by Lance R. Lewand Doble Engineering Company
A previous article provided an introduction to sulfur in transformer systems and some of the negative eff ects that corrosive or reactive sulfur can have on a transformer. However, transformer mineral oil is not the only material containing sulfur, and this article will explore some of the other sources. Sulfur compounds are also present in gaskets, some water-based glues, copper, and paper insulation used in the manufacture of transformers. Sulfur can also be introduced into the transformer through accidental means such as through the use of incompatible hoses.
It is generally accepted that older gaskets used in trans- former applications were made from cork, cork/glyptal, and corkprene. In more recent years, the most oil-compatible gaskets have been nitrile rubbers such as BUNA-N, fl uo- roelastomers such as VITON®, or fl uorosilicone rubbers. Properly made nitrile rubbers of the correct grade (butadiene acrylonitrile) and fl uoroelastomers (fl uorinated hydrocar- bon) are excellent gaskets for use in transformers. In the manufacture of some of these materials such as the nitriles, sulfur is used in the curing process when the formulations are being developed into a hardened material. Th e curing process is supposed to eliminate all sulfur from the fi nished product. Most gasket manufacturers assume the sulfur is eliminated after the curing process. In some cases, the con- centration of sulfur contained in the fi nal gasket product is not monitored.
Doble Engineering performed scanning electron mi- croscopy/energy-dispersive x-ray analysis (SEM/EDX) on numerous gaskets taken from recently manufactured transformers. Each gasket was prepared for analysis by cleaning the outside surface with a sulfur-free hydrocar- bon solvent. Th e gasket was then cut lengthwise to reveal the inside surface. Th e outside and inside surfaces of each gasket were coated with evaporated graphite. Th e samples were then subjected to SEM/EDX analysis in which an electron beam of the scanning electron microscope enters the bulk of a sample producing an x-ray emittance. Th e x-ray peak positions along the energy scale identify the ele-
ments present in the sample and can provide the percentage concentrations of each of these elements, thus providing an elemental breakdown of the material or particles. Results from two gaskets are shown in Tables 1 and 2.
TABLE 1
Elemental Composition of an O-ring Radiator Gasket
ELEMENT Outside Surface Inside Surface
Organic Component ≈80% ≈80% Silicon 8.0% 2.1% Zinc 5.4% 8.3% Sulfur 4.3% 8.2% Titanium 1.2% 0.0% Calcium 1.1% 1.2% Aluminum 0.0% 0.2%
TABLE 2
Elemental Composition of a Butterfl y Valve Flat GasketELEMENT Outside Surface Inside Surface
Organic Component ≈80% ≈80% Silicon 9.5% 9.0% Aluminum 4.9% 5.7% Zinc 2.4% 2.3% Sulfur 2.1% 2.1% Copper 0.4% 0.0% Calcium 0.3% 0.3% Titanium 0.3% 0.3% Iron 0.1% 0.3%
Both gaskets contained a large amount of sulfur, especially the O-ring gasket. Th e SEM/EDX analysis was performed on the inside surface of the gasket to determine if the out- side surface had possibly been contaminated with corrosive sulfur from the oil. It is clear that sulfur is a component of both original gaskets. Th e original formulations for a nitrile rubber, fl uoroelastomer, or a fl uoro-silicone rubber (a fl uoro- polydimethylsiloxane) do NOT contain any sulfur.
In discussions with elastomer manufacturers, it was found that very few manufacturers (except for E.I. DuPont) were performing any chemical testing on the fi nished product to determine what amount of sulfur, if any, remained. Th ere also does not appear to be any standard on what percentage of sulfur should remain in the fi nal product. It then becomes obvious that the onus is on the fi nal user of the material to specify a sulfur-free or low-sulfur material for use or to test the material prior to use.
In light of this information, additional SEM/EDX analy- sis was performed on gasket material available in the Doble Materials Laboratory. One sample was a fl uoroelastomer, and another was a nitrile rubber produced by Parker. Th e results are shown in Tables 3 and 4.
Note: SEM/EDX analysis cannot quantify elements such as fl uorine, nitrogen, oxygen, carbon, hydrogen, and boron but can sometimes give a qualitative indication of the amount in high enough concentrations.
As shown in the two tables, the fl uoroelastomer shows no sulfur on the inside surface and very little on the outside, suggesting that material was cured correctly. Th e opposite is true of the Parker nitrile material, which shows a very high sulfur content on both surfaces, suggesting that the sulfur was not removed after the curing process.
Water-based glues, used to secure the paper insula- tion during manufacture, may sometimes contain sulfur compounds. Th ere has been at least one known instance in which the glue used in the manufacture of the windings has contributed to a corrosive sulfur condition.
Most coppers used in manufacturing transformer wind- ings contain some impurities. Sulfur happens to be one of those impurities, along with silver, arsenic, phosphorous, tellurium, and oxygen. Th e amount of sulfur allowed in most of the electrical grades of copper is 15 parts per million or less. Analyses performed on random copper samples from windings showed that the sulfur contents were very low at fi ve parts per million or less. However, care still has to be taken in the selection of materials used in construction so that copper with a high sulfur content is not used.
