COPPER SULPHIDE LONG TERM MITIGATION AND RISK ASSESSMENT. Working Group A2.40 TUTORIAL

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1 COPPER SULPHIDE LONG TERM MITIGATION AND RISK ASSESSMENT Working Group A2.40 TUTORIAL

2 WG members Members J.Lukic, convenor&tf 01 leader (RS), G.Wilson, TF 02 Leader (UK), F.Scatiggio, TF03 Leader (IT), M.Dahlund, (SE), R.Maina (IT), J.Rasco (USA), L.E.F. de Lemos (BR), A.Peixoto (PT), T.Buchacz (PL), C. Perrier (FR), I.A. Höhlein (DE), A.Skholnik (IL), P.Wiklund (SE), B.Nemeth (HU), H.Ding (UK), A.Lombard, (ZA), Y.Bertrand (FR), J.Van Peteghem (BE), P.Smith (DE), S.Dorieux (FR), M.Facciotti (UK). Corresponding members G.Krikke (NL), M.Grisaru (IL), L.Lewand (USA). Former members L. Eiselstein (USA), S.Laboncz, (HU), J.Tanimura (JP), A.Yamada (JP), J.Yare (UK), T. Amimoto (JP), T.C.S.M Gupta, former TF 01 leader (IN), W.Mc Dermid, (CA). Contributors H.F.A. Verhaart (NL), A.Petersen (AUS), C.N.Fares (UY), M.A.Martins (PT). Publication 2015

3 Outline Introduction Risk Assessment Copper Sulphide Long Term Mitigation Monitoring and Maintenance Procedures Conclusions

4 Copper Sulphide Long-term Mitigation and Risk Assessment Introduction WG A2.40 was set up in May 2009 as continuation of the work of A2.32.: "Copper Sulphide in Transformer Insulation" (TB 378) based on the main topics highlighted in the conclusions and proposed activities for the future work. The scope of A2.40 was to improve understanding of the mechanism of copper sulphide formation and mapping of influential factors in order to provide more precise risk assessment. It was also required that long-term effects of mitigation techniques, like addition of metal passivators and oil treatment processes, should be examined and an evaluation of their efficiency and any side effects be investigated. This report reviews the current best understanding of the mechanism of copper sulphide formation, details of service experiences, reported failures and problems due to copper sulphide and the results of mitigation techniques.

5 Outline Introduction Copper Sulphide Formation Mechanism Risk Assessment Copper Sulphide Long Term Mitigation Monitoring and Maintenance Procedures Conclusions

6 Copper Sulphide Formation Mechanism Deposition of copper sulphide on copper or in the paper is dependant on: Temperature Base oil composition / degree of oil refining Oxygen content Inhibitors in the oil Paper from conductor of failed converter transformer - copper clean, paper contaminated with Copper sulphide Conductors from failed converter transformer: left - inner paper layers, right outer paper layers Copper and paper contaminated with copper sulphide

7 Copper Sulphide Formation Mechanism Mechanism: copper in oil dissolution - absorption in the paper reaction of copper and sulphur and deposition of copper sulphide in the paper Copper sulphide deposition on the copper and transfer to adjacent paper layer Scheme of copper sulphide formation in the paper Legend: R-S-S-R: disulphide (for example dibenzyl disylphide-dbds) Cu: Copper T: Temperature AC/DC: Electrical fields Cudiss.: Dissolved copper compounds (Cu + (O 2 R) - ) ROOH: hydroperoxides R OH: alcohols, phenols and derivates (DBPC, DBPh) CuxS: Copper sulphide

8 Copper Sulphide Formation Mechanism Influence of Temperature Formation of copper sulphide at temperatures from 80 C to above 300 C, from different reactive sulphur compounds. Arrhenius Law for temperature ranges from C up to 200 C the rate of copper sulphide formation approximately doubles with every 10 C increase A first order reaction was suggested for temperatures up to 150 C with estimated activation energy of 123 KJ/mol ; kinetics of DBDS depletion is more complex - rate falls off less rapidly than expected from true first order

