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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.