Shivakumar report for testing for corrosive sulfur effect in transformer oil

1. Testing for Corrosive Sulfur Effects in Transformer oil
ABSTRACT
OVER THE PAST SEVERAL YEARS THERE HAVE BEEN A NUMBER OF FAILURES OF POWER
TRANSFORMERS AND REACTORS WHERE THERE ARE INDICATORS OF COPPER CORROSION
AND FORMATION OF COPPER SULFIDES ON CONDUCTORS AND INSULATING MATERIALS .
THE CONDUCTIVE COPPER SULFIDE REDUCES THE DIELECTRIC BREAKDOWN VOLTAGE
OF THE SOLID INSULATION. THIS CAN RESULT IN A DIELECTRIC PUNCTURE THROUGH THE
PAPER INSULATION . THE FAILURES HAVE OCCURRED AFTER THE APPARATUS HAVE BEEN IN
SERVICE FOR A YEAR OR MORE , OFTEN SEVERAL YEARS . AT NORMAL OPERATING
TEMPERATURES THE CORROSION PROCESS APPEARS TO TAKE TIME TO FORM CRITICAL
AMOUNTS OF CONDUCTIVE SULFIDES . TO DATE THE FAILURES HAVE OCCURRED WITHOUT
PRIOR EVIDENCE OF ABNORMAL GASSING BEHAVIOR ; THEREFORE, IT APPEARS THAT THIS
PROBLEM IS DIFFICULT TO DETECT AND MANAGE .
THIS PROBLEM IS FAIRLY WIDE KNOWN IN THE LARGE POWER UTILITIES BUT LESS SO
WITH INDUSTRIAL AND COMMERCIAL USERS WITH OIL - FILLED ELECTRICAL EQUIPMENT,
BUT THE SAME ISSUES CAN ARISE. FAILURES DUE TO CORROSIVE SULFUR ATTACK HAVE
ALSO BEEN DOCUMENTED IN LOAD TAP -CHANGERS . ONE OF THE MOST FREQUENTLY ASKED
QUESTIONS IS “WHICH APPARATUS IS MOST SUSCEPTIBLE ?” IN RESPONSE TO THIS ,
DOBLE HAS DEVELOPED THE FOLLOWING LIST :
1) UNITS THAT HAVE THE FOLLOWING CHARACTERISTICS
A) MANUFACTURED FROM 1999 TO PRESENT
B) OPERATE AT HIGH TEMPERATURES OVER LONG PERIODS OF TIME SUCH AS GENERATOR
STEP -UP TRANSFORMERS AND REACTORS AND CONTINUOUSLY FULLY - LOADED
TRANSFORMERS
C) ARE EITHER GAS BLANKETED OR HAVE A SEALED CONSERVATOR SYSTEM (A FEW
FAILURES HAVE OCCURRED WITH FREE -BREATHING CONSERVATORS )
D) HAVE OILS THAT FAIL THE CORROSIVE SULFUR TESTS (ASTM D 1275B AND
CCDTESTS )
2) THOSE UNIT THAT DO NOT HAVE COMPLETELY ENAMELED OR VARNISHED CONDUCTORS.
DEPT OF EE 1 GCE RAMANAGARA

2. Testing for Corrosive Sulfur Effects in Transformer oil
CHAPTER 1
INTRODUCTION
Corrosive sulfur and the effects that it has in transformer system can be significant. The
extent of the corrosion damage caused by sulfur, if left unchecked, can be so severe as to
failure of the apparatus. The problems with corrosive sulfur have been recognized for quite
some time. As early as 1948, F.M. Clark and E.L. Raab [1] issued a report on the subject for
method development within what is know now as ASTM committee D 27 and eventually
became ASTM method D 1275. Sulfur is found in many materials of transformer
constriction including the copper, paper insulation, gaskets and oil. Not all sulfur is
considered corrosive but the tendency to operate transformer at substantially higher
temperatures can aggravate an already present corrosive sulfur condition or convert stable
compound into reactive ones that will cause damage.
IN THE FAST SEVERAL YEARS, IN RESPONSE TO THIS PROBLEM, THERE HAS BEEN A GREAT
DEAL OF RESEARCH ACTIVITY IN DEVELOPING NEW TEST TO HELP IDENTIFY OILS WITH THE
POTENTIAL TO HAVE CORROSIVE SULFUR PROBLEMS SOME OF THE TESTS ARE IN VARIOUS
STAGES OF DEVELOPMENT AND ARE BROKEN DOWN INTO TWO GROUPS .
1. OIL TESTS.
2. TRANSFORMER ELECTRICAL TEST.
BUT WE ARE DISCUSSING ONLY ABOUT TRANSFORMER OIL.
DEPT OF EE 2 GCE RAMANAGARA

