The “Corrosion caused by dissolution of metals (C4)” criticality is caused by the corrosive action of oil on metals present in transformers (e.g. copper).Such corrosion results in the dissolution of metals within the oil under normal operating conditions.
The type and speed of metal corrosion depend on the temperature, oxygen concentration and formulation of oil in terms aromatic compounds, polynuclear aromatic compounds (PNA), etheroatomic compounds (e.g. oxygen, nitrogen), antioxidant additives (e.g.DBPC), passive additives (e.g.Irgamet 39, Irgamet 30).
Causes in relation to life cycle phases
| Causes of the “Corrosion caused by dissolution of metals (C4)” criticality | When it may occur (life cycle phases) |
|---|---|
| Lack of purchase requirements of the oils (unused or recycled) | Requirements and purchase |
| Lack of quality control for individual batches or individual supplies of insulating oil | Acceptance of insulating oil |
| Lack of controls and analytical procedures (and sampling) for checking the dissolved metals in oil | Acceptance of insulating oil, factory test, commissioning and pre-energization, operation, aging, post mortem |
| Cross contamination caused by the use of oil, plants, tanks or drums contaminated by metals (due to topping up, impregnations, fillings or treatments) | factory test, commissioning and pre-energization, operation, aging, post mortem (recycling of oil) |
The effect of the electric field inside the transformer amplifies the ionic mobility of organometallic compounds dissolved in the insulating liquids by degrading the dielectric properties of the insulating system (oil and paper).These phenomena can become particularly critical for certain types of transformers, such as rectifier, converter (HVDC) and special (e.g. electric traction of trains) transformers.
Dissolution causes the deposition of metals (e.g. copper) on insulating papers and the formation of insoluble deposits (sludge) inside the casing; the result is the progressive degradation of the dielectric and chemical properties of both oil and paper.
The presence of dissolved copper acts as a catalyst in the oxidation processes of the oil
– accelerating the chemical degradation of oil,
– reducing the oxidation stability properties of oil,
– accelerating the ageing of papers,
– and accelerating the formation of sludge.
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Sea Marconi has experimentally studied these phenomena on various types of insulating liquids.In some typologies a correlation was found between the dielectric dissipation factor (tg delta) and the concentration of copper dissolved in the insulating oil.In other types of oil, due to the copper deposition tendency test developed by Sea Marconi, there was a relationship between the formulation of the oil with particular additives and the tendency to deposit organo-metallic compounds containing copper on the papers.
