testing for corrosive sulfur effect in transformer oil report BY SHIVAKUMAR

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)

DEPT OF EE 1 GCE RAMANAGARA

Testing for Corrosive Sulfur Effects in Transformer oil2) THOSE UNIT THAT DO NOT HAVE COMPLETELY ENAMELED OR VARNISHED

CONDUCTORS.

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.

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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

Testing for Corrosive Sulfur Effects in Transformer oil

Chapter 3 (1) CORROSIVE SULFUR IN OIL TEST

T HIS 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 APRIL 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.THIS IS A VERY ROUTINE TEST.

Figure (1A) Figure (1B)

DEPT OF EE 4 GCE RAMANAGARA

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.

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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.

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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

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 ML 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

Testing for Corrosive Sulfur Effects in Transformer oil

Figure (2A) Figure (2B)

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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. THERE 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

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

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