3. Testing for Corrosive Sulfur Effects in Transformer oil
Chapter 2
OIL TESTS
These tests aid in determining if corrosive sulfur compounds are present in the oil or
identifying sulfur compounds that are presently stable but may become corrosive under
conditions that exist in the transformer (temperature, time, oxygen, etc.). It must be
remembered that some of these tests are accelerated aging tests and do not represent
transformer environment. The purpose of the accelerated aging test is to get a reasonable
expectation of what may occur without testing the oil for years instead of a few days. A
listing of some of the tests used with descriptions follows:
1. ASTM D 1275B, Corrosive Sulfur in Oil.
2. ASTM D 5623, Sulfur Speciation.
3. ASTM D 4294, Total Sulfur in Oil.
4. ASTM D 3227, Mercaptans in Oil.
5. Doble Covered Conductor Deposition (CCD) Test, Doble Test.
6. Dibenzyl disulfide (DBDS in Oil), Doble Test.
DEPT OF EE 3 GCE RAMANAGARA

4. Testing for Corrosive Sulfur Effects in Transformer oil
Chapter 3 (1) CORROSIVE SULFUR IN OIL TEST
THIS TEST AGES 220 MLS OF OIL IN CONTACT WITH A COPPER STRIP IN A SEALED VESSEL
FOR 48 HOURS AT 150°C. THE PRIMARY PURPOSE OF THE TEST IS TO DETERMINE IF ANY
CORROSIVE SULFUR COMPOUNDS IN THE OIL WILL REACT WITH THE COPPER STRIP TO TURN
IT GRAY OR BLACK . IT IS A SUBJECTIVE TEST IN THAT THERE IS COMPARISON OF COLORS OF
THE COPPER STRIP WITH SOME COLORED STANDARDS AND A TABLE OF DESCRIPTIONS
LISTING WHAT IS CORROSIVE AND NONCORROSIVE (FIGURE 1A AND 1B). THE TEST WAS
RECENTLY MODIFIED FROM JUST D 1275 IN A PRIL OF 2006 TO D 1275AAND D 1275B. THE
B METHOD IS MORE RIGOROUS THEN THE OLD METHOD D 1275 OR D 1275A AND WAS
DEVELOPED IN RESPONSE TO THE PROBLEM WITH CORROSIVE SULFUR . THE ISSUE WAS
THAT SOME OF THESE OILS MET THE REQUIREMENT OF THE OIL SPECIFICATIONS USING THE
OLD D 1275 (D 1275A) TEST YET BECAME CORROSIVE WHILE IN SERVICE AND
EVENTUALLY CAUSED FAILURES OF TRANSFORMERS, LTCS, AND REACTORS . ASTM D
27 COMMITTEE RECOGNIZED THIS FLAW AND MODIFIED THE TEST METHOD TO INCLUDE D
1275B.T HIS IS A VERY ROUTINE TEST.
Figure (1A) Figure (1B)
DEPT OF EE 4 GCE RAMANAGARA

5. Testing for Corrosive Sulfur Effects in Transformer oil
(2)SULFUR SPECIATION
The ASTM D 5623 analysis is performed by gas chromatography with sulfur selective
detection and covers the detection of volatile sulfur-containing compounds. The test method
will not identify all individual sulfur components. Detector response to sulfur is linear and
essentially equimolar for all sulfur compounds; thus, both unidentified and known individual
compounds are determined. However, many sulfur compounds, for example, hydrogen
sulfide andmercaptans, are reactive, and their concentration in samples may change during
sampling and analysis. This test can be used to isolate specific sulfur compounds, some of
which may or may not be responsible for corrosive sulfur attack. One of the issues is that
depending on the lab, the database of sulfur compounds that can be analyzed can be small or
large but usually not more than about 70 compounds. There are thousands of sulfur
compounds, and this test does not have the ability to cover them all. Certain types of sulfur
compounds cannot be fully isolated and identified. It is usually not considered a routine test
and is mostly used for research purposes.
DEPT OF EE 5 GCE RAMANAGARA

