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COPPER SULPHIDE LONG TERM MITIGATION AND RISK
ASSESSMENT
Working Group A2.40
TUTORIAL
WG members
Publication 2015
MembersJ.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 membersG.Krikke (NL), M.Grisaru (IL), L.Lewand (USA).
Former membersL. 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).
ContributorsH.F.A. Verhaart (NL), A.Petersen (AUS), C.N.Fares (UY), M.A.Martins (PT).
Outline
Introduction
Risk Assessment
Copper Sulphide Long Term Mitigation
Monitoring and Maintenance Procedures
Conclusions
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.
Copper Sulphide Long-term Mitigation and Risk Assessment
OutlineIntroduction
Copper Sulphide Formation Mechanism Risk Assessment
Copper Sulphide Long Term Mitigation
Monitoring and Maintenance Procedures
Conclusions
Copper Sulphide Formation Mechanism
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 layersCopper and paper contaminated with copper sulphide
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
Copper Sulphide Formation MechanismMechanism: 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: CopperT: TemperatureAC/DC: Electrical fieldsCudiss.: Dissolved copper compounds (Cu+(O2R)- )ROOH: hydroperoxidesR’OH: alcohols, phenols and derivates (DBPC, DBPh)CuxS: Copper sulphide
Copper Sulphide Formation MechanismInfluence 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 80-100°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”
Copper Sulphide Formation MechanismInfluence 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)
Copper Sulphide Formation MechanismInfluence of Base oil Composition and inhibitors
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ent i
n th
e pa
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120°C - 3 days 120°C - 7 days 150°C - 3 days 150°C - 7 days
120⁰C/150⁰C breathingnaphtenic oil A,uninhbited
naphtenic oil A,uninhibited withadded DBPCnaphtenic oil B,inhibited
paraffinic oil ,inhibited
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120⁰C/150⁰C non-breathing naphtenic oil A, uninhbited
naphtenic oil A, uninhibitedwith added DBPCnaphtenic oil B, inhibited
paraffinic oil , inhibited
Uninhibited corrosive oils with 0.3% added DBPC, DBPh and variable oxygen content
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Cop
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onte
nt in
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r, m
g/kg
initial OilDBFDBPC
Naphthenic oils have a slightly higher propensity for copper sulphide deposition on the paper
Addition of inhibitors DBPC and DBPh to uninhibited oils comprised to significant copper sulphide deposition on the paper, which was pronounced in oxygen environment
Copper Sulphide Formation MechanismReactivity of sulphur compounds
Reactivity of different classes of sulphur compounds in the temperature range from 80°C to 180°C
IEC 62535 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 - carbonylcompounds containing sulphur
+
Thiophenes +/-
Reactivity of corrosive sulphur compounds for copper sulphide deposition on the paper according to IEC 62535 is as follows:
ELEMENTAL SULPHUR > DISULPHIDES > MERCAPTANS AND OXIDIZED SULPHUR COMPOUNDS > MONOSULPHIDES AND TIOPHENES
Copper Sulphide Formation MechanismCopper in Oil Dissolution
-0,2
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0,2
0,4
0,6
0,8
1
1,2
1,4
90 100 110 120 130 140 150
Cop
per c
onte
nt (m
g/kg
) in
oil
Temperature, °C
Oil # 1 Oil # 2Oil # 3 Oil # 4Oil # 5 Oil # 6Oil # 7 Oil # 8
0
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90 100 110 120 130 140 150
Cop
per c
onte
nt (m
g/kg
) in
pape
r
Temperature, °C
Oil # 1 Oil # 2Oil # 3 Oil # 4Oil # 5 Oil # 6Oil # 7 Oil # 8
Figure 9 Copper contents in the oil and paper at different temperatures (oils from Table 2).
0
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4,5
Oil # 1 (inhibited)
Oil # 5 (uninhibited)
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onte
nt (m
g/kg
) in
oil
Oil type
Inert atmosphere (Argon)
Oxidative atmosphere (Oxygen)
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Oil # 1 (inhibited)
Oil # 5 (uninhibited)
Cop
per c
onte
nt (m
g/kg
) in
pape
r
Oil type
Inert atmosphere (Argon)
Oxidative atmosphere (Oxygen)
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
Copper Sulphide Formation MechanismCopper 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
OutlineIntroduction
Copper Sulphide Formation Mechanism
Risk AssessmentCopper Sulphide Long Term Mitigation
Monitoring and Maintenance Procedures
Conclusions
Risk AssessmentService experiences: Transmission transformers
copper sulphide deposition precipitated the 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.
