The Evolving Problem of Corrosive Sulfur in

Transformer OilIEEE/PES Transformers

CommitteeMemphis, Tennessee

Knowledge Is PowerSM

Apparatus Maintenance and Power Management for Energy Delivery

Corrosive Sulfur in Oils, and Transformers; Why it is Such a Problem

Lance R. LewandDoble Engineering Company

What is Corrosive Sulfur?

US Definition found in ASTM D 2864 -

“elemental sulfur and thermally unstable sulfur compounds in electrical insulating oil that can cause corrosion of certain transformer metals such as copper and silver”

Why is Corrosive Sulfur such a Problem?

• Reacts on contact with copper

• Does not require heat to promote the reaction

• Heat makes the effect more pronounced

• More pronounced in sealed systems

• May lead to deposition of copper-sulfur compounds in the paper insulation

• Copper-sulfur compound deposition in the paper insulation will lead to a weakened dielectric strength

The Problem

Ø Large power transformer and reactor failures starting in 2000. Doble has recorded about 25+ units that have failed worldwide

ØMany of these are units only 5 to 7 years olds, (represents high asset cost)

ØVery little advance warning:

§ No observable PD in tear downs

§ No generation of combustible gas even on the day before

Effects on Copper

Adverse Effects: Copper Conductor

Initiation of Attack

Copper Conductor

Copper, ≈ 95.5%Sulfur, ≈ 0.5%

Bulbous Structures on Cu Surface

Copper Conductor

Copper, ≈ 85% (80%)Sulfur, ≈ 15% (20%)

Copper(I) Sulfide -aka “Cuprous Sulfide”

Cu2S Layer Buildup

Compacted Copper Sulfide Layer

Area of continual buildup

Adverse Effects: NLTC Contacts

Cu2S Deposition on Conductor

Regular Paper

Kraft Paper Crepe Paper

Adverse Effects: Contaminated Paper

Adverse Effects: Plating on Paper

Effects on dielectric strength

Layer 1: 80 V/milLayer 2: 1050 V/milLayer 3: 1370 V/mil

Mechanism

Ø Process§ Corrosive Sulfur presence or formation§ Attack of metal surfaces, copper sulfide deposition on

conductor § Deposition of copper sulfide in paper insulation

Ø Copper ions migrate to the insulating paper adjacent to the conductor, react with corrosive sulfur compounds (or transfers over to the paper as a copper/sulfur compound).

Ø Mechanism: Reduction of dielectric strength - voltage of the conductor exceeds the insulating capacity of the paper insulation and BIL rating. Result: arcing between two or more turns/discs and a subsequent failure.

Failure

HV Winding Dissection

130TH Disk – near very top of transformer

97TH Disk – area of failure

10 TH Disk – near very bottom of transformer

29th Turn14th Turn1st Turn

Middle

Bottom

Top

Disk 10

Layer 1 Layer 2 Layer 3 Layer 4 Layer 5 Layer 6 Layer 7 Layer 8 Layer 9

0

200

400

600

800

1000

1200

Cu

Co

nce

ntr

atio

n, m

g/k

g

1st Turn,Copper Result, mg/kg14th Turn, Copper Result, mg/kg

29th Turn, Copper Result, mg/kgTurn 29

Turn 14

Turn 1

Disk 97

Layer 1 Layer 2 Layer 3 Layer 4 Layer 5 Layer 6 Layer 7 Layer 8 Layer 9(Crepe)

0

200

400

600

800

1000

1200

Cu

Con

cent

ratio

n, m

g/kg

29th Turn

7th Turn

1st Turn

Disk 130

Layer 1 Layer 2 Layer 3 Layer 4 Layer 5 Layer 6 Layer 7 Layer 8

S1

S30

200

400

600

800

1000

1200

Cu

Co

nte

nt

in P

aper

, mg

/kg

29th Turn

1st Turn

14th Turn

Example Conductor

1136 ppm233 ppm122 ppm

92 ppm91 ppm71 ppm86 ppm136 ppm

Copper Migration/Deposition

CoreLV

WindingHV

Winding

Disk 130

Disk 10

Disk 97

Designed Oil Flow

The oil flow lines in the HV winding take place in every disk just not those shown.

The 3 black dots represent the turn with the highest copper found in the testing. The size of the dot indicates the relative concentration.