9 Copper Sulphide Formation Mechanism Influence of Oxygen and Oxydation proces At temperatures up to 120 C rate of copper sulphide formation higher in sealed conditions comparing to the rate of formation in breathing conditions At temperatures above 150 C Rate of copper sulphide formation in the paper higher in breathing conditions Oxidation process can promote oil corrosiveness, some sulphur species may become more reactive when oxidized (less refined uninhibited oils)

10 Copper Sulphide Formation Mechanism Influence of Base oil Composition and inhibitors Copper content in the paper, ppm ⁰C/150⁰C breathing naphtenic oil A, uninhbited naphtenic oil A, uninhibited with added DBPC naphtenic oil B, inhibited paraffinic oil, inhibited Copper content in the paper, ppm ⁰C/150⁰C non-breathing naphtenic oil A, uninhbited naphtenic oil A, uninhibited with added DBPC naphtenic oil B, inhibited paraffinic oil, inhibited C - 3 days 120 C - 7 days 150 C - 3 days 150 C - 7 days C - 3 days 120 C - 7 days 150 C - 3 days 150 C - 7 days Naphthenic oils have a slightly higher propensity for copper sulphide deposition on the paper Copper content in the paper, mg/kg Uninhibited corrosive oils with 0.3% added DBPC, DBPh and variable oxygen content D D + O2 MO MO + O2 Oils Addition of inhibitors DBPC and DBPh to uninhibited oils comprised to significant copper sulphide deposition on the paper, which was pronounced in oxygen environment initial Oil DBF DBPC

11 Copper Sulphide Formation Mechanism Reactivity of sulphur compounds Reactivity of different classes of sulphur compounds in the temperature range from 80 C to 180 C IEC test in white oil 80 C 100 C 120 C 150 C 180 C Mercaptans (thiols) Monosulphides Disulphides Oxidized sulphur compounds: sulphoxides/sulphones Oxidized hydrocarbons - carbonyl compounds containing sulphur Thiophenes +/- + Reactivity of corrosive sulphur compounds for copper sulphide deposition on the paper according to IEC is as follows: ELEMENTAL SULPHUR > DISULPHIDES > MERCAPTANS AND OXIDIZED SULPHUR COMPOUNDS > MONOSULPHIDES AND TIOPHENES

12 Copper Sulphide Formation Mechanism Copper in Oil Dissolution Copper content (mg/kg) in oil 1,4 1,2 1 0,8 0,6 0,4 0,2 Oil # 1 Oil # 2 Oil # 3 Oil # 4 Oil # 5 Oil # 6 Oil # 7 Oil # 8 Copper content (mg/kg) in paper Oil # 1 Oil # 2 Oil # 3 Oil # 4 Oil # 5 Oil # 6 Oil # 7 Oil # 8 Copper content (mg/kg) in oil 4,5 4 3,5 3 2,5 2 1,5 1 Inert atmosphere (Argon) Oxidative atmosphere (Oxygen) Copper content (mg/kg) in paper 700,00 600,00 500,00 400,00 300,00 200,00 Inert atmosphere (Argon) Oxidative atmosphere (Oxygen) ,5 100,00-0, Temperature, C Figure 9 Copper contents in the oil and paper at different temperatures (oils from Table 2). Temperature, C 0 Oil # 1 (inhibited) Oil # 5 (uninhibited) Oil type 0,00 Oil # 1 (inhibited) Oil # 5 (uninhibited) Oil type Figure 10 Effect of atmosphere on copper content in the oil left and copper content in the paper right at 100 C. Aromatic compounds and oxygen promote copper in oil dissolution and absorption of copper in the paper

13 Copper Sulphide Formation Mechanism Copper in Oil Dissolution Change of paper surface resistivity after ageing of paper/oil with 1 l/h oxygen flow; left inner paper layer, right-outer paper layer Copper compounds absorbed in the paper after ageing with non-corrosive oils were found not to have conducting potential

