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Copper Sulphide in Transformer Insulation
Tutorial of Cigre WG A2-32 Convener: Mats Dahlund, Sweden
Different examples of copper sulphide growth
What is the problem? •Cu2S is a conductor •Extensive growth may form a conducting bridge through the conductor insulation •Lowered PD inception voltage may occur already with moderate amounts of sulphide formation •Local heat evolution may lead to degradation of paper •Other possibilities? Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
3
Extent of problem During the last 15 years: 100 failures of large units? All major transformer manufacturers affected Several different oil suppliers Many observations of copper sulphide - but not always a cause of failures
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
4
WG A2-32 tasks •Understanding the mechanisms of sulphide formation and failures •Find relevant methods to test oil •How to identify units at risk
•Identity mitigation techniques
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
5
Not tasks of A2-32: •Copper corrosion or effects of corrosive sulphur in general •Tap changers/ Selectors •Bushings
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
6
WG • • • • • • • • • • • • •
Mats Dahlund, convenor, Sweden Ivanka Höhlein, (TF 01), Riccardo Maina (TF 02), Italy Nick Dominelli (TF 03), Canada Trond Ohnstad TF 04), Norway Tsuyoshi Amimoto, Japan Yves Bertrand, Xue Chendong, China Clair Claiborne, USA Paul Griffin, USA Jelena Lukic, Serbia Lars Lundgaard, Norway Julie Van Peheghem, Belgium
• • • • • • • • • • • •
Volker Null, Jayme Leite Nunes Jr, Brazil Marit-Helen Ese, Norway Alfonso de Pablo, Spain Christophe Perrier, Fabio Scatiggio, Italy Viktor Sokolov†, Ukraine Kjell Sundkvist, Sweden Yongyuth Vachiratatarapadron, Thailand Junji Tanimura, Japan Peter Smith, Vladyslav Mezhvynskiy,
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
7
Task Forces of WG A2.32 • Task Force 1 New test for detection of corrosive sulphur (new standard IEC 62535) • Task Force 2 Metal ivator – analysis methods and stability • Task Force 3 Sulphur speciation • Task Force 4 Recommendations for s Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
8
WG A2-32 now closed • All original tasks adressed, Broschure No. 378 published • Other related CIGRE and IEC working bodies active now: Oil testing and specification Oil maintenance Sulphur speciation
IEC TC10 MT21 IEC TC10 MT22 IEC TC10 WG37
Copper sulphide - long term mitigation and risk asessement (starts in 2009) CIGRE WG A2-40
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
9
Cu2S formation mechanism •Details of mechanism are not understood - but some progress made •Some active sulphur compounds identified - disulphides in general, DBDS in particular •Influence of oxygen clearly demonstrated - some explanations proposed •Influence of non-corrosive oil components is still not well understood
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
10
Proposed Cu2S formation mechanism DBDS-Cu complex
By-products CH 2 CH 2
Insulating oil
CH
S-S S-
CH 2
CH 2
S-S
Cu
ＤＢＤS
BiBZ
CH
2
2
CH 2
Cu
Dissolution Make particles
Coordination
ＤＢS
Absorption
CH
Copper
Cu
Cu
Cu
Cu
Cu
Cu
Cu
CH
Reaction
CH
2
2
S-S
2
2
S-S
Cu
Cu
CH 2
Diffusion
Step 1
CH
S
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Step 2 Decomposition
Cu2S S Cu
Cu
Insulating paper
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
11
Main influential factors • Corrosive sulphur in oil • High temperature • Oxygen content
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
12
Corrosive oil The presence of corrosive (or potentially corrosive) sulphur is a prerequisite for copper sulphide formation Oil failing IEC 62535 or ASTM D1275-B should be considered as corrosive Metal ivators may block the effects of corrosive components, even under the severe conditions of these tests
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
13
Problems only with uninhibited oil? • Most problems were with uninhibited oil (these need more sulphur content for oxidation stability) • Some oils with inhibitor also caused failures • Inhibitor may influence deposition – but problem not restricted to one type or another!