Varnished wire, no paper

insulation

© C

opyr

ight

200

5 A

BB

All

right

s re

serv

ed

Insert image here

Specification & Testing of Transformer Oils with Respect to Corrosion

Clair Claiborne

ABB Inc, Power Technologies Div., Raleigh, NC

-

Background/History

F.M. Clark – 1962 – “Sulfur compounds are inevitably present in all commercial insulating oils” – Insulating Materials for Design and Engineering Practice

Transformer oils contain varying levels and kinds of sulfur compounds

Procedures were standardized to test for corrosive sulfur in the early 1950’s

Relatively few incidents since these standardized tests

-

Recent History

HVDC Converter Transformers and GSU’s

Reported:

§ CIGRE working group, Paris, 2004

§ ABB Review, 2004

§ IEEE Transformers HVDC section, Las Vegas, 2004

§ Doble Conference, Boston, 2005

§ ASTM D27, Reno, 2005

§ ABINEE, Sao Paulo, 2005

-

Conditions and Evaluations

Design and operation within industry standard practice§ No unusual temperature or other environmental factors

§ Sealed units with relatively low oxygen content in oil

§ Relatively high and constant load

Investigation: Cuprous Sulphide, Cu2S§ On insulation paper

§ On copper conductor

§ Other transformer components

-

0 2 4 6 8 10Energy (keV)

0

2

4

6cps

C

O

CuS

Cu Cu

SEM/EDX - Cu2S Deposition from HVDC Unit

SEM EDX

-

Current Situation

Mineral oil/Transformer oil requirements§ Oxidation stability more important to long life

Sulfur compounds desirable for enhanced stability

Some organic sulfur compounds act as peroxide scavengers in an oxygen-rich oil

§ Life extension of insulation overriding concern

§ Relatively few cases result from corrosive sulfur

Standards were believed to be strong enough to prevent problems

Only recently (last 5-10 years): Standard tests not always conclusive

-

ASTM Test D 1275 Corrosive Sulfur in Electrical Insulating Oils

.

§ ASTM D 1275 used since 1953 – Superseded ASTM D 117

§ D 117: 5 hours at 100oC. D 1275: 19 hours at 140oC.

§ F.M. Clark and E. L. Raab, Proc. ASTM, Vol. 48, 1948, pp. 1201- 1210:

§ Demonstrated inadequacy of D 117 at this low temperature and duration. Could not identify corrosive oils

§ For same level of tarnish, D 117 took 432 hours, but only 5 hours at 140oC (temperature chosen for D 1275).

§ D 117 sensed only free sulfur.

-

Examples of Copper Corrosion Tests

ASTM D 1275: Copper Based Test Oils at 140oC, 19 hrs

New oil

(Plant oil)

Field Unit 1: Very slightly corrosive

Field Unit 2: Slightly corrosive

-

Definition of Corrosive Oil - ASTM D 1275

Appearance of Copper Strip

Noncorrosive:

Orange, red lavender, multicolored with lavender, blue or silver, or both, overlaid on claret red, silvery, brassy or gold, magenta overcast on brassy strip, multicolored with red and green showing (peacock) but no gray

Corrosive:

Transparent black, dark gray or dark brown, graphite or lusterless black, glossy or jet black, any degree of flaking

-

DIN 51353 – Detection of Corrosive Sulfur –Silver Strip Method

§ Prüfung auf korrosiven Schwefel - Silberstreifenprüfung

§ Deutsches Institut für Normung e. V.

§ Used in IEC (mostly outside USA)

§Previous Editions: Jan. 1965, Sept. 1977

§Current Edition: Dec. 1985

§ Adopted in response to many color shades on copper

§ Same sample configuration but silver strip

-

DIN 51353: Silver Based Test Oils at 100oC, 18 hrs

New oil

(Plant oil)

Field Unit 1: Very slightly corrosive

Field Unit 2: Slightly corrosive

Examples of Silver Corrosion Tests

-

Non-corrosive:

No noticeable affect, or a weak golden yellow discoloration

Corrosive:

Light grey or brown shade to a distinct grey up to black

Definition of Corrosive Oil - DIN 51353

Appearance of Silver Strip

-

ASTM D 1275 vs. DIN 51353

Two tests with very nearly the same result

ASTM D 1275

Copper strip in oil

Nitrogen bubbled

1 minute

19 hours @ 140oC

DIN 51353

Silver strip in oil

Loose fitting cap

(oxygen)

18 hours @100oC

-

Alternative Corrosion Tests

n Doble Engineering Extended/Modified ASTM D1275Same parameters as D1275 except:

n 48 hours @ 150oC (replacing 19 hours @ 140oC)

n ASTM D 5623 – Sulfur Compounds in Light Petroleum by Gas Chromatography and Sulfur Selective Detection

n Potentiometric titration method to determine mercaptan level

n ABB Covered Conductor Corrosion & Deposition (CCCD) test

-

Why different tests?

n Not all sulfur compounds react in same way

n Environmental dependences:

Presence/Absence of oxygen

Different temperatures

Presence of passivators

n Not all transformers operate with same conditions

-

Experimental Set-up - CCCD

Gas

Thermostattedblock

Tube with sample

-

Experimental Arrangements - CCCD

Cu + pressboard Cu + paper + excess Cu Cu + layered paper

-

Summary: Methods to Study Problem

n Cu2S deposition reproduced in laboratory

Test materials & environment similar to real transformer service, e.g. temperature, oxygen content etc

n Result: Cu2S deposition can be reproduced

On conductor

On paper facing the conductor

On free cellulose surfaces

Deposition can occur at low temperatures, 80oC and 100oC

Time required for test 12 weeks at 100oC, 3 weeks 120oC

-

Promising Tests

§ Metal strip test method such as D 1275 --higher temperature and longer timeProduces quick screening test

Somewhat sensitive to oxygen

Proposed to ASTM but not accepted – Too few actual cases

§ New ABB CCCD test method preferredProduces results we want to avoid in transformers

Relatively time consuming

Reliable, produces results close to real case

-

Conclusions

n Sulfur and potentially corrosive sulfur have always been present

in transformer oil

n Currently available tests not always completely capable of

finding potential problems

n Further development and verification of new standard methods

should be (and is) being pursued

The Basics of Crude Oil Selection and Refining

IEEE/PES Transformers Committee Fall 2005 Meeting

Crude Oil Selection

Considerations1) Availability/Logistics: How much is there and where is it.

How does it get to the plant?

2) What does it look like: Can we run it? Sweet/Sour vs. Plant Design

3) Will it work?: Target Markets and Specifications

Sulfur evaluations are typically for overall sulfur content only– no speciation of specific sulfur compounds is necessary

Major Refining Steps for Naphthenic OilsStep ObjectiveDistillation Split into desired Fractions for:

ViscosityBoiling RangeVolatilityFlash Point

Hydrotreating Convert Aromatics to Naphthenics for:Better Heat StabilityBetter Color StabilityControl of CompatibilityRemoval of Impurities

Solvent Extraction Removes Aromatics which:Improves StabilityReduces Compatibility

2005 Calumet Lubricants Co.

REDUCEDCRUDE

VACUUMDISTILLATION

CRUDEUNIT

CRUDES

REFINED LUBE PROCESSINGNAPHTHENIC

OR

OR

FINAL DISTILLATION

SO

LV

EN

TE

XT

RA

CT

ION

AROMATICEXTRACTS

SO

LV

EN

TE

XT

RA

CT

IONAROMATICEXTRACTS

HYDRO-GENATION

HYDRO-GENATION

FINISHEDPRODUCTS

OR

OR

IEEE TC – Fall 2005

Corrosive Sulfur in Oil

Joao Baldauf

Two new Transmission lines in the Brazilian Grid :

North/South II

Southeast/Northeast

IEEE TC – Fall 2005

North/South Line:

o 30 single phase reactors 550/v 3 kV, 55 MVAr plus 6 spare units.In service 1.5 years.

12 units with oil “A” and 24 units with oil “B”

IEEE TC – Fall 2005

Southeast/Northeast Line:

o 03+1spare reactors 500/v 3 kV 45.3 MVAro 06+2spare reactors 500/v 3 kV 33.3 MVAro 12+2spare reactors 500/v 3 kV 66.6 MVAro 09+2spare reactors 500/v 3 kV 50.0 MVAr

In service 2.5 years. All units with oil “A”

IEEE TC – Fall 2005

o All reactors were filled with oil tested according to ASTM D 1275 and the result was “non corrosive”.

o After appoximately 6 month in service some units of North/South line filled with oil “A” presented corrosive sulfur when tested with the same method.

o Units filled with oil “B” did not test positive for corrosive sulfur

IEEE TC – Fall 2005

o In march 2005, after approx. one year in service, one of the units of the North/South line with oil “A” returned to factory, without having failed, to be examinated.

o A new heat run test with sensors in the winding indicated a maximum hot spot of 59.8°C ( copper/ambient).

o DP of paper varying from 900 to 1000o The copper of 1/3 of the winding was

contaminated with copper sulfide that migrated to the first two layers of insulating paper.

IEEE TC – Fall 2005

Copper from the middle of upper disc

IEEE TC – Fall 2005

First two layers of paper

IEEE TC – Fall 2005

Test made with reactor conductor samples

IEEE TC – Fall 2005

o August 2005 one of the 33.3 MVAr reactors of the Southeast/Northeast line that failed after 2.5 years in service was opened in the factory.

o All the copper was contaminated with copper sulfide.

o Until the end of september 2005 eight reactors from different manufacturers of this line failed, all with oil “A”

IEEE TC – Fall 2005

Failure in the second disc from the top

IEEE TC – Fall 2005

Contaminated copper and first seven paper layers

IEEE TC – Fall 2005

o There is also a transmission line of another utility with:

6+1 spare reactors 550/v 3 55 MVAr6+1 spare reactors 550/v 3 35 MVAro All filled with oil “A” with corrosive sulfur

and energized since the end 2002 (approx. the same age of the Northeast/Southeast line ).

o No one failed.