14 Outline Introduction Copper Sulphide Formation Mechanism Risk Assessment Copper Sulphide Long Term Mitigation Monitoring and Maintenance Procedures Conclusions

15 Risk Assessment Service experiences: Transmission transformers Free-breathing, 1000MVA, 400/275 kv transmission auto-transformer, in service for 11 years HV winding turn-to-turn failure hotspot was most likely above 100 C for long periods, transformer design was found to have contributed to the failure as the placement of an oil guiding washing reduced oil flow to the top disc especially under ON conditions. copper sulphide deposition precipitated the failure

16 Risk Assessment Service experiences: GSU s inter-turn fault was found in the HV winding Normal working temperatures The transformer failed suddenly in 2010 after 15 years with corrosive oil (containing DBDS) During inspection paper was found in good condition, DP > ) A 192 MVA, 400/15 kv ONAF transformer failure after 8 years in service Suffered from overheating due to problems in design (cooling oil ducts) inter-turn fault in HV winding with copper sulphide deposition

17 Risk Assessment Service experiences: Industrial applications 1) 63MVA, 230 kv industrial transformer evidence of flashovers on the LV winding and extensive evidence of copper sulphide deposition frequent load changes but the average oil temperature was not believed to have exceeded 60 C. 2) 44 kv, 33 MVA industrial rectifier a turn-to-turn failure in the HV winding only nine months after an oil change which was carried out to revitalize insulation system. The transformer was already aged and suffering from overheating problems but the copper sulphide deposition precipitated failure

18 Risk Assessment Service experiences: Industrial applications 3) 93.4 MVA, 66/0.4 kv, cooling ODAF, windings CTC Oil uninhibited, containing DBDS = 159ppm Failure in 2010 without prior warning from DGA Copper sulphide deposits were found on enameled conductors, on the copper and on the paper Flakes on enameled conductors, formation of bubbles

19 Risk Assessment: Failure Cases Statistics Shunt Reactor 11% Transformer Application Industrial 3% Distribution 7% Transmission 25% GSU 36% Rectifier 18% Transformer Failure Type Mechanical Gassing 4% 11% Dielectric 25% Inter turn failure 39% Tap changer fault 21%

20 Risk Assessment: Failure Cases Statistics corrosion + Sulphur Through fault corrosion + 4% Overheating fault 11% Sulphur corrosion + Normal solid insulation ageing 14% Sulphur Transformer Failure Cause Sulphur corrosion + Operation + Maintenance 11% Silver sulphide on OLTC contacts 14% Sulphur corrosion + Abnormal solid insulation ageing 46% DBDS not measured 51% Transformer Failure and DBDS Content DBDS<20 ppm or none 21% DBDS ppm 7% DBDS>100 ppm 21%

21 Risk Assessment:Failure Cases Statistics Transformer Failure and Oil Type Inhibited oil Uninhibited oil Unknown oil Unknown oil 7% Inhibited oil 36% Uninhibited oil 57% Transformer failure and preservation system Unknown 7% Sealed 21% Free breathing 72%

22 Risk Assessment:Failure Cases Statistics Variable 32% Transfromer failure and load profile Unknown 14% Constant high 54% Transformer Failure Discovery Unknown 3% Failed in service by several protections 61% Removed from service by gassing or asset health review 36%

23 Risk Assessment Risk Factors Experiences and failure case statistics showed that copper sulphide deposition and consequential failures were spread over the range of transformer applications. RISK factors for copper sulphide formation related to equipment design and manufacture are: cooling design and design defects sealed or free-breathing system RISK factors related to the state of the paper/oil insulation are: type and amount of corrosive sulphur species, type and amount of additives other than DBDS oil oxidation process and insulation ageing RISK factors related to service conditions are: working temperatures and loading conditions electrical stresses (especially transients in HVDC apparatus and industrial applications).

