Definitions

"Corrosion through dissolution of metals – C4" is the criticality of the corrosive property of the oil that causes dissolution of metals (e.g. copper) inside transformers and other electrical equipment under normal operating conditions.This criticality gradually degrades the dielectric and chemical properties of materials and insulators (oil and paper) and is unrelated to corrosive sulfur (C1, C2, C3).

Corrosion
Disintegration of a metal due to chemical reactions with sulfur and other chemical species in insulating liquids
[Sea Marconi translation of technical standard IEC 62697-1 of 2012, para.3.1.5 – page 10]

 

 

Introduction

The Corrosion through dissolution of metals – C4 criticality was discovered by Sea Marconi through analysis of an extensive body of over 40 years of case histories in its database.These studies allowed us to experimentally correlate the concentration of dissolved metals (e.g. copper) with certain types of insulating fluids used in various electrical appliance families.High concentrations of copper dissolved in oil (up to 500 mg/kg) and degradation of the dielectric properties of the insulating oil (dielectric dissipation factor – DDF up to over 2) were observed.There was also a significant phenomenon of deposition of organo-metallic species on insulating papers which does not depend on the phenomenon of corrosive sulfur.

 

 

At the origins of criticality

Analysis of dissolved metals in insulating liquids was pioneered by Sea Marconi in 1984.Correlated with analysis of particles in oil, this investigation - which Sea Marconi was the first in the world to systematically perform in 1976 - allowed us to diagnose precisely specific criticalities of oils and transformers from the mid-1980s.

[ALT img:Corrosion through dissolution of metals (C4)]

In 1995 Sea Marconi had the opportunity to study the phenomenon of metal dissolution in even greater depth.On that occasion, Sea Marconi was called on by a transformer construction company in South America to investigate the many cases of catastrophic failure.It concerned shunt reactors on a 500 KV network, all between 18 and 24 months.During the analytical investigation, Sea Marconi analysed the factors responsible for the failures but without establishing with certainty the cause of the catastrophic events.That expertise, however, surprisingly showed the trend of some types of copper-based oils in specific transformer families.This study was communicated to Cigré TF 15.01.05 and further detailed at the presentation to Cigre in Paris in August 2000.

The chart above shows an example of statistical analysis with correlation between year of construction and copper concentration for mineral insulating oils on distribution transformers

 

 

Click here to access Sea Marconi's major publications on the topic:

Maina, V. Tumiatti, M. Pompili and R. Bartnikas, Dielectric Loss Characteristics of Copper Contaminated Transformer Oils, IEEE Trans.On Power Delivery, Vol.25, No.3, 2010

Maina, V. Tumiatti, M.C.Bruzzoniti, R.M.De Carlo, J. Lukić, D. Naumović-Vuković, Copper Dissolution and Deposition Tendency of Insulating Mineral Oils Related to Dielectric Properties of Liquid and Solid Insulation, ICDL 2011, Trondheim, June 26-30 2011

M.C.Bruzzoniti, R.M.De Carlo, C. Sarzanini, R. Maina, V. Tumiatti, Determination of copper in liquid and solid insulation for large electrical equipment by ICP-OES.Application to copper contamination assessment in power transformers, Talanta, Vol. 99, 2012, 703-711

M. De Carlo, M.C.Bruzzoniti; C. Sarzanini, R. Maina; V. Tumiatti, Copper Contaminated Insulating Oils-Testing and Investigations, IEEE Trans.On Dielectrics and Electrical Chim.Dr. Riccardo Maina, Via Tiraboschi, 25 10149 Turin (TO) Insulation, Vol. 20, No.2, 2013, 557-563

R.M. De Carlo, C. Sarzanini, M.C.Bruzzoniti; R. Maina; V. Tumiatti; Copper-in-oil Dissolution and Copper-on-Paper Deposition Behavior of Mineral Insulating Oils, IEEE Trans.On Dielectrics and Electrical Insulation, Vol. 21, No.2, 2014, 666-673

 

 

Regulatory framework

• IEC 60422:2013, Mineral insulating oils in electrical equipment – Supervision and maintenance guidance
• ASTM D 7151, Standard Test Method for Determination of Elements in Insulated Oils by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES)
• IEC 60422:2013, Mineral insulating oils in electrical equipment – Supervision and maintenance guidance
• CIGRE Brochure 378:2009, Copper sulphide in transformer insulation
• CIGRE Brochure 413:2010, Insulating Oil Regeneration and Dehalogenation
• CIGRE Brochure 625:2015, Copper Sulphide long term mitigation and risk assessment

 

 

Causes

The Corrosion through dissolution of metals – C4 criticality is caused by the corrosive action of oil on metals present in transformers (e.g. copper).Such corrosion results in the dissolution of metals within the oil under normal operating conditions.

