When corrosive oils are present (see criticalities C1, C2, C3, C4) Sea Marconi implements the Selective Depolarisation from corrosive sulphur Chedcos®.
Our solution is effective, economic and conclusive since it eliminates the cause of the problem. This means removing from the oil both the DBDS and the other corrosive sulphured compounds (DBDS < 10 mg/Kg). At the same time, the Depolarisation by Sea Marconi reactivates the functional properties of the oil in compliance with the IEC standards 60422 Ed. 4-2013, CIGRE 378 – 2009 art. 4.2.
Total pieces of equipment decontaminated from corrosive sulphur through December 2014: 372
Total mass of oil decontaminated from corrosive sulphur through December 2014 ca. 6.560 ton
The Chedcos® selective Depolarisation uses:
Treatment plants totally developed in-house designated as DMU (Decontamination Modular Unit),
(S/CHED®) without which the treatment wouldn’t be as effective
A unique operational methodology since it is carried out on-site with a closed-loop, meaning that the transformer is not drained not even partially.
Appropriate safety measures also in the “On-load” mode, meaning that the transformer can be kept in operation and under load during the duration of the entire treatment.
Estratto "corrosive sulfur" da tab. 5 della IEC 60422 ed. 4-2013
Recommended action limits
Recommended action (b, c)
O, A, B, C, D, E, F
For corrosive oil:- conduct risk assessment
- reduce the corrosivity of the oil by adding a copper passivator (e.g. triazole derivates)
- remove the source of corrosivity by changing the oil (see Clause 12) or by removing corrosive compounds by mean of suitable oil treatments.
The necessity of corrective actions is to be carefully evaluated on the base of a risk assessment study (CIGRÉ Brochure no. 378, 2009 .After passivation of the oil, regular monitoring the concentration of the passivator is necessary.
In case of continuous depletion of concentration of the passivator remove the cause of corrosivity accordingly with recommended actions
(a) Category of equipment (see Table 2).
(b) Ensure sampling was properly done, sampling valve was properly cleaned before sampling (Clause 6) and transportation and storage periods in the laboratory were short.
(c) No action should be taken on the basis of one result and one property. All results shall be confirmed by a repeat sample especially if the result appears abnormal compared to the trend of the previous results obtained.
(d) If the oil test for corrosivity is positive and DBDS is present, follow the recommendations of CIGRÉ Brochure 378:2009  for the appropriate mitigation actions.
(e) If there is a tendency for particles to increase, it may be necessary to make a determination of metals and other elements dissolved in the oil (see Table B.1).
The depolarisation from corrosive sulphur is among the countermeasures indicated by the standards
Taken from IEC 60422 Ed. 4 2013
Una tecnica eclusiva ed efficace
The extraordinary results of the interventions by Sea Marconi are the product of the expertise accrued since 1968 from the use of Decontamination Modular Units (DMU) engineered and developed in-house and from the formulation of specific reagents (S/CHED®, etc.) used depending upon the different operational scenarios.
The DMU is connected to the transformer by hoses then the decontamination process is started: the insulating fluid runs through the DMU in which it is:
depolarised (by patented special reagents)
then returned into the transformer that is never drained, not even partially.
The continuous circulation of the insulating oil creates a constant flow of the fluid in the transformer enhancing the elimination of deposits (sludge) on the paper elements and the tank.
The activity is performed under safe conditions thanks to
low processing temperatures,
the use of patented reagents that do not create explosion or fire dangers,
the use of special hoses (SpillGuard®) capable of cutting off the process in the event of accidental spills and also thanks to
a monitoring software system checking in real time that the process operates under safe and quality control conditions 24/7, even without the presence of an operator.
The interventions are performed with the On-Load option keeping the transformer in operation, energised and under load that is without any loss of production (international experiences even in nuclear power plants and on transformers up to 760 MVA and up to 500 KV).
Two way travel of specialised technicians, treatment plants (DMU), tooling, reagents and consumables from the Sea Marconi location to the transformer site;
Installation and hydraulic and electrical connection of the depolarisation mobile units (DMU);
Sampling, analysis and diagnosis (Deosvision®) prior to the treatment (possible);
Depolarisation from acidity treatment (Chedcos®);
Quality control of the treatment with field measuring kits SM-TCS;
Physical decontamination treatment (degassing and dehydration under vacuum);
Monitoring of the treatment parameters;
Change of the oil in the tap changer (possible);
Check of the levels and breathers;
Change of the silica gel drying salts;
Sampling, analysis and diagnosis (Deosvision®) at the end of the intervention;
Inhibition of the oil with anti-oxidant additive DBPC (possible) in case the initial conditions of the oil are such to suspect that the oil lost the resistance to oxidation.
– Removal of the sulphured corrosive compounds (DBDS and others)
– Removal of the inorganic polar compounds (metals)
– Removal of the soluble and insoluble oxidation compounds (acids)
– Removal of the water and physical contaminants in the oil
– Reconditioning of the dielectric properties of the oil
besides, naturally, the advantages offered by all the treatments with the Sea Marconi methodology:
Thanks to the use of a patented solid reagent, at the low process temperatures (80-100 °C 175-210°F), avoiding emptying the transformer and the use of decontamination units (DMU) developed in-house equipped with an advanced monitoring system and 24/7 automatic safety system.
With a total 372 pieces of equipment decontaminated from corrosive sulphur and a total of about 6,500 tons of oil decontaminated from corrosive sulphur (data recorded at December 2014). )
The Chedcos® treatment avoids the logistic and operational criticalities related to the change of the oil. The new oil would be in any case contaminated by the DBDS (or other sulphured corrosive compounds) present in the “old” oil remaining impregnated in the paper insulations that would be released after the change. In addition, the change of the oil involves the machine down condition of the transformer and functional risks (e.g. electric discharges) due to air bubbles and atmospheric moisture.