Definitions

"Corrosive sulfur without DBDS - C2" is the criticality of 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 caused by the presence of sulfur compounds (other than DBDS) such as polysulfides and disulfides."Corrosive sulfur without DBDS - C2" occurs at normal operating conditions of the transformer.

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
• 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

[ALT img:Corrosive sulfur without DBDS (C2)]

The "Corrosive sulfur without DBDS - C2" criticality is caused by the presence in oil of some corrosive sulfur compounds such as polysulfides and disulfides.The latter can be used as antioxidant additives in some types of insulating oils and can produce effects equivalent to more well-known DBDS.The sulfur compounds present in corrosive oils react with copper and silver surfaces inside the transformer.The result is the formation of copper sulfide or silver sulfide.

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 of energy and power arcs.

[ALT img:Corrosive sulfur without DBDS (C2)]

However, copper sulfide can also form on 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.This circumstance may also cause partial energy discharges and power arcs (without any specific signs or symptoms) eventually resulting in a catastrophic failure.

Causes of corrosive sulfur without DBDS - C2 criticality | 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)

Surprisingly it has been found that some types of oils (especially with paraffinic base) containing initially non-corrosive sulfur compounds have demonstrated corrosive behaviour during the life cycle of the transformer; this is due to oxidation, or degradation in general, under particular electrical or thermal stress conditions (e.g. dibenzothiophene - DBT).

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)

[ALT img:Corrosive sulfur without DBDS (C2) | case11 IFED ]

Oil fingerprint.The presence of sulfur compounds responsible for the corrosivity of the oil is observed

The main symptom of the "Corrosive sulfur without DBDS - C2" criticality is related to the presence of corrosive sulfur compounds in the oil.In particular, the main diagnostic indicator for this criticality is:

TCS – Total Corrosive Sulfur (IEC 62697-2).

Total corrosive sulfur can be expressed as the sum of all corrosive sulfur compounds or as the 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)
• Oil fingerprint: profile GC-AED/GC-MS (IEC 62697-1)
• Additives:Passivators (BTA, Irgamet 39, Irgament 30); oxidation inhibitors (DBPC, DBP)
• Total sulfur (IP 373)
• Particles (IEC 60970)
• Evaluate T1, T2 and T3 through dissolved gas analysis (DGA – IEC 60599)
• Total acidity TAN (IEC 62021-1 or IEC 62021-2)

DBDS analysis methods are unable to determine the corrosiveness of sulfur compounds responsible for the "Corrosive sulfur without DBDS – C2" 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 sulphide product (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.

Corrosiveness of different families of compounds at different temperatures
[ALT img:Corrosive sulfur without DBDS (C2)]

Rate of conversion into corrosive sulfur of 22 sulfur compounds (calculation based on the TCS test)
[ALT img:Corrosive sulfur without DBDS (C2)]

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

Sea Marconi employs its own diagnostic metric for diagnosing "DBDS corrosive sulfur – C2″ criticality:

• visual signs on the transformer are interpreted, in this case following an inspection after failure of twin machines,
• through analysis of the oil, symptoms are identified, namely the specific indicators (TCS - Total Corrosive Sulfur), and related concentrations

| 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 - following 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 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

The only way to evaluate contamination of paper (see causes above) is by determining the amount of Total Corrosive Sulfur in the oil correlated with the speed of conversion of these compounds into copper sulfide.Obviously, the higher the speed of conversion the higher the risk, and consequently the greater the priority of action to implement the necessary countermeasures.

Click to see a practical example

Step-up transformer in thermoelectric plant
Constant load profile 7500 h/year
Oil type = mineral oil with naphthenic base
Oil mass = 50,000 kg
Age = year 2000
DBDSeq = 200 mg/kg in 2000 DBDSeq = 150 mg/kg in 2005 means 50 mg/kg of DBDSeq were converted into copper sulfide!!!

DBDS = 120 mg/kg in 2006 means that the criticality worsened considerably because the speed increased on an annual basis from 10mg/kg to 30mg/kg

This example enables implementation of the best maintenance strategy: for twin machines it is advisable to take adequate measures starting with the higher conversion speed of DBDS into copper sulfide.

50,000 kg of oil in the transformer and DBDS at 200mg/kg means having 10 kg of DBDS in the transformer oil mass.After 5 years with contamination at 150 mg/kg, this means that 7.5 kg of DBDS remain in the transformer, and consequently 2.5 kg of DBDS have reacted with the copper components inside the transformer, forming up to around 1.9 kg of copper sulfide.These are not uniformly distributed, but accumulate in the hottest areas of the transformer.
If there is a hot spot (e.g. T2), it is clear that in that area the speed of formation of copper sulfide is higher (Arrhenius equation), which determines a weak point from the point of view of electrical insulation and thus that with the highest probability of evolving (in less time) into an electrical failure with the power arc.

N.B.In the presence of oils with added passivators (e.g.Irgamet 39) their degradation rate must be evaluated in correlation with the rate of degradation of corrosive sulfur compounds.

Irgamet is typically added to oil with concentrations of 100 mg/kg but it has been seen that after about one year its concentration has decreased to 90%.

Prevention

1. It is recommended that strategic information be updated with a "dynamic inventory" of oils and transformers contaminated by corrosive sulfur compounds.
2. It is recommended that maintenance practices be changed:
   – purchasing new oils free from corrosive sulfur compounds and checking the oils in the acceptance phase of supplies
   – asking that factory tests be carried out using oils free of corrosive sulfur and monitoring that oil and machine treatments are not a source of cross-contamination, both in this phase of the life cycle and in subsequent phases

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:Corrosive sulfur without DBDS (C2) | MDU module 3 ]

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 with 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

1. Oil sampling must be carried out by qualified professionals according to procedures
2. laboratory analyses must be carried out using methods set by reference standards and procedures, as guaranteed by accredited laboratories
3. countermeasures for "Corrosive sulfur without DBDS – C2" criticality, namely depolarisation treatments, must be performed
   - using technologies that are safe and fit for purpose,
   - using staff with specific skill and training,
   - relying on operators able to demonstrate a wide application case history.