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
14
Temperature •The rate of all chemical reactions is governed by temperature
•The rate of sulphide formation reactions seems to approximately double for every 10ºC increase of temperature
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
15
Oxygen content •Oxygen promotes copper transport to paper, but there seems to be an optimal range •Some sulphur compounds become more active when oxidized, or is there an effect of other oxidation products in oil helping solubilize copper? •Paper surface more efficient sorbent for intermediates when oxidized? •At very high O2 content precipitation of oxidation products becomes significant •Conclusion so far: low to intermediate oxygen content seems worse than high (or absent O2) Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
16
Closed vs. open breathers Initial failure statistics seemed to be dominated by closed units, but we also see cases with open breathers. Closed: •Major population works under high load Open breathers: •Oxygen in moderate amounts is promoting Cu2S formation •But process could also be slower due to competitive oxidation reactions A constant (high) load means the transformer is not breathing, even if it is nominally a ”free breather” The problem is thus not restricted to one type or another, high load is the dominant risk factor Actual oxygen content is more important than ”closed” or ”open” type
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
17
Applications •Failure statistics were initally dominated by - Shunt Reactors - Generator step-up transformers - HVDC transformers
•Growing number of failures in other groups, e.g. - Industrial (rectifiers) - Transmission and distribution (varying sizes)
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
18
Test methods for detecting corrosive sulphur in oil • Metal strip tests • Tests involving copper and paper • Chemical analysis (“speciation”)
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
19
Metal strip tests • Silver strip tests DIN 51353
100˚C, 18 hours
• Copper strip tests ASTM D1275-A (ISO 5662)
140˚C, 19 hours
ASTM D1275-B
150˚C 48 hours
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
20
Test involving paper Objectives of Task Force A2-32.01: • The test should reflect the environment in a real transformer • To provide relevant results in an accelerated mode •
To cover the existing failure modes
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
21
Proposed method from TF A2-32.01 • Carried out in a 20 ml head-space vial • 15 ml air saturated oil + 5 ml air • 3 cm copper conductor 8 mm x 2 mm, 1 paper layer wound „gap to gap“ • 72 hours at 150°C • Now IEC 62535 Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
22
Evaluation of copper and paper after the test The result is “corrosive” if one or both of the following is found: -dark grey, dark brown or black discoloration of copper or -shiny deposits on paper
corrosive
noncorrosive
Specific sulphur compounds Some researchers have pointed out the presence of a dominant sulphur containing compound in many corrosive oils. This dominant compound was identified as dibenzyldisulfide (DBDS) DBDS has been shown to be a strong copper sulphide forming agent, present in most (but not all) oils involved Most oils introduced recently do not contain detectable amounts of this substance Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
24
Gas Chromatography with sulphurspecific detector (e.g. AED) used to reveal the presence of DBDS
Dominant peak found in sulphur chromatogram, identified as DBDS
Oil with DBDS
Oil without DBDS
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
25
Sulphur speciation TF A2-32.03 recommended methods • Dibenzyl disulphide (DBDS) - GC-ECD - GC-MS - GC-AED • Sum of disulphide and mercaptan sulphur - potentiometric titration with Ag/Ag2S electrode • More details in separate report • Work is carried on by IEC TC10 WG37 Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
26
DGA •Not useful for direct detection of Cu2S formation •However, DGA may be a valuable component in overall risk assessment, by detecting conditions that lead to higher risks of Cu2S formation: - oxygen depletion - overheating
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
27
Oil tests – conclusions • Qualitative tests have highest priority: - IEC 62535 covered conductor test - ASTM D1275-B copper strip test • Determination of content of corrosive components can be useful additional information • Limited possibilitites to detect Cu2S formation - though depletion of DBDS and build-up of known by-products may give some indication
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
28