24 Risk Assessment - Diagnostic methods Standardized tests: IEC (corrosion of paper wrapped conductors) DIN (silver plate corrosion) ASTM D 1275 B (copper corrosion) IEC (DBDS quantification) Non-standardized tests: Determination of elemental sulphur using gas chromatography with Electron capture detector under development in IEC TC 10 WG 37 Determination of DBDS ad other sulphur species reactive to silver using gas chromatography with Photometric Flame detector A method for Total Corrosive Sulphur content in mineral insulating oil under development in IEC TC 10 WG 37 Detection of total amount of disulphides, mercaptans and elemental sulphur using potentiometric titration Detection of benzyl mercaptan, benzyl-group (Benzyl alcohol, Benzaldehydes, Benzoic acid), toluene and elemental sulphur using Gas Chromatography with Mass Spectrometry Detector Change in sulphur and copper concentrations in corrosive oils as detected by XRF

25 Risk Assessment Service experiences: Silver Corrosion Silver corrosion of OLTC silver plated contacts reported in number of transformers In majority of the cases copper was intact Oils with DBDS, but also oils without DBDS were involved Dominantly failures caused by silver corrosion were reported after (improper) oil reclamation In most of the cases silver corrosion did not lead to a failure, indications were obtained from DRM and DGA; corroded contacts replaced/cleaned and re-silvered Scant reports of failures of transformers during service due to silver corrosion; no warning or weak indications from DGA, failures due to detachment of conductive silver sulphide particles, electric arcs/flashover.

26 Risk Assessment Source of Silver Corrosion Formation of S 8 from different sulphur containing species (at high temperatures, above 200 C) during oil reclamation /reactivation of adsorbent, heat dissipated from electric arcs Once formed S 8 easily reacts with silver at lower temperatures (from 80 C onwards) S8 Ag + S = Ag 2 S 1% reclaimed oil in non-corrosive oil Traces of S 8 are enough to cause corrosion to silver 5% reclaimed oil in non-corrosive oil 10% reclaimed oil in non-corrosive oil 0% reclaimed oil in non-corrosive oil Figure 40b Tap selector contact heated in mixture of reclaimed corrosive oil and non-corrosive oil.

27 Risk Assessment Silver Corrosion: Failure in Service Irgamet 39 added in 2007 after 2 years of service, oil corrosive, DBDS = 150ppm. No depletion of metal passivator was detected. Failure in 2010, silver corrosion - Ag 2 S particles had flaked off and contaminated the windings. Irgamet 39 was found not to be effect in protection of silver surfaces. A 150MVA, 230/130 kv transmission transformer

28 Risk Assessment Service experiences: Silver Corrosion Example of silver corrosion following reclamation (l) prior to reclamation (c) contacts one year after reclamation, (r) three years after original switch out. Examples of coatings on silver plated selector contacts

29 Risk Assessment Silver Corrosion: Detection of target compounds DIN GC ECD - IEC (DBDS content) GC ECD - Detection and quantification of S 8 under development in IEC TC 10 WG 37 GC FPD method for detection of sulphur species reactive to silver

30 Risk Assessment Silver Corrosion Diagnostics In Service Diagnostics: DGA - detection of fault gases mainly ethylene (indicator of overheating of contact contaminated with Ag 2 S), good indicator of problem to prevent failure Dynamic Resistance Measurements sensitive to detect changes in resistances, due to overheating of contacts contaminated with Ag 2 S, good indication of silver sulphide problem Resistance diagrams from oscillograms following resistance measurements of healthy (left) and likely contaminated contacts (right)

31 Outline Introduction Copper Sulphide Formation Mechanism Risk Assessment Copper Sulphide Long Term Mitigation Monitoring and Maintenance Procedures Conclusions

32 Copper Sulphide Long Term Mitigation Mitigation Techniques Survey Distribution of mitigation survey responses among countries It is estimated that more than units around the world had been subjected to mitigation against copper sulphide Replies came from 16 countries, more than 1200 cases were reported

