, oxygen and sludge) low and reduce any catalysing effects such as metals in the oil.
Suggested actions include:
Depolarisation by Sea Marconi
It is a process performed on site, keeping the transformer in service (and under load) without having to empty it. The operation is carried out using a Modular Decontamination Unit (DMU) specifically created by Sea Marconi. The transformer is connected to the DMU by flexible hoses; the oil contaminated with DBDS is sucked from the lower part of the transformer and transferred into the DMU, which heats it, filters it, degasses it, dehumidifies and depolarises it before pumping it back into the upper part of the transformer. This creates a closed loop and every time the oil is circulated the degradation compounds are removed and at the same time the oil returns to optimal conditions. (read more)
For example, IEC 60422 considers the acidity parameter critical if > 0.15, > 0.20, > 0.30 mgKOH/g depending on the different categories of transformers.However, acidity ranging from 0.07 to 0.10 mg KOH/g has already shown phenomena of corrosion by dissolved metals (C4) and dangerous sludge formations.It would thus be advisable to intervene with a depolarisation treatment before the oil reaches the indicated acidity thresholds and which contributes to the reduction in the thermal life of the insulating papers.
Regeneration through percolation
The process involves three phases:
1) the oil extracted from the bottom of the transformer is heated and circulated through a filter to remove particles.
2) l’the oil is then circulated through one or more cartridges containing fuller’s earth or other material suitable for removal of soluble polar contaminants.
3) the oil is finally circulated through an oil treatment system (vacuum or centrifugal dehydration) to remove water and gases.
This treatment is not effective for certain species of organic compounds, nor for PCBs, nor for the corrosive sulfur compounds, which require specific chemical reactions (e.g. hydrogenation) to be removed.Also, when treatment involves the reactivation of fuller’s earth, the “corrosive sulfur from sulfur combustion by-products (C3)” criticality may occur.
Assess treatments in terms of mass balance, energy balance, emissions balance, cost-benefit, cost-effectiveness in the given time.
What are the prevention actions to be taken on electrical equipment with insulating liquids other than mineral ones?
Concerning natural ester oils and synthetic esters, the prevention actions are the same, but it is advisable to choose countermeasures after careful assessment of cost-benefit, cost-effectiveness and environmental impact (biodegradability and fire safety).For silicone oils in operation, the treatments recommended by the standard (IEC 60944:1988) are “vacuum treatment and filtration” and “molecular sieves and filtration”.
