Copper Sulphide in Transformer Insulation - Cigrea2.cigre.org/content/download/65343/3094823/version/1/file/A2.32... · •Identity mitigation techniques. ... (e.g. FDS, Power Factor

Copper Sulphide in Transformer Insulation

Tutorial of Cigre WG A2-32Convener: Mats Dahlund, Sweden

Different examples of copper sulphide growth

Copper Sulphide in Transformer Insulation - Tutorial from Cigre WG A2.32 3

•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?

What is the problem?


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


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


Not tasks of A2-32:

•Copper corrosion or effects of corrosive sulphur in general

•Tap changers/ Selectors

•Bushings


• Volker Null, Germany• Jayme Leite Nunes Jr, Brazil• Marit-Helen Ese, Norway• Alfonso de Pablo, Spain• Christophe Perrier, France • Fabio Scatiggio, Italy• Viktor Sokolov†, Ukraine• Kjell Sundkvist, Sweden• Yongyuth Vachiratatarapadron, Thailand• Junji Tanimura, Japan• Peter Smith, Germany• Vladyslav Mezhvynskiy, Germany

WG Members• Mats Dahlund, convenor, Sweden• Ivanka Höhlein, (TF 01), Germany• Riccardo Maina (TF 02), Italy• Nick Dominelli (TF 03), Canada• Trond Ohnstad TF 04), Norway • Tsuyoshi Amimoto, Japan • Yves Bertrand, France• Xue Chendong, China• Clair Claiborne, USA• Paul Griffin, USA• Jelena Lukic, Serbia• Lars Lundgaard, Norway• Julie Van Peheghem, Belgium


Task Forces of WG A2.32• Task Force 1

New test for detection of corrosive sulphur (new standard IEC 62535)

• Task Force 2Metal passivator – analysis methods and stability

• Task Force 3Sulphur speciation

• Task Force 4 Recommendations for users


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 IEC TC10 MT21Oil maintenance IEC TC10 MT22 Sulphur speciation IEC TC10 WG37

Copper sulphide - long term mitigation and risk asessement (starts in 2009) CIGRE WG A2-40


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


ＤＢＤS

Insu

lating

oil

Cu

S-S CH222 S- 2CH

Cu Cu Cu Cu Cu Cu Cu

Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu

Step 2

Decomposition

DissolutionMake particles

Insulating paper

Absorption

Step 1

Copp

er

S-SCH2CH2

S-SCH2CH2CH2

CH2CH2

CoordinationCu2S

S

Cu Cu

Cu Cu

S-SCH2CH2

S-SCH2CH2CH2

CH2CH2

Cu Cu

S-SCH2CH2

S-SCH2CH2CH2

CH2CH2

Cu Cu

S CH222 2CH

CH222 2CH

ＤＢS

BiBZ

By-products

Reaction

DBDS-Cu complex

DiffusionＤＢＤS

Insu

lating

oil

CuCu

S-S CH222 S- 2CH S-S CH222 S- 2CH

CuCu CuCu CuCu CuCu CuCu CuCu CuCu

CuCu CuCu CuCu CuCu CuCu CuCu CuCu CuCu CuCu CuCu

Step 2

Decomposition

DissolutionMake particles

Insulating paper

Absorption

Step 1

Copp

er

S-SCH2CH2

S-SCH2CH2CH2

CH2CH2

S-SCH2CH2

S-SCH2CH2CH2

CH2CH2

S-SCH2CH2CH2

CH2CH2

S-SCH2CH2CH2

CH2CH2CH2

CoordinationCu2S

S

Cu Cu

Cu2S

SS

CuCu CuCu

CuCu CuCu

S-SCH2CH2

S-SCH2CH2CH2

CH2CH2

Cu Cu

S-SCH2CH2

S-SCH2CH2CH2

CH2CH2

S-SCH2CH2CH2

CH2CH2

S-SCH2CH2CH2

CH2CH2CH2

CuCu CuCu

S-SCH2CH2

S-SCH2CH2CH2

CH2CH2

Cu Cu

S-SCH2CH2

S-SCH2CH2CH2

CH2CH2

S-SCH2CH2CH2

CH2CH2

S-SCH2CH2CH2

CH2CH2CH2

CuCu CuCu

S CH222 2CH S CH222 2CH

CH222 2CH CH222 2CH

ＤＢS

BiBZ

By-products

Reaction

DBDS-Cu complex

Diffusion

Proposed Cu2S formationmechanism


Main influential factors

• Corrosive sulphur in oil

• High temperature

• Oxygen content


Corrosive oilThe 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 passivators may block the effects of corrosive components, even under the severe conditions of these tests


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!