The type and speed of metal corrosion depend on the temperature, oxygen concentration and formulation of oil in terms aromatic compounds, polynuclear aromatic compounds (PNA), etheroatomic compounds (e.g. oxygen, nitrogen), antioxidant additives (e.g.DBPC), passive additives (e.g.Irgamet 39, Irgamet 30).

 

 

Causes in relation to life cycle phases

The effect of the electric field inside the transformer amplifies the ionic mobility of organometallic compounds dissolved in the insulating liquids by degrading the dielectric properties of the insulating system (oil and paper).These phenomena can become particularly critical for certain types of transformers, such as rectifier, converter (HVDC) and special (e.g. electric traction of trains) transformers.

Dissolution causes the deposition of metals (e.g. copper) on insulating papers and the formation of insoluble deposits (sludge) inside the casing; the result is the progressive degradation of the dielectric and chemical properties of both oil and paper.

The presence of dissolved copper acts as a catalyst in the oxidation processes of the oil
– accelerating the chemical degradation of oil,
– reducing the oxidation stability properties of oil,
– accelerating the ageing of papers,
- and accelerating the formation of sludge.

On some types of high-grade aromatic oils, concentrations of copper dissolved in oil up to 500 mg/kg (in comparison with a typical value < of 0.80 mg/kg) and a dielectric dissipation factor DDF (tan delta) greater than 2 (compared with a typical value < of 0.10) were observed.

Copper contamination with concentrations up to 2700 mg/kg (compared with a typical value of 50 mg/kg) was observed on the papers.

Sea Marconi has experimentally studied these phenomena on various types of insulating liquids.In some typologies a correlation was found between the dielectric dissipation factor (tg delta) and the concentration of copper dissolved in the insulating oil.In other types of oil, due to the copper deposition tendency test developed by Sea Marconi, there was a relationship between the formulation of the oil with particular additives and the tendency to deposit organo-metallic compounds containing copper on the papers.

 

 

Signs (visual inspection) – Symptoms (analysis)

Signs (visual inspection)

Signs of the criticality only become apparent through an internal inspection of the transformer, following a failure for example.In the presence of this criticality, grey/brown insoluble deposits (sludge) are found on the bottom of the casing, papers, windings, and oil circulation and winding cooling channels.Another sign may be the progressive increase in oil temperature with respect to the environment at same load (symptom of a reduction in thermal exchange).

 

Representative sampling

Should it be decided to carry out an internal inspection of the transformer, following a failure or in order to carry out a thorough inspection, it is strongly recommended to take samples of the insulating papers in accordance with relevant protocols and procedures.In particular, it is advisable to select the paper at the top, bottom, and middle of the both primary and secondary windings for each phase, taking multiple paper samples in areas with greatest darkening or embrittlement of the papers themselves.

During external inspection of the transformer, it is necessary to take samples of insulating oil in accordance with the reference standard and the operating instructions attached to the sampling kits.

 

Symptoms (analysis)

The main symptom of "C4" criticality is linked to the presence of:
metals dissolved in insulating liquids (e.g. copper) (ASTM D7151)

There are other co-factors useful for completing the diagnostic picture:

• Appearance (ISO 2049)
• Colour (ISO 2049)
• Particles (IEC 60970)
• Dielectric dissipation factor – tan delta (IEC 60247)
• CDT test (Copper Deposition Tendency Test) (internal method)
• Total acidity – TAN (IEC 62021-1 or IEC 62021-2)
• Additives:Passivators (BTA, Irgamet 39, Irgament 30); oxidation inhibitors (DBPC, DBP)
• Oil fingerprint (internal method)

 

[ALT img:Corrosion through dissolution of metals (C4)]
Corrosion of different compound families at different temperatures
[ALT img:Corrosion through dissolution of metals (C4)]
Corrosive sulfur conversion rate of 22 sulfur compounds (calculation based on the TCS test)

 

