02-01-2013, 03:31 PM
testing for corrosive sulfur effect in transformer oil
testing for corrosive sulfur.docx (Size: 1.03 MB / Downloads: 91)
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.
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.
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.
(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 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.