Free-breathing, 1000MVA, 400/275 kV transmission auto-transformer, in service for 11 years
HV winding turn-to-turn failure
Risk AssessmentService experiences: GSU’s
During inspection paper was found in good condition, DP > 600.
Normal working temperaturesThe transformer failed suddenly in 2010 after 15 years with corrosive oil (containing DBDS)
2) 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
inter-turn fault was found in the HV winding
Risk AssessmentService experiences: Industrial applications
1) 63MVA, 230 kV industrial transformerevidence of flashovers on the LV winding and extensive evidence of copper sulphide depositionfrequent load changes but the average oil temperature was not believed to have exceeded 60°C.
2) 44 kV, 33 MVA industrial rectifiera 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
Risk AssessmentService 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
Risk Assessment: Failure Cases Statistics
Transformer Failure Type
Gassing11%
Inter‐turn failure39%
Tap‐changer fault21%
Dielectric25%
Mechanical4%
Transformer ApplicationDistribution
7%
GSU36%
Rectifier18%
Transmission25%
Shunt Reactor11%
Industrial3%
Risk Assessment: Failure Cases StatisticsTransformer Failure CauseSulphur
corrosion + Through fault
4%Sulphur
corrosion + Overheating
fault 11%
Sulphur corrosion + Normal solid insulation ageing14%
Sulphur corrosion + Operation + Maintenance
11%
Sulphur corrosion + Abnormal
solid insulation ageing46%
Silver sulphide on
OLTC contacts14%
Transformer Failure and DBDS Content
DBDS not measured
51%
DBDS<20ppm or none21%
DBDS>100ppm21%
DBDS 20‐50 ppm7%
Risk Assessment:Failure Cases Statistics
Transformer failure and preservation system
Free breathing
72%
Sealed21%
Unknown7%
Transformer Failure and Oil Type
Inhibited oil36%
Uninhibited oil57%
Unknown oil7%
Inhibi ted oi l Uninhibi ted oi l Unknown oi l
Risk Assessment:Failure Cases Statistics Transfromer failure and load profile
Variable32%
Constant high54%
Unknown14%
Transformer Failure Discovery
Failed in service by several
protections61%
Unknown3%
Removed from service by
gassing or asset health review
36%
Risk AssessmentRisk Factors
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 DBDSoil oxidation process and insulation ageing
RISK factors related to service conditions are:
working temperatures and loading conditionselectrical stresses (especially transients in HVDC apparatus and industrial applications).
Experiences and failure case statistics showed that copper sulphide deposition and consequential failures were spread over the range of transformer applications.
Risk Assessment - Diagnostic methods
Standardized tests:
IEC 62535 (corrosion of paper wrapped conductors)
DIN 51353 (silver plate corrosion)
ASTM D 1275 B (copper corrosion)
IEC 62697 (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 chromatographywith 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
Risk AssessmentService experiences: Silver CorrosionSilver 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.
Risk AssessmentSource of Silver Corrosion
Ag + S = Ag2S
Formation of S8 from different sulphur containing species (at high temperatures, above 200°C) during oil reclamation /reactivation of adsorbent, heat dissipated from electric arcs
Once formed S8 easily reacts with silver at lower temperatures (from 80°C onwards)
Traces of S8 are enough to cause corrosion to silver
Figure 40b Tap selector contact heated in mixture of reclaimed corrosive oil and non-corrosive oil.
S8
1% reclaimed oil in non-corrosive oil
5% reclaimed oil in non-corrosive oil
10% reclaimed oil in non-corrosive oil
0% reclaimed oil in non-corrosive oil
Risk AssessmentSilver Corrosion: Failure in Service
A 150MVA, 230/130 kV transmission transformer
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 - Ag2S particles had flaked off and contaminated the windings.
Irgamet® 39 was found not to be effect in protection of silver surfaces.
