Definitions

 

"Corrosive sulfur from sulfur combustion by-products without DBDS – C3″ is the criticality characterised by the corrosive property of oil, and other insulating liquids, with respect to the metallic surfaces of which some components (e.g. copper conductors or silver contacts) inside transformers and other electrical equipment are made.This criticality is specifically caused by sulfur combustion by-products such as H2S (hydrogen sulfide), mercaptans, elemental sulfur, and so on, originated by oil regeneration treatments that use thermal re-activation of fuller's earth (or other particulate matter) by combustion.

Corrosive sulfur – Free sulfur or compounds of corrosive sulfur identified when subjecting metals, such as copper, to contact with an insulating liquid under regulated conditions [Sea Marconi's interpretation of technical standard IEC 62697-1 of 2012, para.3.1.6 – page 10]

 

 

Introduction

 

The introduction of technical standard IEC 62697-1 of 2012 (page 7) states that:

• Sulfur can be found in insulating liquids (used in transformers and other electrical equipment) in various forms
• Total sulfur concentration depends on origin of the liquid, refining process, formulation and the presence of additives
• There are non-corrosive sulfur compounds and others that are extremely corrosive for metal surfaces, such as those inside transformers
• The presence of these corrosive sulfur species has been directly linked to failures involving equipment used in the generation, transmission and distribution of energy for several decades.
• For this reason, the IEC standard has ruled that both new insulating oils and in-service oils must be free of these corrosive sulfur compounds

 

Along with the discovery of DBDS as the principle cause of the corrosive sulfur phenomenon (July 2005), Sea Marconi has also studied the corrosive action of both sulfur compounds that are normally found in oil and of products of the degradation of additives.

 

 

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

 

M. Pompili, F. Scatiggio, V. Tumiatti, R. Maina, M. Tumiatti, Corrosive sulfur in insulating oils: its detection and correlated power apparatus failures, IEEE Trans.On Power Delivery, Vol.23, No.1, 2008

V. Tumiatti, R. Maina, F. Scatiggio, M. Pompili and R. Bartnikas, In Service Reduction of Corrosive Sulfur Compounds in Insulating Mineral Oils, ISEI 2008, Toronto, June 2008

F. Scatiggio, V. Tumiatti, R. Maina, M. Tumiatti, M. Pompili and R. Bartnikas, Corrosive Sulfur Induced Failures In Oil-Filled Electrical Power Transformers And Shunt Reactors, IEEE Trans.On Power Delivery, Vol.24, No.3, 2009

R. Maina, V. Tumiatti, M. Pompili and R. Bartnikas, Corrosive Sulfur Effects in Transformer Oils and Remedial Procedures, IEEE Trans.On Dielectrics and Electrical Insulation, Vol.16, No.6, 2009

R. 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

F. Scatiggio, R. Maina, V. Tumiatti, M. Pompili and R. Bartnikas, Long Term Stability of Insulating Mineral Oils Following their Corrosive Sulfur Removal, ISEI 2010, San Diego, June 2010

R. Maina, V. Tumiatti, F. Scatiggio, M. Pompili and R. Bartnikas, Transformers Surveillance Following Corrosive Sulfur Remedial Procedures, IEEE Trans.On Power Delivery, Vol.PP, Issue 99, 2011

M.C.Bruzzoniti, C. Sarzanini, R.M.De Carlo, R. Maina, V. Tumiatti, Guasti in trasformatori di potenza impregnati in olio minerale isolante e potenziali danni ambientali.Indagine su fenomeni di corrosione correlati a contaminazione da sostanze corrosive, Proc.XII National Congress of the Division of Environmental Chemistry and Cultural Heritage, Taormina (IT), September 2010, http://www.socchimdabc.it/joomla/documenti/atti_XII_congr.pdf

R. 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

R. 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.Dott. Riccardo Maina 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

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

 

 

Regulatory framework

– IEC 60296:2012, Fluids for electrotechnical applications – Unused mineral insulating oils for transformers and switchgear
– IEC 60422:2013, Mineral insulating oils in electrical equipment – Supervision and maintenance guidance
– IEC 62697-1:2012, Test method for quantitative determination of dibenzyldisulfide (DBDS)
– CIGRE Brochure 413:2010, Insulating Oil Regeneration and Dehalogenation

 

 

Causes

 

[ALT img: case 14 treatment of earth]
[ALT img: case 14 silver contact]
[ALT img: case 14 arc signs]

The criticality of "Corrosive sulfur from sulfur combustion by-products" is caused by oil regeneration treatments that involve the re-activation of fuller's earth (and other particulate adsorbents) by a combustion process.This uncontrolled thermo-oxidation process (> 370°C) degrades the sulfur present in the oil, producing three distinct criticalities:

A. contamination of regenerated oil by the formation of highly corrosive by-products (H2S-hydrogen sulphide, mercaptans, elemental sulfur, etc.)
B. corrosion of copper and silver parts with copper sulfide and silver sulfide formation inside the transformer impregnated with regenerated oil (e.g. contacts of the on-load tap-changer)
C. emissions of CO2 and contaminants such as H2S and PCDD-Dioxins and PCDF-Furans into the environment in the event of contamination by PCB-Polychlorinated biphenyls and other chlorinated and persistent organic pollutants (POPs)

 

 

Cause of criticality "Corrosive sulfur from sulfur combustion by-products - C3" | When it can occur (life cycle phases)

Lack of requisites for purchase of oils (new or recycled) | Requisites and purchase

Defciencies in quality control for individual lots or individual supplies of insulating oil | Acceptance of insulating oils

Deficiencies in analytical procedures for the verification of corrosive sulfur compounds | Oil acceptance, factory test, installation and pre-energisation, operation, old age, post-mortem

Cross-contamination through use of oil, plants, tanks or containers contaminated by corrosive sulfur compounds (for toppings up, impregnations, fillings or treatments) | Factory test, installation and pre-energisation, operation, old age, post-mortem (oil recycling)

 

 

To understand the damageing effects of oil regeneration treatments that reactivate fuller's earth by combustion, it is necessary to know more about how reactivation takes place.

 

 

Further reading

 

The purpose of oil "regeneration" treatments is to restore the physical-chemical properties of the oil itself (e.g. acidity, dielectric dissipation factor)
Oil regeneration treatments are performed using different techniques and system solutions.Some of these "regeneration" operations make the oil flow through columns containing fuller's earth (or other adsorbent particulate matter).The oil passes through the earth at a temperature of 60-80°C
Fuller's earth is unable to decontaminate PCB, DBDS or other corrosive sulfur compounds
When the fuller's earth becomes saturated, it can be replaced (with production of waste to be disposed of) or reactivated by combustion.The C3 criticality is generated precisely by this phase.
To reactivate the earth, the oil flow in the column is interrupted and oil drainage is continued

N.B.A significant aliquot of oil after drainage remains impregnated in fuller's earth cavities.

The next step is combustion.In detail, the following occurs in this phase:

A. heating of one end of the column up to the ignition temperature (about 350-400°C);
B. injection, at the opposite end of the column, of the combustive agent (oxygen of air) under pressure;
C. the real combustion of oil impregnated in fuller's earth until complete exhaustion of the fuel (oil).

During combustion, the flame front (700-800°C) moves progressively from the trigger point towards the opposite side of the column.At the end of combustion, input of the combustive agent is stopped and the column and the particulate media inside are cooled

The duration of reactivation is about 12-18 hours

 

 

Real example
A column with a volume of 200 litres can contain about 150 kg of fuller's earth (dry); fuller's earth can retain oil up to 50% of its weight.Consequently, in spite of the drainage of oil, 75 kg of oil still remain trapped in the fuller's earth to be reactivated.Assuming a total sulfur concentration of 10,000 mg/kg, this means that there are 750,000 mg of sulfur (that is, 750 g) in the oil aliquot!

In conclusion, in order to reactivate the fuller's earth, 75 kg of oil will in fact be burned with 750 g of sulfur, generating highly corrosive by-products in the transformer oil mass and generating dangerous emissions into the environment.

 

 

Failure mechanisms

Contamination of corrosive sulfur compounds in regenerated oil creates an uncontrolled phenomenon of cross-contamination on the transformer pool with high probability of failure due to the formation of copper sulfide and silver sulfide (e.g. on-load tap changers or switches) .Copper sulfide increases as the temperature rises, reaching its peak in the presence of localised hot spots.The result is the formation of deposits and macroparticles that can circulate dangerously in the oil causing partial discharges and power arcs.
However, copper sulfide can also be formed from windings which are also made of copper.In this case there is a progressive migration of copper sulfide from the conductors on the windings to the layers of paper around them.Copper sulfide crystals push against the layers of paper, gradually arriving at the outermost layer until causing the loss of its insulating properties.Partial discharges and power arcs can also be generated in this case up to catastrophic failure.

 

Corrosion can increase if the starting oil contains significant concentrations of chlorinated organic compounds (e.g.PCB, trichlorobenzene) which, when subjected to thermal degradation, tend to form highly toxic by-products (PCDD-dioxins, PCDF-furans) as well as other compounds with free chlorine or hydrochloric acid (HCl).

