The “Water in the transformer (paper and oil)” criticality is caused mainly by mechanisms resulting from external influences, with moisture entering from the atmosphere, and internal influences, resulting from the degradation mechanisms of solid and liquid insulators that generate water (hydrolysis of cellulose).
Causes in relation to life cycle phases
| Causes of the “Water in the transformer (papers and oil)” criticality | When it may occur (life cycle phases) |
|---|---|
| Lack of transformer drying requirements in the construction phase (< 0. 5% -1%) | Requirements and purchase |
| Lack of quality control for individual batches or individual supplies of paper and oil (example:Initial paper DP before impregnation, water content in oil) | Acceptance of insulating papers and oil |
| Lack of controls and analytical procedures (and sampling) for checking the water content of oil | Oil acceptance, factory test, installation and pre-energisation, operation, old age |
| Loss of protection gas and moisture accumulation on solid insulators | Transport and installation of the transformer |
| Deficiencies in paper dehumidification treatments (e. g. vapour processes phase). A good insulating paper has a water impregnated value of between 0. 5 and 1% by mass | Construction, transport, installation and pre-energisation, operation, old age |
| Accumulation of air and moisture (for example, during oil change or other electromechanical maintenance) | Factory test, installation and pre-energisation, operation, old age |
Main sources of water contamination
- Residual moisture in “thick structures”
The residual moisture in a new transformer after its construction should be less than 1% with a target of 0. 5%. An excessive residual moisture of – 4% may persist in some thick insulation components, particularly in laminated and plastic cardboards - Entry from the atmosphere
1. Absorption of water from the atmosphere due to direct exposure of air insulation (especially during installation or maintenance operations)
2. Entry of moisture in the form of molecular flux (Knudsen) due to the difference in the concentration of water in the atmosphere and that in the oil in the casing (negligible)
3. Viscous flow of moist air into the transformer due to the difference between atmospheric pressure and pressure in the casing (main contamination) - Decomposition of cellulose due to ageing
- Decomposition of oil due to ageing
- Release of “bound” water from oil and particles
