Removal (depolarisation) of acidity of oil

Friday March 17th, 2017

For insulating liquids that are either acidic or have polar compounds, Sea Marconi has devised a service called Selective Depolarization against acidity (Chedcos®).This treatment is aimed at removing polar compounds and oxidation products that are responsible for increasing the total acidity (TAN), increasing the Dielectric Dissipation Factor (tan delta) and decreasing interfacial tension.At the same time, thanks to Sea Marconi’s Depolarization treatment, it is possible to restore the other physical-chemical properties of the mineral insulation oil in accordance with IEC 60422 Ed.4 2013.

Power transformers treated against acidity until the 2017: 626
Mass of oil decontaminated against acidity until the 2017: 6.600 ton

See the recommended actions according to the standard

on the “acidity” and “interfacial tension” parameters taken from IEC 60422 Ed.4 2013
Extract of “acidity” e “intefacial tension” from tab. 5 of IEC 60422 ed. 4-2013

Oil oxidation accelerates the deterioration process of the papers and reduces the life of the transformer, firstly forming acidic products (light and heavy acids), which chemically react up to corrosively dissolve the metals (C4), and secondly forming sludge, insoluble compounds that accumulate on the papers of the reels and reduce their heat exchange properties.The consequence is a significant increase in localized temperature and therefore quicker deterioration of the papers until their failure. This mechanism is explained by Arrhenius law which states that the reaction rate doubles every 6/8°C.

It is a similar phenomenon to the prolonged accumulation of fat in veins, causing an increase in local stress (increased blood pressure) and reducing the life expectancy of the patient.

Thus from an asset management point of view it is recommended to intervene early on with a depolarization treatment against acidity and not to wait for the oil to reach the thresholds specified in IEC 60422 (critical acidity at > 0.15 > 0.20 > 0.30 mgKOH/goil depending on the different types of transformers), in fact even with an acidity between 0.07 and 0.10 mgKOH/goil dissolved metal corrosion (C4) has been recorded and dangerous formation of sludge.

Operating mode

Sea Marconi’s extraordinary results are based upon its experience in the industry since 1968, the use of Decontamination Modular Units (DMU), designed and manufactured internally, and the formulation of specific reactants (S/CHED®, etc.) used depending on the operating scenario.

The DMUs are connected to the transformer via flexible hoses, after which the decontamination process is started: the insulating fluid circulates through the DMUs where it is: heated, degassed, dehumidified, and filtered and depolarised (through special patented reactants), to then returned to the transformer, which is never emptied, not even partially.

The continuous circulation of the insulating liquid creates a constant flow of fluid in the transformer that favors the elimination of deposits (sludge) on papers and in the case.

The process can be done safely thanks to

  • low process temperatures,
  • the use of patented reactants that are not explosive or flammable,
  • the use of special piping (SpillGuard®) which can stop the process if there are any accidential leaks but also thanks to
  • a software control system able to check in real time that the process is taking place safely as well as guaranteeing quality, 24 hours a day even without the supervision of an operator.

The treatment is performed with the On-Load option, keeping the transformer in service, under tension and filled, i.e. without any loss of production (with international experience also in nuclear power stations and transformers up to 760 MVA and 500 KV).

Sea Marconi’s closed loop treatments offer the advantage of avoiding difficult operational problems related to replacing fluids, which involves putting the machine out of service and functional risks (e.g. electrical discharges) due to air bubbles and humidity.

Open for more details

The service includes the following activities:

  • Transport before/after of specialized technicians, treatment units (DMU), equipment, reactants and disposable objects from the Sea Marconi office to the sites of the transformers;
  • The Hydraulic and electrical installation and set up of the mobile depolarisation units (DMU)
  • Sampling, analysis and diagnosis (Deosvision®) before treatment (if any);
  • Depolarization treatment against acidity (Chedcos®);
  • Treatment quality control with on site measurement kit SM-TAN;
  • Physical decontamination treatment (degassing and dehydration in a vacuum);
  • Monitoring the treatment parameters;
  • Changing of the oil of the OLTC (if necessary);
  • Level and vent controls;
  • Replacement of desiccant salts in silica gel;
  • Sampling, analysis and diagnosis (Deosvision®) at the end of the process;
  • Oil inhibition with antioxidant additive DBPC (if necessary), should the initial conditions of the oil be such as to arouse suspicion that the oil has lost resistance to oxidation

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  • Removal of soluble and insoluble oxidation compounds (acids) in the oil
  • Removal of any water and physical contaminants from the oil
  • Restoration of the oil’s dielectric properties

and of course all the benefits that come with using Sea Marconi treatments:

Operational safety

due to; using a solid patented reactant, low process temperature (80-100 °C), the lack of emptying the transformer, the use of a modular decontamination unit (MDU) created by Sea Marconi and equipped with an advanced monitoring system, providing 24 hour a day automatic safety

Environmental saftey

because Chedcos® avoids both the replacement, disposal, transfer and handling of enormous amounts of oil that can still be used, and the risks associated with handling contaminated oil

The treatment units' small dimensions

mean they are operational even under extreme logistical constraints (e.g . in bunkers, and mines)

Functional recovery of oil and transformers

by means of a unique closed-loop operating mode

Reference regulation

IEC 60422 Ed. 4 2013

Strong case studies

with 135 total pieces of equipment decontaminated against acidity and approx. 975 ton of total oil decontaminated against acidity (data only from 2017)


The Chedcos® Selective Depolarization against acidity is cheap and effective, because it guarantees a total acidity of < 0.05 mgKOH/goil and a dielectric dissipation factor (Tan delta at 90 ° C) of < 0.1 as well as restoring the other functional properties in accordance with IEC 60422 Ed.4 2013 and CIGRE 378 Art. 4.2.

Supplementary worksheet


Sea Marconi Treatments vs Oil replacement (354.1 KiB)


Treatments by Sea Marconi vs fuller earth treatments (378 KiB)

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