Other ways to detect copper sulphide formation? • Dielectric response methods (e.g. FDS, Power Factor Tip-Up, PDC, FRA?) may give some guidance • Very limited experience so far • Interpretation difficult • May indicate the presence of conducting contaminants, but location and amounts of contaminants is difficult to assess Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
29
FDS (or DFR) Dielectric Response Method Frequency Domain Spectroscopy or Dielectric Frequency Response –Measurement of dielectric properties: (capacitance, loss, tangent delta/power factor) over a range of frequencies (typical frequency range: 1mHz – 1000Hz) –Typical applied voltage: 140V rms –Interpretation with modeling of contamination in insulation system requires knowledge of geometric design parameters of insulation system, dielectric characteristics of the oil and oil-impregnated cellulose under different moisture and temperature conditions Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
30
FDS Analysis of Winding to Core Shield Insulation •Measurement deviates from normal unit model •Estimated extent of contamination: –1-2% volume of solid insulation
•Estimated conductivity of contaminant: –2E-10 – 5E-11 S/m (clean paper: approx 10-16S/cm) Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
31
Power Factor tip-up
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
32
Inspection of windings • Copper sulphide deposits are normally not seen unless oil is removed • Deposits may be very localized, both axially and radially in windings (and in paper layers!) • Carry out all inspection in an unbiased manner
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
33
Mitigation techniques • Oil additives (metal ivators) • Oil treatment • Oil exchange • Modified operating conditions
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
34
Metal ivators • Experiences • Consumption/ Monitoring • Side effects • Effects of oil treatment
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
35
Metal ivators • Efficiency demonstrated in many different corrosivity test set-ups • Extensive experience - have been used for a long time (e.g. in Japan) • Procedures developed to add on site • Limited life in some cases, especially for aged oils – recommended to monitor the ivator content
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
36
Metal ivators • •
Monitoring , analysis HPLC method developed by TF02
Final results from RRT:
BTA 50 ppm
Irgamet39 50 ppm
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
37
Metal ivator - depletion
Sampling date
Only some initial decrease (absorption by paper?)
20 0 18/01/2007
09/09/2008
01/06/2008
22/02/2008
14/11/2007
06/08/2007
28/04/2007
18/01/2007
10/10/2006
02/07/2006
0
40
29/11/2006
20
60
10/10/2006
40
80
21/08/2006
60
100
02/07/2006
80
120
13/05/2006
100
140
24/03/2006
Irgamet 39 conc. (mg/kg)
120
24/03/2006
Irgamet 39 conc. (mg/kg)
2 different cases
Sampling date
Steady rate of depletion
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
38
ivator - side effects and caution 1000
800
H2 (ppm)
• Stray gassing!
600
400
200
08-06-01
08-04-12
08-02-22
08-01-03
07-11-14
07-09-25
07-08-06
07-06-17
0
• Treatments on ivated oil: - reclaiming will remove ivator - reconditioning OK (with care)
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
39
Oil treatments for sulphur removal continuous on-line treatment with sorbent “selective depolarization” (a combination of reagents and sorbents) mobile on-line reclaiming, with reactivating sorbent treatment with KOH/PEG, similar to established PCB removal technology liquid-liquid extraction
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
40
Treatment of oil – general concerns •Do not expect every combination of oil and treatment to work
•Not only corrosive sulphur is affected
•Always the result by established oil tests and the new severe tests for corrosive sulphur
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
41
Oil exchange • A complete change to non-corrosive oil is obviously desirable • Exchange is never truly complete in real life • Due to contamination from old oil, exchange may sometimes not totally eliminate further corrosive reactions – test before, with a pessimistic ratio of old oil:new oil
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
42
Operating conditions •Keep temperature down - load restrictions - improved/forced cooling
Winding Hot Spot Temperature 108 106 104 102 100 °C 98 96 94 92 90 12.00
Forced Air Cooling Natural Cooling
15.00
18.00
21.00
0.00
3.00
6.00
9.00
12.00
t ime
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
43
Recommendations •Guidelines for data collection and ranking •Decision making scheme, that uses - oil tests (trends) - maintenance history - operating conditions Selection made in present range of available mitigation techniques
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
44
