copper sulphide long term mitigation and risk...

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



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

0

5000

10000

15000

20000

25000

30000

35000

40000

Cop

per

cont

ent i

n th

e pa

per,

ppm

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

0

2000

4000

6000

8000

10000

12000

14000

16000

Cop

per

cont

ent i

n th

e pa

per,

ppm

120°C - 3 days 120°C - 7 days 150°C - 3 days 150°C - 7 days

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

0

5000

10000

15000

20000

25000

30000

35000

40000

D D + O2 MO MO + O2Oils

Cop

per c

onte

nt in

the

pape

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

0

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

200

400

600

800

1000

1200

1400

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

0,5

1

1,5

2

2,5

3

3,5

4

4,5

Oil # 1 (inhibited)

Oil # 5 (uninhibited)

Cop

per c

onte

nt (m

g/kg

) in

oil

Oil type

Inert atmosphere (Argon)

Oxidative atmosphere (Oxygen)

0,00

100,00

200,00

300,00

400,00

500,00

600,00

700,00

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


Risk AssessmentCopper Sulphide Long Term Mitigation


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




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


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

200

300

400

500

600

700

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

100

150

200

250

300

350

400

450

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

400

500

600

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

5.00

10.00

15.00

20.00

25.00

30.00

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

0,3

0,5

0,8

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


NO

Probability of coppersulfide deposition:

NULL


LOW


MEDIUM


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


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


Risk Assessment


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


Risk Assessment



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?

copper sulphide long term mitigation and risk...

Documents