33 Copper Sulphide Long Term Mitigation Mitigation Techniques Survey More than 20 electrical utilities mainly operating in: generation (24%) transmission (70%) distribution (1%) industrial applications (3 %) All kinds of power apparatus (transformers, auto-transformers, shunt-reactors, rectifiers, etc.) free-breathing (64%) sealed units (36%) naphthenic oils (98%) paraffinic oils (2%) uninhibited oils (85%) inhibited oils (15%)

34 Copper Sulphide Long Term Mitigation Mitigation Techniques Spread of mitigation actions, according to TF 03 survey

35 Copper Sulphide Long Term Mitigation Metal Passivators Metal Deactivator - Ciba IRGAMET 39 (liquid pure reagent) Cobratec TT100 Tolutriazole (solid pure reagent) Nynas AB - Nypass (pre-blend of 10% passivator and a transformer oil base stock) Shell Diala Concentrate P (10% concentrate) DSI Sulphur Inhibitor liquid concentrate mixture of sulphur stabilizer, metal passivator and phenolic antioxidant. Metal Deactivator - Ciba IRGAMET 30 Concentration of metal passivator typically suggested is 100 mg/kg, but amounts around 200 mg/kg are also recommended to achieve higher buffering effect and decrease costs of on-site re-passivation. Concentrations < 50 mg/kg of metal passivator are considered ineffective (IEC 60422) - stringent rec. with higher safety margin.

36 Copper Sulphide Long Term Mitigation Metal Passivator Efficiency TF03 survey: Around 1100 units were subjected to metal passivator addition Around 20 units were reported to have failed after addition of metal passivator, including units reported in early period of copper sulphide problem ( ) before WG A2.40 was established These unsuccessful experiences were attributed to late addition of metal passivator, but service conditions and design weaknesses had an additional contributory effect towards failure; several cases from this group were attributed to silver corrosion which was unsuccessfully mitigated with Irgamet 39 Irgamet 39 was found to be inefficient to counteract silver corrosion, while Irgamet 30 was found to be inefficient to counteract copper corrosion DSI mixture was found to be efficient to counteract silver corrosion tried on lab scale, but can not be recommended for real applications, since the chemical composition of the mixture is unknown

37 Copper Sulphide Long Term Mitigation Metal Passivator Efficiency and Oil Condition Oil condition strongly affects performance of metal passivator Different oils have consumed metal passivator in different time spans, nevertheless thickness of metal passivator layer on the copper was the same in all investigated oils

38 Copper Sulphide Long Term Mitigation Metal Passivator Thermal Stability Metal passivator Ion profiles in vacuum for BTA and TTA ions in different oils, copper surface treated with 100 ppm of Irgamet 39 Temperature profile of metal passivator was very similar for all investigated oils In real service conditions, in air, with conductors immersed in oil, the boundary temperature would above 100 C

39 Copper Sulphide Long Term Mitigation Metal Passivator Thermal Stability Figure 58 SSIMS images (tolyltriazole green, copper red, sulphur blue) of a reference copper sample treated with Irgamet 39 in mint conditions (left), outmost conductor of the top (middle) and bottom (right) disc of a 400/275 kv scrapped autotransformer. Transformer had suffered prolonged overheating of the top discs (design and cooling issues) Copper Surface was found not to be efficiently protected with metal passivator

40 Copper Sulphide Long Term Mitigation Metal Passivator Interactions with Solid copper dissolved in the oil 800 amino methyl susbst.tta conentration in the oil concentration of amino subst.tta in the paper*, mg/kg Insulation amino methyl subst. TTA concentration in the oil, mg/kg Copper dissolved in the oil, mg/kg test duration, h subst. TTA in the paper*, mg/kg Concentration of amino methyl Simultaneous decrease of metal passivator in the oil and increase of metal passivator in the paper at 150 C over 350 h, without formation of copper dissolved in the oil High Ox ygen Low O xygen Metal passivator absorbed in the paper suppress copper in oil dissolution for some time - prolonged protection of conductors Copper dissolved in the oil, ppb Low oxygen aged oil C I Low oxygen aged oil C II Low oxygen new oil C I Low oxygen new oil C II High oxygen new oil C I High oxygen new oil C II High oxygen aged oil C I High oxygen aged oil C II a) C I aged oil 120h b) C I aged oil 120h c) C I new oil 120h d) C I new oil 120h e) C II aged oil 120h f) C II aged oil 120h 0 g) C II new oil 120h h) C II new oil 120h test duration, h Copper in the oil and images of paper wrapped conductors heated in corrosive oils for 120 h at 140 C