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


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)


Closed vs. open breathersInitial 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


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)


Test methods for detecting corrosive sulphur in oil

• Metal strip tests

• Tests involving copper and paper

• Chemical analysis (“speciation”)


• Silver strip tests

DIN 51353 100˚C, 18 hours

• Copper strip tests

ASTM D1275-A 140˚C, 19 hours(ISO 5662)

ASTM D1275-B 150˚C 48 hours

Metal strip tests


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


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

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

noncorrosive

corrosive

Evaluation of copper and paper after the test


Some researchers have pointed out the presence of a dominant sulphur containing compound in many corrosive oils.

This dominant compound was identified as dibenzyl-disulfide (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

Specific sulphur compounds


Gas Chromatography with sulphur-specific detector (e.g. AED) used to reveal the presence of DBDS

Oil with DBDSDominant peak found in sulphur chromatogram, identified as DBDS

Oil without DBDS


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


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


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


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


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

FDS (or DFR) Dielectric Response Method


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)


Power Factor tip-up


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


• Oil additives (metal passivators)

• Oil treatment

• Oil exchange

• Modified operating conditions

Mitigation techniques


• Experiences

• Consumption/ Monitoring

• Side effects

• Effects of oil treatment

Metal passivators


• 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 passivator content

Metal passivators


• Monitoring , analysis• HPLC method developed by TF02

Metal passivators

Irgamet3950 ppm

BTA50 ppm

Final results from RRT:


Metal passivator - depletion

0

20

40

60

80

100

120

24/0

3/20

06

02/0

7/20

06

10/1

0/20

06

18/0

1/20

07

28/0

4/20

07

06/0

8/20

07

14/1

1/20

07

22/0

2/20

08

01/0

6/20

08

09/0

9/20

08

Sampling date

Irga

met

39

con

c. (

mg/

kg)

0

20

40

60

80

100

120

140

24/0

3/20

06

13/0

5/20

06

02/0

7/20

06

21/0

8/20

06

10/1

0/20

06

29/1

1/20

06

18/0

1/20

07

Sampling dateIr

gam

et 3

9 c

onc.

(m

g/kg

)

2 different cases

Only some initial decrease(absorption by paper?)

Steady rate of depletion


0

200

400

600

800

1000

07-0

6-17

07-0

8-06

07-0

9-25

07-1

1-14

08-0

1-03

08-0

2-22

08-0

4-12

08-0

6-01

H2

(ppm

)• Stray gassing!

• Treatments on passivated oil:

- reclaiming will remove passivator- reconditioning OK (with care)

Passivator - side effects and caution


Oil treatments for sulphur removalcontinuous 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


Treatment of oil – general concerns

•Do not expect every combination of oil and treatment to work

•Not only corrosive sulphur is affected

•Always verify the result by established oil tests and the new severe tests for corrosive sulphur


• 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

Oil exchange


Winding Hot Spot Temperature

9092949698

100102104106108

12.00 15.00 18.00 21.00 0.00 3.00 6.00 9.00 12.00t ime

°C

Forced Air CoolingNatural Cooling

Operating conditions•Keep temperature down

- load restrictions- improved/forced cooling


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


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

•Remember:

Presence of Cu2S does not necessarily lead to failure


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


Brochure 378 available at www.e-cigre.org

Thank you for your attention!

Documents

Copper Sulphide in Transformer Insulation - Cigrea2.cigre.org/content/download/65343/3094823/version/1/file/A2.32... · •Identity mitigation techniques. ... (e.g. FDS, Power Factor