Th e pulping process for coated transformer paper, such as electrical Kraft paper, converts wood chips to cellulose by removing the majority of lignin (95-98.5 percent) and other impurities. Th ere are two basic processes:
• Th e sulfi te process is considered an acidic process and uses sulfur dioxide, sulfuric acid, and calcium bisulfi te. • Th e main process used today and the one that is used
to produce electrical grade coated transformer papers is the sulfate process, which is also called “alkaline pulp- ing.” Sodium hydroxide and sodium sulfi de are used in what is termed the “cooking process.” Th e cooking process under conditions of heat, pressure, and chemi- cals (pulping liquors) removes the lignin and impuri- ties from the wood chips in order that only cellulose remains. Th e pulping liquor is removed and recycled for use again, and the remaining cellulose pulp is washed several times to remove as much of the pulping liquor as possible from the cellulose pulp.
Th e Kraft process is slightly diff erent, in that the same chemicals are used, but the pulp is intentionally undercooked and results in the darker color of the paper as well as ex- ceptional mechanical strength. Th e pulp fi bers in the Kraft process do absorb some of the sulfur compounds that can- not be removed via the washing/rinsing process. Tests were performed to determine how much total sulfur remains in the fi nished paper products. Th e fi rst analysis performed was SEM/EDX analysis of new Kraft and thermally-upgraded (TU) Kraft papers from United States manufacturers. Th ese results are listed in Table 5 and are for the surface of the paper only.
TABLE 3
Elemental Composition of a Fluoroelastomer Gasket
ELEMENT Outside Surface Inside Surface
Organic, Fluorine Component >90% >90% Calcium 6.88% 7.40% Magnesium 1.06% 2.47% Chlorine 0.31% 0.13% Silicon 0.40% 0.0% Phosphorous 0.40% 0.0% Aluminum 0.36% 0.0% Sulfur 0.33% 0.0% Potassium 0.26% 0.0%
TABLE 4
Elemental Composition of a Parker Nitrile Gasket
ELEMENT Outside Surface Inside Surface Organic Component >95% >95% Sulfur 2.46 2.73 Zinc 1.79 1.85 Aluminum 0.40 0.21 Silicon 0.35 0.21
Insulating Oils Handbook
49
In addition, several diff erent samples of Kraft paper insulation were analyzed for total sulfur and total sulfate content. Th e results are present in Table 6.
TABLE 5
Surface Composition of Kraft and TU Kraft
ELEMENT KRAFT TU-KRAFT
Organic Component >95% >95% Calcium 3.6% 2.5%
Sulfur 0.4% 1.1%
Silicon 1.1% 1.4%
TABLE 6
Sulfur Composition in Various Electrical Papers
PAPER Total Sulfur
Content* Total Sulfate Content*
Kraft Paper-1 700 ppm 205 ppm Kraft Paper-2 300 ppm <7.5 ppm TU-Kraft 700 ppm 158 ppm Kraft Crepe Paper-1 600 ppm 93 ppm Kraft Crepe Paper-2 500 ppm 30 ppm
*Total sulfur analysis was performed by ASTM Method 129, and total sulfate analysis was performed by EPA Method 300.0
As shown in Table 6, the amount of sulfur varies between electrical paper manufacturers — sometimes considerably. Th e amount of sulfur present is fairly signifi cant in most of the samples. Th e amount of reactive or corrosive sulfur in relation to the total is unknown, although it is assumed that the amount of sulfates in the sample is at least the minimum amount.
Accidental contamination of the transformer oil with corrosive and reactive sulfur compounds can occur by use of incompatible materials or contaminated processing equip- ment to transfer oil. For example, hoses made from natural rubber or gasoline hoses both contain high amounts of sulfur easily transferred to the oil being pumped through them. Extra care must be exercised in the selection of hoses so that no incompatibility exists. Oil-processing equipment runs the risk of being contaminated from processing a transformer with corrosive and reactive sulfur, and thus contaminating the next transformer to be processed. Th e best safeguard is to check the remaining oil left in the processing equipment prior to its next use.
As described above, several materials in the transformer will contain sulfur such as copper, paper, and oil. In some cases, the sulfur species in question are stable or are so tightly bound in the material that they would not be available for reactions. In other cases some of the sulfur compounds are
corrosive or reactive. In these cases, appropriate material compatibility testing should screen out these materials before they are used in transformer construction.
Conclusions
Many internal and external sources of sulfur exist within a transformer besides the oil. Internal sources include copper, paper, gaskets, glues, and possibly other materials. External sources usually include inadvertent contamination from incompatible materials such as oil-transfer hoses.
References
Casey, James P., ed. Pulp and Paper: Chemistry and Chemi-
cal Technology, Interscience Publishers, Inc., New York,
1952.
Lance Lewand received his BS degree at St. Mary’s College of Maryland in 1980. He has been employed by the Doble Engineering Company since 1992 and is currently the Laboratory Manager for the Doble Materials Laboratory and Product Manager for the DOMINO®
product line. Prior to his present position at Doble, he was the Manager of the Transformer Fluid Test Laboratory and PCB and Oil Services at MET Electrical Testing in Baltimore, MD. Mr. Lewand is a member of ASTM committee D 27.