“Insoluble deposits (sludge)” criticality is mainly caused by mechanisms of normal ageing and thermal stress of both the oil and the papers. Concauses also include problems of cross-contamination due to improper handling of solvents and transformers. The aforementioned improper practices have an impact on the life cycle of the electrical equipment with insulating fluids:
| Causes of the “Insoluble deposits (sludge)” criticality | When it may occur (life cycle phases) |
|---|---|
| Lack of purchase requirements for new insulating liquid (check oxidation stability according to IEC 60296) | Requirements and purchase |
| Deficiencies in quality control for individual lots or individual supplies of insulating oil | Acceptance of oil and paper |
| Deficiency in analytical procedures for checking chemical degradation of oil and degradation of paper | Oil acceptance, factory test, installation and pre-energisation, operation, old age, post-mortem |
| Cross-contamination through the use of oil, plants, tanks or containers contaminated by oxidised, polar and/or incompatible compounds (for topping up, impregnating or filling) | Transformer construction, factory test, installation and pre-energisation, operation, old age |
| Recycling of oil and other materials contaminated by oxidation products or polar compounds | Post-mortem |