IEEE TC – Fall 2005

o To finish three conclusions and one question:

1- The copper sulfide migrates from the conductor to the insulation paper reducing it´s dielectric strenght.

2- The corrosion of the copper conductor begins with temperatures below those allowed by the standards and is more significant the higher the temperature.

3- The contamination (copper and paper) increases with the time.

o 1- Why some TL are more subjected to failures than others? (Transients ?)

Tamyres Luiz Machado Junior

Corrosive Sulphur

23.10.2005

Since December 2004 , 12 Single Phase Shunt Reactors 525 kV of the North – Southeast Brazilian Interconnection failed due to the problems with corrosive sulfur (7 units delivered by Siemens and 5 units by another Brazilian manufacturers)

Recently, a second single phase GSU units of Nuclear PowerStation – Angra 2 failed.The ASTM special tests was performed in the oil of all four units and it was detected the presence of corrosive sulfur in the oil of the two failed units.The failure investigation showed a high degree of copper sulphide contamination on the paper insulation of the tap leads and HV winding conductors . The most likely cause of the failure is still in discussion.

Corrosive Sulfur

23.10.2005

Single Phase Shunt Reactors 50 MVAr and 66 MVAr

525 kV

23.10.2005

Reactor core&coil assembly

23.10.2005

Reactor Failure

23.10.2005

Reactor Failure

23.10.2005 Inner strand of twin

outer strand of twin Inner strand of twin

Outer strand of twin

Turn at the outer diameter Turn at the inner diameter

Disc conductors from the upper part

Reactor Failure

23.10.2005

Copper Sulphide deposits at different disc locations

Reactor Failure

23.10.2005

GSU Transformer Failure

Source : EPRI Report 09/2005 to Eletronuclear – Mr. Nichols C. Abi-Samra

23.10.2005

GSU Transformer Failure

Source : EPRI Report 09/2005 to Eletronuclear – Mr. Nichols C. Abi-Samra

23.10.2005

GSU Transformer Failure

Source : EPRI Report 09/2005 to Eletronuclear – Mr. Nichols C. Abi-Samra

23.10.2005

GSU Transformer Failure

Source : EPRI Report 09/2005 to Eletronuclear – Mr. Nichols C. Abi-Samra

23.10.2005

GSU Transformer Failure

Source : EPRI Report 09/2005 to Eletronuclear – Mr. Nichols C. Abi-Samra

23.10.2005

GSU Transformer Failure

Source : EPRI Report 09/2005 to Eletronuclear – Mr. Nichols C. Abi-Samra

23.10.2005

Copper Sulphide deposit mechanism

2. Catalytic deposition of Cu2S

1. Superficial corrosion of copper

23.10.2005

Failure Mechanism

COPPER

COPPER

PAPER

PAPER

23.10.2005

Failure Mechanism

COPPER

COPPER

PAPER

PAPER

23.10.2005

Sulfur Source

Fonte: Nynas

Source: Nynas

SourceS: EPRI Report 09/2005 to Eletronuclear – Mr. Nichols C. Abi-SamraNynas report

23.10.2005

Corrosive Sulfur Test

23.10.2005 Source : EPRI Report 09/2005 to Eletronuclear – Mr. Nichols C. Abi-Samra

23.10.2005

Oil Passivator (Nypass)

The passivator protects the copper surface against to the new attack of the corrosive sulfur.

The passivator can stop the cooper sulphide generation but it is not able to remove the existing deposits on the paper insulation.

Conductor surface

23.10.2005

Recommended Actions

•Oil Corrosive Sulfur test performs using the ASTM modified test (150 ºC and 48h with O2 removed).

•Addition of Passivator where the corrosive sulfur attack are still not critical, according to oil supplier recommendations.

23.10.2005

Open questions

• Critical operating temperature level and surround conditions.

• Breakdown mechanism

•Diagnostic method to evaluate the contamination degree and the insulation strength reduction.

• Long term performance of the Passivator.

Corrosive Sulfur Tests ResultsSalt River Project

By Tom Lundquist

Transformer Oil Test ResultsModified ASTM D-1275

• 198 transformer tested

• 19% (38) failed the Modified test

• 2.0% (4) failed the standard D-1275 test

• All were built between 1998 & 2004

• 4 manufacturers transformers involved

Transformer Oil Test ResultsModified ASTM D-1275

• No Significant Indicators from oil screen tests

• 218,000 gallons of oil

• $22 million transformers at risk

• At least 2 oil refiners involved one unknown

Transformer Oil Test ResultsModified ASTM D-1275

• Both inhibited and uninhibited oils

• To stop the corrosive sulfur issue– adding metal deactivators to the transformer

oil at 100 PPM.

– Require any new oil shall pass the Modified D1275 test before receipt.