What if I already have Cu2S deposits? •Minimize risk of further Cu2S formation •Avoid overvoltages •Use experience from “brothers and sisters” to assess seriousness •Plan for repairs/ replacements •: Presence of Cu2S does not necessarily lead to failure Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
45
Conclusions •Copper sulphide is still a real problem, due to large numbers of transformers having corrosive oil •A growing range of mitigation techniques is at hand •Relevant test methods for corrosive sulphur in oil are now available •Precise diagnostics and risk assessment methods are still not well developed – WG A2-40 will carry on the work
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
46
Thank you for your attention! Brochure 378 available at www.e-cigre.org
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
47
Tutorial of Cigre WG A2-32 Convener: Mats Dahlund, Sweden
Different examples of copper sulphide growth
What is the problem? •Cu2S is a conductor •Extensive growth may form a conducting bridge through the conductor insulation •Lowered PD inception voltage may occur already with moderate amounts of sulphide formation •Local heat evolution may lead to degradation of paper •Other possibilities? Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
3
Extent of problem During the last 15 years: 100 failures of large units? All major transformer manufacturers affected Several different oil suppliers Many observations of copper sulphide - but not always a cause of failures
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
4
WG A2-32 tasks •Understanding the mechanisms of sulphide formation and failures •Find relevant methods to test oil •How to identify units at risk
•Identity mitigation techniques
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
5
Not tasks of A2-32: •Copper corrosion or effects of corrosive sulphur in general •Tap changers/ Selectors •Bushings
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
6
WG • • • • • • • • • • • • •
Mats Dahlund, convenor, Sweden Ivanka Höhlein, (TF 01), Riccardo Maina (TF 02), Italy Nick Dominelli (TF 03), Canada Trond Ohnstad TF 04), Norway Tsuyoshi Amimoto, Japan Yves Bertrand, Xue Chendong, China Clair Claiborne, USA Paul Griffin, USA Jelena Lukic, Serbia Lars Lundgaard, Norway Julie Van Peheghem, Belgium
• • • • • • • • • • • •
Volker Null, Jayme Leite Nunes Jr, Brazil Marit-Helen Ese, Norway Alfonso de Pablo, Spain Christophe Perrier, Fabio Scatiggio, Italy Viktor Sokolov†, Ukraine Kjell Sundkvist, Sweden Yongyuth Vachiratatarapadron, Thailand Junji Tanimura, Japan Peter Smith, Vladyslav Mezhvynskiy,
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
7
Task Forces of WG A2.32 • Task Force 1 New test for detection of corrosive sulphur (new standard IEC 62535) • Task Force 2 Metal ivator – analysis methods and stability • Task Force 3 Sulphur speciation • Task Force 4 Recommendations for s Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
8
WG A2-32 now closed • All original tasks adressed, Broschure No. 378 published • Other related CIGRE and IEC working bodies active now: Oil testing and specification Oil maintenance Sulphur speciation
IEC TC10 MT21 IEC TC10 MT22 IEC TC10 WG37
Copper sulphide - long term mitigation and risk asessement (starts in 2009) CIGRE WG A2-40
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
9
Cu2S formation mechanism •Details of mechanism are not understood - but some progress made •Some active sulphur compounds identified - disulphides in general, DBDS in particular •Influence of oxygen clearly demonstrated - some explanations proposed •Influence of non-corrosive oil components is still not well understood
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
10
Proposed Cu2S formation mechanism DBDS-Cu complex
By-products CH 2 CH 2
Insulating oil
CH
S-S S-
CH 2
CH 2
S-S
Cu
ＤＢＤS
BiBZ
CH
2
2
CH 2
Cu
Dissolution Make particles
Coordination
ＤＢS
Absorption
CH
Copper
Cu
Cu
Cu
Cu
Cu
Cu
Cu
CH
Reaction
CH
2
2
S-S
2
2
S-S
Cu
Cu
CH 2
Diffusion
Step 1
CH
S
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Step 2 Decomposition
Cu2S S Cu
Cu
Insulating paper
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
11
Main influential factors • Corrosive sulphur in oil • High temperature • Oxygen content
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
12
Corrosive oil The presence of corrosive (or potentially corrosive) sulphur is a prerequisite for copper sulphide formation Oil failing IEC 62535 or ASTM D1275-B should be considered as corrosive Metal ivators may block the effects of corrosive components, even under the severe conditions of these tests
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
13
Problems only with uninhibited oil? • Most problems were with uninhibited oil (these need more sulphur content for oxidation stability) • Some oils with inhibitor also caused failures • Inhibitor may influence deposition – but problem not restricted to one type or another!