Risk AssessmentService 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
Risk AssessmentSilver Corrosion: Detection of target
compoundsDIN 51353
GC ECD - IEC 62697 (DBDS content)
GC ECD - Detection and quantification of S8 – under development in IEC TC 10 WG 37
GC FPD method for detection of sulphur species reactive to silver
Risk AssessmentSilver Corrosion Diagnostics
Resistance diagrams from oscillograms following resistance measurements of healthy (left) and likely contaminated contacts (right)
In Service Diagnostics:
DGA - detection of fault gases – mainly ethylene (indicator of overheating of contact contaminated with Ag2S), good indicator of problem to prevent failure
Dynamic Resistance Measurements – sensitive to detect changes in resistances, due to overheating of contacts contaminated with Ag2S, good indication of silver sulphide problem
OutlineIntroduction
Copper Sulphide Formation Mechanism
Risk Assessment
Copper Sulphide Long Term MitigationMonitoring and Maintenance Procedures
Conclusions
Copper Sulphide Long Term MitigationMitigation Techniques Survey
Distribution of mitigation survey responses among countries
It is estimated that more than 10.000 units around the world had been subjected to mitigation against copper sulphide
Replies came from 16 countries, more than 1200 cases were reported
Copper Sulphide Long Term MitigationMitigation 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%)
Copper Sulphide Long Term MitigationMitigation Techniques
Spread of mitigation actions, according to TF 03 survey
Copper Sulphide Long Term MitigationMetal 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.
Copper Sulphide Long Term MitigationMetal 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 (2004-2007) 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
Copper Sulphide Long Term MitigationMetal 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
Copper Sulphide Long Term MitigationMetal Passivator Thermal Stability
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
Metal passivator Ion profiles in vacuum for BTA and TTA ions in different oils, copper surface treated with 100 ppm of Irgamet® 39
Copper Sulphide Long Term MitigationMetal 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
Copper Sulphide Long Term MitigationMetal Passivator Interactions with Solid
Insulation
50 100 150 200 250 300 350 400
0
5
10
15
20
25
copper d issolved in the o il am ino methyl susbst.T TA conentration in the o il concentration of am ino subst.TTA in the paper*, m g/kg
test duration, hamin
o m
ethy
l sub
st. T
TA c
once
ntra
tion
in th
e oi
l, m
g/kg
Cop
per d
isso
lved
in th
e oi
l, m
g/kg
100
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500
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800
Concentration of am
ino methyl
subst. TTA in the paper*, m
g/kg
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
Metal passivator absorbed in the paper suppress copper in oil dissolution for some time -prolonged protection of conductors
20 40 60 80 100 120
0
50
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Copp
er dis
solve
d in t
he oi
l, ppb
test duration, h
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
High Ox ygen Low O xygen
a) C I a ged oi l 120h
b) C I a ged oi l 120h
c) C I ne w oil 120h
d) C I ne w oil 120h
e) C II a ge d oi l 120h
f) C II a ge d oi l 120h
g) C II ne w oi l 120h
h) C II ne w oil 120h
Copper in the oil and images of paper wrapped conductors heated in corrosive oils for 120 h at 140°C
Copper Sulphide Long Term MitigationMetal Passivators - Side effects
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
stray gassing13%
fast depletion15%
normal depletion
17%
without stray gassing and
depletion55%
Copper Sulphide Long Term MitigationMetal Passivators – Stray Gassing
Generation of hydrogen by elimination of hydrogen from benzo triazole molecule
Decomposition of metal passivator yield H2, CO and CO2.
H2 (Metal passivator 500 ppm)
H2 (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.
Copper Sulphide Long Term MitigationMetal Passivators –Service experiences
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.
0,00
0,25
0,50
0,75
1,00
Sta
rt-up
Pas
siva
tion
2nd
Pas
siva
tion
Stil
l in
serv
ice
0 12 15 18 19 20 23 26 32 33 36 40 44 46
operation time (months)
p.u.
Corrosiveness test (ASTMD1275-B)
DBDS
Irgamet 39
H2
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
EXAMPLE OF TRANSFORMER WITH METAL PASSIVATOR ADDED
Copper Sulphide Long Term MitigationMetal Passivators – Partition of Furans
200
300
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700
800
900
1000
1100
1200
initi al 3 days 6 days 9 days 12 days
DP
Oil 1Oil 1+ IR39Oil 2Oil 2 + IR39Oil 3Oil 3 + IR39Oil 4Oil 4 + IR 39
0.00
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initial 3 days 6 days 9 days 12 days2-
FAL
, pp
m
Oil 1Oil 1+ IR39Oil 2Oil 2 + IR39Oil 3Oil 3 + IR39Oil 4Oil 4 + IR 39
Figure 68 Change of 2-FAL concentrations in different corrosive oils before and after addition metal passivator during IEC 62535 ageing test set up for 12 days, wet paper at 140°C
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
Service Experiences - no significant change of furans
Copper Sulphide Long Term MitigationOil Change
0,00
0,25
0,50
0,75
1,00
Start-up Oil change Still inservice
0 12 14 15 24 32
operation time (months)
p.u.