6. Testing for Corrosive Sulfur Effects in Transformer oil
(3)TOTAL SULFUR IN OIL
THERE ARE ACTUALLY SEVERAL ASTM TESTS THAT CAN BE USED FOR THE DETECTION OF
TOTAL SULFUR IN OIL . THIS JUST HAPPENS TO BE THE ONE THAT IS USED BY OUR
LABORATORY . THIS TEST DETERMINES THE TOTAL AMOUNT OF SULFUR IN THE OIL BUT
DOES NOT DETERMINE WHETHER OR NOT THE COMPOUNDS BEING DETECTED ARE
CORROSIVE . TRANSFORMER OIL IS MADE FROM PETROLEUM CRUDE THAT HAS
NATURALLY OCCURRING SULFUR . DEPENDING ON WHERE IN THE WORLD THE OIL IS FROM
DICTATES THE AMOUNT OF SULFUR IN THE CRUDE THE PROCESS OF REFINING THE CRUDE
TO TRANSFORMER OIL AND OTHER PRODUCTS ATTEMPTS TO REMOVE REACTIVE
(CORROSIVE) SULFUR COMPOUNDS BY CONVERTING THEM TO HYDROGEN SULFIDE GAS
WHICH IS EASILY REMOVED FROM THE PROCESS . THE REFINING PROCESS CAN ALSO
CONVERT SOME OF THE LESS STABLE SULFUR COMPOUNDS INTO MORE STABLE UNCREATIVE
SULFUR COMPOUNDS THAT CAN PROVIDE BENEFITS IN THE FINAL REFINED PRODUCT. THE
AMOUNT OF CONVERSION AND REMOVAL DEPENDS ON THE CRUDE AND THE REFINING
PROCESS ITSELF, AS EACH REFINER HAS ITS OWN DISTINCTIVE PROCESS. MOST MODERN
TRANSFORMER OILS HAVE A FINAL SULFUR CONTENT OF LESS THAN 1500 PPM, AND SOME
ARE EVEN LESS THAN 10 PPM. THIS IS A ROUTINE TEST, BUT IT ONLY PROVIDES
INFORMATION ON HOW MUCH SULFUR IS IN THE OIL, NOT WHETHER IT IS DELETERIOUS OR
NOT.
DEPT OF EE 6 GCE RAMANAGARA

7. Testing for Corrosive Sulfur Effects in Transformer oil
(4)MERCAPTANS IN OIL
MERCAPTANS ARE SULFUR COMPOUNDS THAT CAN BE VERY REACTIVE . THIS IS A
POTENTIOMETRIC TITRATION TEST THAT IS USED TO DETERMINE THE CONCENTRATION OF
MERCAPTANS AS A CLASS OF COMPOUNDS IN THE OIL, BUT IT WILL NOT BE ABLE TO
IDENTIFY SPECIFIC COMPOUNDS . THIS TEST IS USED IN THE FUEL AND LUBRICATION
INDUSTRIES OFTEN AND IS STARTING TO BE USED IN THE TRANSFORMER OIL INDUSTRY .
ONE OF THE PROBLEMS WITH MERCAPTANS IS THAT SOME OF THEM ARE HIGHLY VOLATILE
AND THUS NOT EASILY SAMPLED FOR . IT IS PRESENTLY NOT A ROUTINE TEST, BUT IT IS
BEING USED MORE AND MORE.
DEPT OF EE 7 GCE RAMANAGARA