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
14
Temperature •The rate of all chemical reactions is governed by temperature
•The rate of sulphide formation reactions seems to approximately double for every 10ºC increase of temperature
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
15
Oxygen content •Oxygen promotes copper transport to paper, but there seems to be an optimal range •Some sulphur compounds become more active when oxidized, or is there an effect of other oxidation products in oil helping solubilize copper? •Paper surface more efficient sorbent for intermediates when oxidized? •At very high O2 content precipitation of oxidation products becomes significant •Conclusion so far: low to intermediate oxygen content seems worse than high (or absent O2) Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
16
Closed vs. open breathers Initial failure statistics seemed to be dominated by closed units, but we also see cases with open breathers. Closed: •Major population works under high load Open breathers: •Oxygen in moderate amounts is promoting Cu2S formation •But process could also be slower due to competitive oxidation reactions A constant (high) load means the transformer is not breathing, even if it is nominally a ”free breather” The problem is thus not restricted to one type or another, high load is the dominant risk factor Actual oxygen content is more important than ”closed” or ”open” type
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
17
Applications •Failure statistics were initally dominated by - Shunt Reactors - Generator step-up transformers - HVDC transformers
•Growing number of failures in other groups, e.g. - Industrial (rectifiers) - Transmission and distribution (varying sizes)
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
18
Test methods for detecting corrosive sulphur in oil • Metal strip tests • Tests involving copper and paper • Chemical analysis (“speciation”)
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
19
Metal strip tests • Silver strip tests DIN 51353
100˚C, 18 hours
• Copper strip tests ASTM D1275-A (ISO 5662)
140˚C, 19 hours
ASTM D1275-B
150˚C 48 hours
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
20
Test involving paper Objectives of Task Force A2-32.01: • The test should reflect the environment in a real transformer • To provide relevant results in an accelerated mode •
To cover the existing failure modes
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
21
Proposed method from TF A2-32.01 • Carried out in a 20 ml head-space vial • 15 ml air saturated oil + 5 ml air • 3 cm copper conductor 8 mm x 2 mm, 1 paper layer wound „gap to gap“ • 72 hours at 150°C • Now IEC 62535 Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
22
Evaluation of copper and paper after the test The result is “corrosive” if one or both of the following is found: -dark grey, dark brown or black discoloration of copper or -shiny deposits on paper
corrosive
noncorrosive
Specific sulphur compounds Some researchers have pointed out the presence of a dominant sulphur containing compound in many corrosive oils. This dominant compound was identified as dibenzyldisulfide (DBDS) DBDS has been shown to be a strong copper sulphide forming agent, present in most (but not all) oils involved Most oils introduced recently do not contain detectable amounts of this substance Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
24
Gas Chromatography with sulphurspecific detector (e.g. AED) used to reveal the presence of DBDS
Dominant peak found in sulphur chromatogram, identified as DBDS
Oil with DBDS
Oil without DBDS
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
25
Sulphur speciation TF A2-32.03 recommended methods • Dibenzyl disulphide (DBDS) - GC-ECD - GC-MS - GC-AED • Sum of disulphide and mercaptan sulphur - potentiometric titration with Ag/Ag2S electrode • More details in separate report • Work is carried on by IEC TC10 WG37 Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
26
DGA •Not useful for direct detection of Cu2S formation •However, DGA may be a valuable component in overall risk assessment, by detecting conditions that lead to higher risks of Cu2S formation: - oxygen depletion - overheating
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
27
Oil tests – conclusions • Qualitative tests have highest priority: - IEC 62535 covered conductor test - ASTM D1275-B copper strip test • Determination of content of corrosive components can be useful additional information • Limited possibilitites to detect Cu2S formation - though depletion of DBDS and build-up of known by-products may give some indication
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
28
Other ways to detect copper sulphide formation? • Dielectric response methods (e.g. FDS, Power Factor Tip-Up, PDC, FRA?) may give some guidance • Very limited experience so far • Interpretation difficult • May indicate the presence of conducting contaminants, but location and amounts of contaminants is difficult to assess Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
29