M.C.Bruzzoniti, R.M.De Carlo, C. Sarzanini, R. Maina, V. Tumiatti, Stability and Reactivity of Sulfur Compounds against Copper in Insulating Mineral Oil:Definition of a Corrosiveness Ranking, Ind.Eng.Chem.Res., 2014, DOI: dx.doi.org/10.1021/ie4032814

 

 

Diagnosis

For diagnosis of the "Corrosion through dissolution of metals – C4" criticality, Sea Marconi uses its own diagnostic metrics, namely:

• the visual signs of external observation of the transformer are interpreted, and those from any internal inspection after failure on twin machines;
• by analysing the oil, the symptoms, that is, the specific indicators (e.g. dissolved copper), are identified;
  For example, for a new transformer to be energised or after energisation, the typical concentration of copper dissolved in oil is "not detectable", that is, < 0.1 mg/kg.For a transformer in operation instead, the reference value must be calculated on a statistical basis of the reference population at the 90th percentile (e.g.0.80 mg/kg)
• the database is used to study family or subjective case histories in the search for failures in similar machines (same oil, same manufacturer, same type of equipment, same operating profile, similar age),
• factors of uncertainty, speed and evolution over time (trend) for each symptomatic indicator are examined and monitored
• on the basis of assessment of these key factors, specific criticalities are classified according to type and priority, and type and priority of corrective actions (treatments) are identified at the same time

 

 

Prevention

• It is recommended that oil treatments be avoided with processes that reactivate fuller's earth through combustion and in any case
• It is always advisable to avoid "burnt oil" contaminating the transformer oil mass.

 

Treatments

The actions recommended by IEC 60422 Ed.4-2013

In the presence of "corrosive sulfur" are:

• perform a risk assessment
• and then alternatively choose to:
• A. reduce the corrosiveness of the oil by adding a copper passivator or
  [NOTE – After passivation of the oil, a regular check of the concentration of the passivator is required.In the event of continuous depletion of the passivator, remove the cause of the corrosiveness as below]
• B. remove the source of corrosiveness by changing the oil or
• C. remove the source of corrosiveness by removing the corrosive compounds through appropriate oil treatments.

 

 

A. Passivation

Passivation consists of adding a substance to the oil that should protect the copper inside the transformer from the corrosive action of DBDS.Analyses carried out on passivated oils in machinery have shown a decrease in passivator content in the first few days after it is added.In other cases, instead, the protective action of the passivator in relation to copper has been shown to be uneven, thus allowing the formation of copper sulfide in some areas.

The case of the Brazilian electricity grid in August 2005, reported in the CIGRE 378:2009 brochure, shows that 50% of passivated reactors suffered a failure - the first 33 days after passivation and the last 590 days after passivation.(read more)

 

 

B. Oil change

Despite changing the oil, 10-15% of the old contaminated oil load remains absorbed in the transformer papers, which release it over time (the time it takes to integrate is about 90 days).The old oil thus contaminates the new oil, and consequently it is impossible to completely remove the DBDS with a single oil change.(read more)

 

 

C. Removal of corrosive compounds, depolarisation (read more)

[ALT img:Corrosion through dissolution of metals (C4)]

The countermeasure devised and employed by Sea Marconi is included in this category.This is a selective DBDS depolarisation process implemented on site while the transformer remains in service (and under load), with no need to empty it.The operation is carried out using a Modular Decontamination Unit (MDU) specifically created by Sea Marconi.The transformer is connected to the MDU by flexible tubes; the oil contaminated with DBDS is sucked from the lower part of the transformer and ends up in the MDU, which heats it, filters it, degasses it, dehumidifies it and decontaminates it before pumping it back into the upper part of the transformer.This creates a closed loop and every time the oil is circulated the corrosive sulfur compounds are removed (< 10 mg/kg expressed as DBDS equivalent)

 

 

Warnings

A qualified operator must be able to propose various solutions for the treatment of oils, highlighting the advantages and disadvantages of each intervention.In this case it is advisable to check that the operator/supplier is full aware of the dangers inherent in oil treatment processes with reactivation of fuller's earth by combustion.

 

 

Successful cases that we have worked on

[ALT img:Corrosion through dissolution of metals (C4)]

Corrosive sulfur in insulating oils, recent failures and possible corrosive corrosive sulfur by DBDS (C1)/corrosive sulfur through metal dissolution (C4)