41 Copper Sulphide Long Term Mitigation Metal Passivators - Side effects stray gassing 13% without stray gassing and depletion 55% fast depletion 15% normal depletion 17% Stray gassing production of hydrogen, carbon oxides Depletion of metal passivator up to 40%/year from initial concentration is considered as normal absorption in cellulose materials Fast depletion of metal passivator from the oil more then 40%/year from initial concentration

42 Copper Sulphide Long Term Mitigation Metal Passivators Stray Gassing Decomposition of metal passivator yield H 2, CO and CO 2. Generation of hydrogen by elimination of hydrogen from benzo triazole molecule H 2 (Metal passivator 500 ppm) H 2 (Metal passivator 100 ppm) Stray gassing due to metal passivator addition is not a fault condition, but may interfere with diagnostics using DGA. Improved DGA interpretation affords the correct identification of this phenomenon.

43 Copper Sulphide Long Term Mitigation Metal Passivators Service experiences EXAMPLE OF TRANSFORMER WITH METAL PASSIVATOR ADDED p.u. 1,00 0,75 0,50 Corrosiveness test (ASTM D1275-B) DBDS 0,25 0,00 Irgamet 39 Start-up Passivation 2nd Passivation Still in service H operation time (months) Stray Gassing was observed to level off after one to two years after addition of metal passivator Fast depletion of metal passivator from the oil absorption and degradation of metal passivator No abnormal change of oil properties were observed after addition of metal passivators No correlation of oil properties (acidity, dielectric dissipation factor) to stray gassing was found.

44 Copper Sulphide Long Term Mitigation Metal Passivators Partition of Furans Service Experiences - no significant change of furans Experiments in the lab accelerated high temperature test Rise of furans changed partition of 2-FAL after Irgamet 39 addition and absorption in the paper DP Oil 1 Oil 1+ IR39 Oil 2 Oil 2 + IR39 Oil 3 Oil 3 + IR39 Oil 4 Oil 4 + IR 39 initial 3 days 6 days 9 days 12 days 2-F AL, pp m Oil 1 Oil 1+ IR39 Oil 2 Oil 2 + IR39 Oil 3 Oil 3 + IR39 Oil 4 Oil 4 + IR39 initial 3 days 6 days 9 days 12 days Figure 68 Change of 2-FAL concentrations in different corrosive oils before and after addition metal passivator during IEC ageing test set up for 12 days, wet paper at 140 C

45 Copper Sulphide Long Term Mitigation Oil Change 1,00 0,75 p.u. 0,50 0,25 Corrosiveness Test (ASTM D1275-B code) DBDS 0,00 Start-up Oil change Still in service operation time (months) Residual oil volume can be kept within the range of 5-10%. Single rinse of the bottom of the tank with a small extra-amount of unused oil as well as the use of the hot-spray technique to rinse both the tank and the windings is adequate.

