The technical-scientific article describing the corrosive sulfur depolarization treatment (DBDS removal) on giant transformers that Sea Marconi is carrying out in Saudi Arabia has seen the publication (October 2022) in the main sector magazine, Transformer Magazine.
The article is entitled: “On-load integrated treatment of giant power transformers in a severe scenario“. The article, subjected to careful technical review by Transformer Magazine’s international consultants, was written in collaboration with GCC Lab and the Saudi national power grid.

Regarding the readers, this article is of absolute interest to owners of electrical transformers all over the world who are faced with the problem of corrosiveness of oils (contained in transformers).
The text does not exclusively describe the countermeasure adopted, but goes into detail on the decision-making process, supported by sector regulations, which guided the holder in choosing the correct asset management and “corrosiveness” risk management strategy. In particular, the actions recommended by the technical guides are described, starting from the CIGRE Technical Brochure 625 “Copper sulphide long term mitigation and risk assessment“, considered the most complete document in this area.
In a nutshell (I leave all the insights to reading the article), considering the operating conditions of the transformers, highly stressed by high loads and high ambient temperatures, the options suggested by the guide in case of corrosivity are: oil change or removal of the corrosive sulfur (oil treatment).

In light of the numerous advantages offered by oil treatment compared to its replacement (see CIGRE TB 625 – Table 12 – Mitigation techniques ranking), the customer then independently explored and tested some oil treatment techniques, obtaining unsatisfactory results. This consequently led to the choice of the solution by Sea Marconi: the selective depolarization treatment from DBDS called Chedcos.
This treatment, in addition to removing the DBDS, the main cause of the “pathology” Corrosive sulfur with DBDS (C1), is able to block the formation of copper sulphide, which typically determines the failures attributed to corrosive sulfur. Sea Marconi depolarization is also able to restore the other oil parameters, bringing them back to values similar to a new oil (unused). In addition to this, the main strength of our treatment is the guarantee of long-term results, in fact, as the article clearly shows, in this case Sea Marconi guaranteed the DBDS <10 mg/kg both at the end of the treatment (almost it always goes below 5 mg / kg) and one year after treatment.
About a year after the start of the treatment campaign, 8 transformers were successfully decontaminated (36% of the total), corresponding to approximately 809 tons of insulating oil (47% of the total).

Sea Marconi recently intervened to treat a transformer in a hydro-electric production plant in the north of Portugal, owned by EDP (Energias de Portugal), one of the largest producers of energy in Europe and one of the most important industrial groups in Portugal. The brilliant results of this activity earned Sea Marconi formal qualification by EDP (Labelec) for the “regeneration” of insulating oil in electrical transformers.

On a more operational level, the transformer treated is from 1953, with a power of 7.5 MVA, a voltage of 70 KV and an oil quantity of 6,300 kg. The activity performed on the transformer was necessary due to the presence of the criticality “corrosive sulphur from DBDS – C1“. The main indicators of the criticality encountered were the content of DBDS in the oil, specifically 76 mg/kg, and the CCD test, which proved corrosive.

Sea Marconi resolved the criticality through a DBDS Selective Depolarisation intervention, a form of dialysis on the transformer. In essence, thanks to the use of modular plants (DMU) constructed by Sea Marconi, the oil is cyclically extracted from the transformer, decontaminated inside the DMU using patented reactants, and once again pumped into the transformer in improved conditions. In this way, the transformer is never emptied of its oil load, and thanks to the constant flow of oil inside the transformer box, the latter benefits from a “cleaning” on the surface of the insulation and on the base of the box.

The Sea Marconi Selective Depolarisation treatment developed in Portugal, which lasted from 25th June to 2nd July 2015, was executed on-load, or rather with the transformer in service and under load for the entire duration of the activity.

The client followed all phases of the treatment, and through its laboratory (Labelec) re-analysed the oil at the end of the treatment. Specifically, it was noted that the DBDS passed to<5 mg/kg, the CCD test became “non-corrosive” and all other parameters were compliant with the IEC 60422 standard.

The results surprised and completely satisfied EDP, who as mentioned earlier, formally qualified the Sea Marconi process for the “regeneration” of insulating oil in electrical transformers.