FDS (or DFR) Dielectric Response Method Frequency Domain Spectroscopy or Dielectric Frequency Response –Measurement of dielectric properties: (capacitance, loss, tangent delta/power factor) over a range of frequencies (typical frequency range: 1mHz – 1000Hz) –Typical applied voltage: 140V rms –Interpretation with modeling of contamination in insulation system requires knowledge of geometric design parameters of insulation system, dielectric characteristics of the oil and oil-impregnated cellulose under different moisture and temperature conditions Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
30
FDS Analysis of Winding to Core Shield Insulation •Measurement deviates from normal unit model •Estimated extent of contamination: –1-2% volume of solid insulation
•Estimated conductivity of contaminant: –2E-10 – 5E-11 S/m (clean paper: approx 10-16S/cm) Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
31
Power Factor tip-up
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
32
Inspection of windings • Copper sulphide deposits are normally not seen unless oil is removed • Deposits may be very localized, both axially and radially in windings (and in paper layers!) • Carry out all inspection in an unbiased manner
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
33
Mitigation techniques • Oil additives (metal ivators) • Oil treatment • Oil exchange • Modified operating conditions
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
34
Metal ivators • Experiences • Consumption/ Monitoring • Side effects • Effects of oil treatment
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
35
Metal ivators • Efficiency demonstrated in many different corrosivity test set-ups • Extensive experience - have been used for a long time (e.g. in Japan) • Procedures developed to add on site • Limited life in some cases, especially for aged oils – recommended to monitor the ivator content
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
36
Metal ivators • •
Monitoring , analysis HPLC method developed by TF02
Final results from RRT:
BTA 50 ppm
Irgamet39 50 ppm
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
37
Metal ivator - depletion
Sampling date
Only some initial decrease (absorption by paper?)
20 0 18/01/2007
09/09/2008
01/06/2008
22/02/2008
14/11/2007
06/08/2007
28/04/2007
18/01/2007
10/10/2006
02/07/2006
0
40
29/11/2006
20
60
10/10/2006
40
80
21/08/2006
60
100
02/07/2006
80
120
13/05/2006
100
140
24/03/2006
Irgamet 39 conc. (mg/kg)
120
24/03/2006
Irgamet 39 conc. (mg/kg)
2 different cases
Sampling date
Steady rate of depletion
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
38
ivator - side effects and caution 1000
800
H2 (ppm)
• Stray gassing!
600
400
200
08-06-01
08-04-12
08-02-22
08-01-03
07-11-14
07-09-25
07-08-06
07-06-17
0
• Treatments on ivated oil: - reclaiming will remove ivator - reconditioning OK (with care)
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
39
Oil treatments for sulphur removal continuous on-line treatment with sorbent “selective depolarization” (a combination of reagents and sorbents) mobile on-line reclaiming, with reactivating sorbent treatment with KOH/PEG, similar to established PCB removal technology liquid-liquid extraction
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
40
Treatment of oil – general concerns •Do not expect every combination of oil and treatment to work
•Not only corrosive sulphur is affected
•Always the result by established oil tests and the new severe tests for corrosive sulphur
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
41
Oil exchange • A complete change to non-corrosive oil is obviously desirable • Exchange is never truly complete in real life • Due to contamination from old oil, exchange may sometimes not totally eliminate further corrosive reactions – test before, with a pessimistic ratio of old oil:new oil
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
42
Operating conditions •Keep temperature down - load restrictions - improved/forced cooling
Winding Hot Spot Temperature 108 106 104 102 100 °C 98 96 94 92 90 12.00
Forced Air Cooling Natural Cooling
15.00
18.00
21.00
0.00
3.00
6.00
9.00
12.00
t ime
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
43
Recommendations •Guidelines for data collection and ranking •Decision making scheme, that uses - oil tests (trends) - maintenance history - operating conditions Selection made in present range of available mitigation techniques
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
44
What if I already have Cu2S deposits? •Minimize risk of further Cu2S formation •Avoid overvoltages •Use experience from “brothers and sisters” to assess seriousness •Plan for repairs/ replacements •: Presence of Cu2S does not necessarily lead to failure Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
45
Conclusions •Copper sulphide is still a real problem, due to large numbers of transformers having corrosive oil •A growing range of mitigation techniques is at hand •Relevant test methods for corrosive sulphur in oil are now available •Precise diagnostics and risk assessment methods are still not well developed – WG A2-40 will carry on the work
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
46
Thank you for your attention! Brochure 378 available at www.e-cigre.org
Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32
47
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