8. Testing for Corrosive Sulfur Effects in Transformer oil
(5)Doble Covered Conductor Deposition (CCD) Test, Doble Test
THERE ARE TWO OTHER VARIATIONS , ONE DEVELOPED BY ABB AND THE OTHER BY
SIEMENS, WHICH PRECEDED THE DOBLE METHOD. A VARIATION OF THE SIEMENS METHOD
IS BEING DEVELOPED INTO AN IEC TEST. THIS NEW TEST IS VERY IMPORTANT IN THAT THE
PURPOSE OF THE TEST NOT ONLY DETERMINES IF THE CORROSIVE SULFUR WILL ATTACK
THE COPPER AND FORM COPPER SULFIDE , BUT ALSO IF COPPER SULFIDE FORMATIONS WILL
DEVELOP IN THE PAPER INSULATION . THE TEST IS CONDUCTED BY TAKING AN ABRADED
COPPER ROD AND WRAPPING NEW KRAFT PAPER INSULATION AROUND THE ROD . TWO RODS
ARE PREPARED FOR EACH SAMPLE, AND THEY ARE PLACED IN 20 M L HEADSPACE VIALS .
OIL IS ADDED AND THEN THE VIALS ARE SEALED . A STAINLESS STEEL NEEDLE OF A
PARTICULAR DIAMETER IS THEN PIERCED THROUGH THE SEPTUM OF ONE VIAL AND LEFT
THERE TO ALLOW AIR INGRESS DURING AGING (SEE FIGURE 2A AND 2B IN NEXT PAGE I. E.
PAGE NO 9). THE VIALS ARE THEN AGED AT 140°C FOR FOUR DAYS . AT THE END OF THE
AGING CYCLE , THE VIALS ARE REMOVED AND THE COPPER RODS WITH PAPER RETRIEVED.
THE PAPER IS THEN REMOVED FROM THE COPPER , AND BOTH ARE WASHED IN A SOLVENT
TO REMOVE THE OIL . THE ROD IS INSPECTED TO DETERMINE IF IT HAS BEEN TARNISHED BY
CORROSIVE SULFUR , AND THE PAPER IS INSPECTED TO DETERMINE IF DEPOSITION HAS
OCCURRED . THE PRESENCE OF DEPOSITION IN THE PAPER IS IMPORTANT AS MOST OF THE
RECENT TRANSFORMER FAILURES HAVE BEEN DUE TO COPPER SULFIDE DEPOSITS IN THE
PAPER CAUSING A SEVERE REDUCTION IN DIELECTRIC STRENGTH . THE RESULTING
"DEPOSITION, "WHETHER HEAVY , MODERATE, OR LIGHT , MEANS THAT THE INTERACTION
OF THE COPPER AND OIL WITH THE PAPER INSULATION OVER THE DURATION OF THE TEST
RESULTED IN AN OBVIOUS DEPOSIT OF COPPER SULFIDE, COPPER , OR OIL/ PAPER
DEGRADATION BY -PRODUCTS ONTO OR IN THE PAPER INSULATION . THIS IS BECOMING A
VERY ROUTINE TEST.
DEPT OF EE 8 GCE RAMANAGARA

9. Testing for Corrosive Sulfur Effects in Transformer oil
Figure (2A) Figure (2B)
DEPT OF EE 9 GCE RAMANAGARA

10. Testing for Corrosive Sulfur Effects in Transformer oil
(6)DIBENZYL DISULFIDE (DBDS IN OIL), DOBLE TEST
DBDS IS A SULFUR COMPOUND FOUND IN CERTAIN TRANSFORMER OILS . DBDS IS NOT
THERMALLY STABLE AT HIGHER TEMPERATURES AND BREAKS DOWN INTO BENZYL
MERCAPTAN WHICH IS VERY CORROSIVE AND ATTACKS THE COPPER QUITE QUICKLY . IT
SHOULD BE REMEMBERED THAT DBDS IS NOT THE ONLY COMPOUND TO CAUSE SEVERE
COPPER SULFIDE DEPOSITION ON THE COPPER . T HERE ARE OTHER COMPOUNDS THAT CAUSE
CORROSIVE SULFUR ATTACK BUT AS OF YET HAVE NOT BEEN IDENTIFIED. THIS IS NOW A
VERY ROUTINE TEST.
DEPT OF EE 10 GCE RAMANAGARA

11. Testing for Corrosive Sulfur Effects in Transformer oil
CONCLUSION
CORROSIVE SULFUR ATTACK CAUSING FAILURE IN OIL- FILLED ELECTRICAL APPARATUS IS
OCCURRING MORE FREQUENTLY ALTHOUGH THE NUMBER OF FAILURES ARE STILL A LOW
PERCENTAGE OF THE POPULATION . HOWEVER, THE LOSS OF REVENUES FROM THESE UNITS
AS WELL AS CAPITAL EQUIPMENT IS SIGNIFICANT . AS A RESULT, NUMEROUS RESEARCHERS
ARE ENGAGED IN ACTIVITIES TO DETERMINE AND ELIMINATE THE SOURCE(S ) OF THE
CORROSIVE SULFUR . BECAUSE OF THIS , MANY TESTS ARE BEING USED IN CONCERT WITH
EACH OTHER TO DETERMINE THE CORROSIVE SULFUR ASPECTS OF THE OILS USED IN THE
ELECTRICAL APPARATUS . THESE TESTS WILL BE USED TO SCREEN OUT OR IDENTIFY OILS
THAT MAY NOT BE SUITABLE.
DEPT OF EE 11 GCE RAMANAGARA

12. Testing for Corrosive Sulfur Effects in Transformer oil
References
 Claiborne, c.clair “recent increases in transformer failure due to
corrosive sulfur,”
 Leward, lance .r “passivators- what they are and how they work”
 Petro- Canada lubricants hand book, Mississauga, Ontario, 2007.
 American society for testing & materials.
DEPT OF EE 12 GCE RAMANAGARA