“Degradation of insulating papers” is an irreversible process that can, however, be prevented or mitigated through specific actions.
By arranging appropriate operational practices (e.g. analytical oil control, oil and paper treatment, load profile management, cooling of the machine), it is possible to reduce the probability of failure and prolong the operating life of the transformer under examination.If the transformer is a member of a family of equipment affected by failure due to the same criticality, ad hoc operational practices can be defined, optimising the various critical factors (metaphorically, it is like suggesting a personalised diet combined with increased physical activity to a person suffering from diabetes).
The replacement cost of a 250 MVA, 400 kV, generation step-up transformer (GSU) can be realistically of the order of € 2,000,000, including dead time, oil emptying, logistics for decommissioning and waste disposal, purchase of a new transformer (and new oil), transport, installation and testing.
Prevention actions during the life cycle of the transformer
- 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:
For example, IEC 60422 considers the acidity parameter critical if > 0.15, > 0.20, > 0.30 mg KOH/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.
Application of cartridges for dehumidifying the transformer
Impregnating oil cannot be fully drained (typically 10-15% remains inside the transformer, about 6-7% absorbed by the paper, and in the interstices and dead points of the machine); consequently, in the case of an oil change, the new filling oil is contaminated by the old undrained oil
Assess any criticalities linked to compatibility/miscibility resulting from the use of liquids other than those of the original impregnation
- It is also recommended that maintenance practices be modified as regards:
- the drafting of purchase requirements for both oils and electrical equipment for specific applications with particular regard to design/sizing criteria
- the acceptance of oils and equipment using the best supervisory and control practices, in accordance with the prescribed methods.Ask the supplier for a certificate of compliance of the properties of oil and insulating papers
- It is recommended that strategic information be updated through a “dynamic inventory” of oils and transformers, indicating the values of symptomatic markers.
In case of failure of a twin transformer, an internal inspection of the transformer being analysed is recommended.In fact, as a result of paper sampling, subsequent laboratory analysis and interpretation of results, it is possible to identify the causes of the failure and prevent the same event on machines of the same family.On the latter, it is also advisable to undertake an in-depth investigation that also includes electrical and thermal tests in order to detect any defects in design or construction of the transformer.
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”.