 

 

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.Where the criticality is present, grey deposits are typically observed on copper conductors (copper sulfide) or silver contacts (silver sulfide).On the other hand, copper sulfide contamination shows up on insulating papers as grey spots and streaks.

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 norm and the operating instructions attached to the sampling kits.

 

Symptoms (analysis)

The main symptom of the "Corrosion by sulfur combustion by-products – C3" criticality is related to the presence of corrosive sulfur compounds as by-products of the combustion of fuller's earth (see causes).

The main diagnostic indicator for this criticality is
TCS - Total Corrosive Sulfur (IEC 62697-2)
Total corrosive sulfur can be expressed as sum of all corrosive sulfur compounds or as a concentration of DBDS equivalent.If the TCS concentration, expressed as DBDS equivalent, exceeds the recommended values ​​(see table below in the section on diagnosis), the required treatments must be performed.

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

• Potentially corrosive sulfur – CCD Test (IEC 62535)
• Corrosive sulfur (IEC 62535, ASTM D1275 Method B, DIN 51353)
• Additives:Passivators (BTA, Irgamet 39, Irgament 30); oxidation inhibitors (DBPC, DBP)
• Elemental sulfur

DBDS analysis methods are unable to determine the corrosiveness of sulfur compounds responsible for the "Corrosive sulfur from sulfur combustion by-products – C3" criticality.
To determine total corrosive sulfur, in particular that not due to DBDS, Sea Marconi has invented, developed, industrialised (and patented No. 0001394617 of 2008) the method called TCS – Total Corrosive Sulphur.This analytical technique is independent of individual corrosive compounds, but evaluates the effects equivalent to DBDS in terms of the amount of copper sulfide produced (under the same test conditions).
This method will be included in the IEC 62697 standard, Part 2 "Test Methods for Quantitative Determination of Total Corrosive Sulphur (TCS)" currently in the Committee Draft for Voting (CDV) phase.The round robin tests performed were excellent and formed the basis for the IEC working group.

Development of this method has experimentally shown that the conversion of the different sulfur compounds into total corrosive sulfur (TCS) occurs differently depending on the temperature and the molecular characteristic of the compounds themselves.

Further support for understanding and diagnosing C3 criticality will be provided by the results of the working group on IEC 62697 "Part 3 - Test Methods for Quantitative Determination of Total Mercaptans and Disulfides (TMDs) and Other Targeted Corrosive Sulfur Species", currently in the Draft Committee (DC) phase.For the time being, a round robin test has been completed on samples of regenerated oil, finding sulfur concentrations of over 100 mg/kg.The samples used are related to regenerated oils through reactivation of fuller's earth and combustion.

 

[Corrosive sulfur from sulfur combustion by-products (C3)]
Corrosion of different compounds families at different temperatures
[Corrosive sulfur from sulfur combustion by-products (C3)]
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 "Corrosive sulfur from sulfur combustion by-products – C3″ criticality, Sea Marconi uses its own diagnostic metrics, namely:

• visual signs on the transformer are interpreted, in this case following an inspection after failure of twin machines;
• by analysing the oil, symptoms are identified, that is, specific indicators (Total Corrosive Sulfur);

 

| Recommended DBDS value | Reference standard

For new insulating oils | “non detectable (< 5 mg/kg)” | [IEC 60296 Ed.4-2012, table 2, page 17]

for insulating oils in operation – before energisation | “non detectable (< 5 mg/kg)” | [IEC 60422 Ed.4-2013, table 3, page 24]

for insulating oils in operation – after energisation | (< 5 mg/kg)” If the concentration of DBDS is greater than the recommended threshold, a risk assessment must be carried out and mitigating actions applied; table 5, note d – these include a selective depolarisation treatment to effectively remove corrosive sulfur from the oil 11.4.4.| [IEC 60422 Ed.4-2013, table 5, page 31]

for insulating oils in operation | (< 10 mg/kg)” – in this case selective depolarisation to remove
effectively from oils is also one of the mitigation techniques 4.2 page 25 | [CIGRE 378 Fig. 9 page 31]

 

• the database is used to study family or subjective case histories (in the search, for example, for failures in twin machines);
• factors of uncertainty, speed and evolution over time (trend) for each symptomatic indicator are examined and monitored

 

 

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)

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:Corrosive sulfur from sulfur combustion by-products (C3) | case 14 arc signs]

Removal of corrosive sulfur due to sulfur combustion by-products (C3) – Uruguay 2010
corrosive sulfur from sulfur combustion (C3)