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.

After about two years from the start of the research and development project finalised toward the evaluation of the Selective Depolarisation project used to solve the problem of corrosive sulfur in Brazil, on 28th October 2010, a meeting was held in Rio de Janeiro to share the results of the research.

The event has factually validated the Selective Depolarisation process by Sea Marconi (with on-load application) as the best available technique against the problem of corrosive sulfur by DBDS on the transformers and reactors of the Brazilian grid (up to 500 kV) controlled by TAESA. With a clear and full letter of reference in favour of Sea Marconi, TAESA formalised the validation of the Selective depolarisation and the benefits obtained in operational terms, both economically and environmentally.

Let’s take a step backward.

TAESA (Transmissora Aliança de Energia Elétrica S.A.), ex Terna Participações S.A.is currently one of the largest groups for the transmission of electric power in Brazil. After suffering several failures caused by corrosive sulfur and after discovering they had 76 (out of 112) transformers and reactors contaminated by DBDS (totalling 2,093,358 kg of corrosive oil), they decided to implement different countermeasures, including the Selective Depolarisation by Sea Marconi. Prior to procuring a DMU (Decontamination Mobile Unit) from Sea Marconi, other countermeasures were tried on the Brazilian grid: passivation, changing the oil, replacing the windings on the worst contaminated units.

The research and development project, the results of which have been divulged last October in Rio de Janeiro, have evaluated on one side the effectiveness of the Selective Depolarisation process by Sea Marconi, on the other hand it was compared with the alternative solutions mentioned above, both on the technical-functional level and the financial and environmental sides. A synthesis of the project is provided here below.

In summary:

• The Selective Depolarisation process represents an assurance for the removal of DBDS; all the forms of application of the process showed being effective: with energised or de-energised equipment, or with the oil in tanks; the duration of the treatment varies depending upon the type of application.
• The process does not change the chemical-physical properties of the oil. However, it is necessary to add an anti oxidant additive (DBPC) to the oil after the treatment.
• Under an environmental point of view, choosing the change of the oil, especially when thousands of tons of oil are involved, produces a large volume of waste to be disposed of, whereas the Selective Depolarisation process provides in fact the full recovery of the oil, preventing its change.
• Also, still on the environmental side, it must be pointed out that the passivation with BTA can cause environmental problems at the end of life of the equipment.
• On the financial side, the Depolarisation, with respect to the change, has a lower total cost.
• In the TAESA instance (~ 1.100 tons of contaminated oil), the Depolarisation, with respect to the change of oil, saved 1,600,000 Euro (without considering the costs for the disposal and the penalties for loss of supply of power).
• A financial comparison with passivation has not been done, since the latter can only be considered as a temporary solution for the action of the DBDS on copper, but it does not eliminate the DBDS.
• Selective Depolarisation is the option generating the least operational risks: in case of emergency, it is possible to obtain the immediate availability of the equipment.
• With respect to the other measures for DBDS in oil (change of the oil and passivation) it is possible to state that the Selective Depolarisation process represents currently the best available technique and the best environmental practice (in the case of TESA it prevented the disposal of 1,167,000 kg of oil).