46 Copper Sulphide Long Term Mitigation Oil treatment Oil reclamation using specific adsorbents Chemical treatment combined with reclamation, processes based on PCB Technologies Solvent extraction process Oil reclamation and chemical treatment applied on site with success p.u. 1,0 0,8 0,5 0,3 0,0 depolarization still in service Corrosiveness test (ASTM D1275-B) DBDS Acidity BDV DDF months

47 Copper Sulphide Long Term Mitigation Oil treatment- Side effects Formation of free sulphur after oil reclamation with reactivation of adsorbent Rise of ethylene in the oil after reclamation process

48 Copper Sulphide Long Term Mitigation Oil treatment- Side effects During oil reclamation, reactivation of adsorbent, at high T > 300 C thermal and catalytic crakying (adsorbent alumosilicate is catalyst) reactions can occur from various sulphur based compounds, not only DBDS : T>300 C, air, catalyst Hydrocarbons & Sulphur containing compounds Olefins (C 2 H 4, C 3 H 8,..), H 2 S, Elemental S Created elemental sulphur attack bare copper and silver surfaces and produce silver sulphide and/or copper sulphide: Cu + S = Cu 2 S Ag + S = Ag 2 S Elemental sulphur is more prone to react with silver and at lower temperatures

49 Copper Sulphide Long Term Mitigation Cost Effective Risk Mitigation Strategies Mitigation Techniques ranking CATEGORY METAL PASSIVATOR ADDITION OIL CHANGE OIL TREATMENT SIMPLICITY / TIME CONSUMING / ON LOAD APPLICATION Not applicable EFFICIENCY / / OIL PROPERTIES RESTORATION LONG TERM PERFORMANCE ENVIRONMENTAL Unknown COST /

50 Recommendations Addition of metal passivators (Irgamet 39) would be an efficient solution, especially if performed in the early days of service with corrosive oil. In order to make a decision whether to perform a second addition of metal passivator, monitoring oil corrosiveness and concentration of reactive sulphur compounds is important: If the total disulphide content is depleted to a value below 5 mg/kg, or the DBDS content is below 10 mg/kg, there is no need to apply a second addition of metal passivator, due to low probability of failure (oils are either non-corrosive or deposit traces of copper sulphide acc. to IEC 62535). In all other cases a second addition of metal passivator should be performed If after second addition of metal passivator, the passivator depletes rapidly from the oil (acc. to IEC poor condition), while concentration of DBDS in the oil is above 10 mg/kg other mitigation actions are recommended, such as removal of corrosive sulphur from the oil, or oil change as long-term solutions. Transformers with oils corrosive to silver (DIN 51353) can not be mitigated by addition of Irgamet 39, as according to service experiences Irgamet 39 was found inefficient to counteract silver corrosion.

51 Recommendations Removal of corrosive sulphur from the oil, or oil change are recommended as long-term solutions for: Units with constant high load or frequent changes of load (usually shunt reactors and industrial applications), Units with frequent and intensive electrical stresses, Units with indications of overheating (cooling defects, thermal faults -DGA), Units with intensive oil oxidation and consumption of oxygen, and combination of these conditions Units with oils corrosive to silver acc. to DIN (addition of metal passivator Irgamet 39 was found not to be efficient). The choice of technique will be dependant on the condition of transformer, criticality and costbenefit evaluation.

52 Start from here After reading XXXXX NO Is the oil corrosive according to IEC 62535? YES YES Are copper conductors fully enamelled? NO Recommendations Revised TB 378 Flow Chart NO Is the equipment passivated? NO Probability of copper sulfide deposition: NULL YES Is DBDS content in oil > 20 mg/kg? or Is total disulfides + mercaptans content in oil > 5 mg(s)/kg? YES Read Note 2!! NO YES Probability of copper sulfide deposition: LOW Is the equipment passivated? YES Was the equipment passivated during the 1 st year of service? NO NO Is the transformer highly loaded / with high thermal range? NO NO Probability of copper sulfide deposition: MEDIUM Is there any symptom of local or diffused overheating from DGA? YES Does the oil have a low oxygen content (due to atmosphere segregation or oxygen consumption)? As new YES When was the Equipment passivated? During 1 st year of service YES Is the equipment passivated? After 1 st year of service Probability of copper sulfide deposition: HIGH actions actions actions actions No action 1) Respect the loading 1) Avoid overloading 1) Reduce the thermal stress guide 2) Add a metal passivator by decreasing the loading 2) If a metal passivator, keep under monitoring if not already done or change/reclaim the oil or/and by improving the cooling passivator s concentration 3) Keep under monitoring passivator s concentration if present 2) Change/reclaim the oil or passivate the oil as a temporary action if not already done (see note 3 under the FC) 3) Keep under monitoring passivator s concentration if present YES NO YES Is passivator concentrations stable in time? NO YES Is passivator concentrations stable in time? NO YES After change/reclaim, is the oil still corrosive? NO

