High power transformer primary and secondary coils are placed in a tank filled with a dielectric fluid that provides two essential operating functions:
- An insulating function that prevents an electric arc from forming between the different live parts
- A cooling function using the fluid’s heat transport role and heat exchangers built into the transformer tank or outside it (passive radiators, air or water cooled exchangers, etc.)
This dielectric fluid is usually oil that can be of mineral or plant origin, of the silicone and/or synthetic type. Over time these fluids deteriorate:
- By accumulating humidity through accidental air contact and/or desorption of the insulation
- Formation of dissolved gases, especially due to local heating
- Presence of impurities linked to the decomposition of certain types of insulation
- Oil oxidation making it increasingly more acid
Oil quality must therefore be monitored by regular analysis, as its ageing alters its dielectric properties. The insulating properties of the fluid deteriorate and the impurities and/or gas bubbles it contains create passages that are propitious to the propagation of electric arcs. This phenomenon, which tends to increase as the transformer ages, will lead to micro-breakdowns and hot spots which will speed up the deterioration of the oil and, in the long term, cause destructive short circuits and even cause the transformer to burn.
It is therefore essential to treat these dielectric fluids as soon as an analysis shows that a parameter has exceeded an acceptable threshold (defined by regulations in certain countries).
Depending on the oil condition two types of treatment are available:
- The standard treatment which consists in purifying the oil by eliminating solid particles using filtering and removing water and dissolved gasses by evaporation in a vacuum at a temperature of from 70 to 90°C (depending on the fluid’s technical specifications)
- Oil regeneration, which follows on from the standard treatment will restore the original physical-chemical properties of the fluid (acidity, colour, resistivity, etc.). This process is carried out by passing the oil through resins (commonly called « fuller’s earth »). This processing mode does not require heavy investment but has significant operating costs due to resin consumption and the cost of processing used resins.
- The new Reg’N process significantly reduces operating costs by making it possible to regularly activate the resins being processed using an original process.
These regeneration treatments are an economic and ecological alternative to the « refilling » process which consists in replacing the oil with new oil. Besides used oil recycling issues and the consumption of a high volume of generally fossil origin product (large transformers can contain up to 100 tonnes of oil), the « refilling » method requires the extended halt of the transformer to be treated. This is not the case for regeneration which can be carried out with the transformer running, thereby avoiding shutting down the installation. Another significant advantage of regeneration is that oil acidity is more fully treated compared to situations in which the load is replaced where it is common, after a few weeks, to see the acidity level rise due to the used oil remaining in the insulation after drainage.