CorrosivenessTest (ASTMD1275-B code)
DBDS
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.
Copper Sulphide Long Term MitigationOil treatment
0,0
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1,0
depo
lariz
atio
n
still
in s
ervi
ce
0 6 18 30 42 60
months
p.u.
Corrosivenesstest (ASTMD1275-B)DBDS
Acidity
BDV
DDF
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
Copper Sulphide Long Term MitigationOil treatment- Side effects
Formation of free sulphur after oil reclamation with reactivation of adsorbent
Rise of ethylene in the oil after reclamation process
Copper Sulphide Long Term MitigationOil treatment- Side effects
Created elemental sulphur attack bare copper and silver surfaces and produce silver sulphide and/or copper sulphide:
Cu + S = Cu2S Ag + S = Ag2S
Elemental sulphur is more prone to react with silver and at lower temperatures
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 :
Hydrocarbons & Sulphur containing compounds Olefins (C2H4, C3H8,..), H2S, Elemental S
T>300°C, air, catalyst
Copper Sulphide Long Term MitigationCost Effective Risk Mitigation Strategies
CATEGORYMETAL
PASSIVATOR ADDITION
OIL CHANGE OIL TREATMENT
SIMPLICITY /
TIME CONSUMING /
ON LOAD APPLICATION Not applicable
EFFICIENCY / /
OIL PROPERTIES RESTORATION
LONG TERM PERFORMANCE
ENVIRONMENTAL Unknown
COST /
Mitigation Techniques ranking
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 60422 “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.
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 51335 (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 cost-benefit evaluation.
Rec
omm
enda
tions
R
evis
ed T
B 3
78 F
low
Cha
rtIs the oil corrosive
according to IEC 62535 ?
Are copper conductors fully enamelled ?
YES
Is there any symptom of local or diffused overheating from DGA ?
Does the oil have a low oxygen content (due to atmosphere segregation
oroxygen consumption) ?
NO
Is the equipment passivated ?
NO
Is the equipment passivated ?
NO
Probability of coppersulfide deposition:
NULL
Probability of coppersulfide deposition:
LOW
Probability of coppersulfide deposition:
MEDIUM
Probability of coppersulfide deposition:
HIGH
NO
Is DBDS content in oil > 20 mg/kg ?
orIs total disulfides +
mercaptanscontent in oil > 5 mg(S)/kg ?
YES
Was the equipmentpassivated during the 1st year of service ?
YES
YES
YES NO
YES
NO
Is the equipment passivated ?
NONO
When was the Equipment passivated ?
YES
As new
YES
During 1st year of service
After 1st year of service
NO
YES
actions actions actions actions
1) Respect the loading guide
2) If a metal passivator, keep under monitoring passivator’sconcentration
1) Avoid overloading2) Add a metal passivator
if not already done or change/reclaim the oil
3) Keep under monitoring passivator’sconcentration if present
No action 1) Reduce the thermal stress by decreasing the loading or/and by improving the cooling
2) Change/reclaim the oil orpassivate 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
Is passivator concentrations stable in time ?
YES
NOIs passivator
concentrations stable in time ?
YES
NOAfter change/reclaim,
is the oil still corrosive ?
NO
YES
Start from hereAfter reading XXXXX
Read Note 2!!
Is the transformer highly loaded / with high thermal range ? YESNO
OutlineIntroduction
Copper Sulphide Formation Mechanism
Risk Assessment
Copper Sulphide Long Term Mitigation
Monitoring and Maintenance ProceduresConclusions
Monitoring and Maintenance ProceduresMONITIRINGCorrosive sulphur test IEC 62535, DIN 51353 and ASTM D 1275 B
DBDS concentration IEC 62697
Elemental Sulphur and total disulphides and mercaptans
Monitoring Irgamet ® 39 content - IEC 60666
MAINTENANCE (Oil reclamation processes):
Thorough rinse of transformer active part from residual corrosive oil, by circulating oil through heatedtransformer active part
On board corrosive sulphur tests at the end of oil treatment (IEC 62535 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 62535 and DIN 51353.
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
Incomplete removal of DBDS after oil reclamation
OutlineIntroduction
Copper Sulphide Formation Mechanism
Risk Assessment
Copper Sulphide Long Term Mitigation
Monitoring and Maintenance Procedures
Conclusions
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.
ConclusionsInvestigation 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.
Thank you for attentionQuestions?
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