53 Outline Introduction Copper Sulphide Formation Mechanism Risk Assessment Copper Sulphide Long Term Mitigation Monitoring and Maintenance Procedures Conclusions

54 Monitoring and Maintenance Procedures MONITIRING Corrosive sulphur test IEC 62535, DIN and ASTM D 1275 B DBDS concentration IEC Elemental Sulphur and total disulphides and mercaptans Monitoring Irgamet 39 content - IEC MAINTENANCE (Oil reclamation processes): Incomplete removal of DBDS after oil reclamation Thorough rinse of transformer active part from residual corrosive oil, by circulating oil through heated transformer active part On board corrosive sulphur tests at the end of oil treatment (IEC and DIN 51353) Optionally, determination of DBDS in the oil during or after treatment (for oils containing DBDS) Re-inhibition with DBPC after the oil is verified as non-corrosive, according to IEC and DIN In case of process with reactivation of adsorbent investigate/revise procedures to avoid cross contamination (buffer tanks, columns, storage vessels) Verify adsorbent performance in laboratory prior on-site treatment

55 Outline Introduction Copper Sulphide Formation Mechanism Risk Assessment Copper Sulphide Long Term Mitigation Monitoring and Maintenance Procedures Conclusions

56 Conslusions Metallic sulphides deposited in the paper insulation or detached from metal surfaces significantly reduce dielectric status of transformer active part and cause, or contribute to transformer failures. Deposits on bare metallic surfaces can also be the cause of overheating. Postulated mechanism of copper sulphide formation involve the dissolution of copper, diffusion and absorption of an intermediate complex in or on paper or board, and subsequent reaction with sulphur compounds to form Cu2S and other by-products. Electrical fields seem to promote copper sulphide formation. They may play an important role in the initiating step of the reaction: formation of copper cations. Detachment of fine copper sulphide particles from copper surfaces and transport to the adjacent paper layer is also proposed. Temperature and concentration of reactive sulphur are the main risk factors. Oxygen determines the rate of copper-in-oil dissolution, diffusion and absorption of intermediate copper complexes in the paper. Copper sulphide is formed in broad range of oxygen contents, corresponding to both sealed and free breathing transformer application. At lower temperatures, the risks of copper sulphide formation is lower in a high oxygen environment, while at higher temperatures (overloading conditions, localized or diffused overheating) risks of copper sulphide formation in the paper are higher at higher oxygen levels.

57 Conclusions Investigation of sources of corrosive sulphur coming from rubbers and gaskets showed that these materials have no corrosive potential. Most of the failed transformers and reactors that have operated with high winding temperatures and with oils having a high concentration of reactive sulphur compounds, regardless of the transformer preservation system. The addition of metal passivator to the oil is still the most commonly applied, which is likely to be related to the low cost and simplicity of its use, but it has limitations; the poor stability at elevated temperatures, in less refined and aged oil is the most important drawback. Rapid depletion of metal passivator from the oil and evolution of gases after metal passivator has been added are the most common side effects (fast depletion of metal passivator in 15 % and 13% cases of units with oil stray gassing). Gassing of the oil is not considered a fault condition and it was recognized to level off after a time, typically one to two years. A more radical solution involves removal of the sulphur either through changing the oil, or removing the reactive sulphur from the oil in situ; if carried out properly, the long-term effects seem to be good for both options. Stringent procedures and monitoring of the corrosive sulphur on-site during reclamation have to be performed to ensure good results. In some cases, improved cooling or reducing the load are suggested.

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59 Thank you for attention Questions?