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Pub
lished

by InterN
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na

l Electrica
l Testing

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cia
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n

Insulating Oils
Handbook

Published by
InterNational Electrical Testing Association

The Anatomy of a
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Page 56

48 Insulating Oils Handbook

Both gaskets contained a large amount of sulfur, especially
the O-ring gasket. � 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. � e original formulations for a nitrile
rubber, � uoroelastomer, or a � uoro-silicone rubber (a � 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 � nished product to
determine what amount of sulfur, if any, remained. � ere also
does not appear to be any standard on what percentage of
sulfur should remain in the � nal product. It then becomes
obvious that the onus is on the � 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 � uoroelastomer,
and another was a nitrile rubber produced by Parker. � e
results are shown in Tables 3 and 4.

Note: SEM/EDX analysis cannot quantify elements
such as � 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 � uoroelastomer shows
no sulfur on the inside surface and very little on the outside,
suggesting that material was cured correctly. � 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. � 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. � 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
� 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.

� 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. � ere are two basic processes:

• � e sul� te process is considered an acidic process and
uses sulfur dioxide, sulfuric acid, and calcium bisul� te.

• � 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 sul� de are used
in what is termed the “cooking process.” � 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. � 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.

� e Kraft process is slightly di� 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. � e pulp � 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 � nished paper products. � e � rst analysis performed was
SEM/EDX analysis of new Kraft and thermally-upgraded
(TU) Kraft papers from United States manufacturers. � 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

Page 57

Insulating Oils Handbook 49

In addition, several di� erent samples of Kraft paper
insulation were analyzed for total sulfur and total sulfate
content. � 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.
� e amount of sulfur present is fairly signi� cant in most
of the samples. � 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. � 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.

Page 111

Setting the Standard

MET Electrical Testing, LLC .......................................................................... William McKenzie
National Field Services ...................................................................................... Eric Beckman
Nationwide Electrical Testing, Inc. ...............................................................Shashikant B. Bagle
North Central Electric, Inc. ...............................................................................Robert Messina
Northern Electrical Testing, Inc. .......................................................................... Lyle Detterman
Orbis Engineering Field Service, Ltd. ....................................................................... Lorne Gara
Pacific Power Testing, Inc. ...................................................................................Steve Emmert
Phasor Engineering ........................................................................................... Rafael Castro
Potomac Testing, Inc. ........................................................................................... Ken Bassett
Power & Generation Testing, Inc. ......................................................................... Mose Ramieh
Power Engineering Services, Inc. ..................................................................... Miles R. Engelke
POWER PLUS Engineering, Inc. ...................................................................Salvatore Mancuso
Power Products & Solutions, Inc. ........................................................................ Ralph Patterson
Power Services, LLC ........................................................................................ Gerald Bydash
Power Solutions Group, Ltd ...........................................................................Barry Willoughby
Power Systems Testing Co. ............................................................................... David Huffman
Power Test, Inc. ..............................................................................................Richard Walker
POWER Testing and Energization, Inc. ...............................................................Chris Zavadlov
Powertech Services, Inc. ................................................................................... Jean A. Brown
Precision Testing Group .................................................................................... Glenn Stuckey
PRIT Service, Inc. ........................................................................................ Roderic Hageman
Reuter & Hanney, Inc. ...................................................................................... Michael Reuter
REV Engineering, LTD ................................................................................ Roland Davidson IV
Scott Testing, Inc................................................................................................Russ Sorbello
Shermco Industries ...............................................................................................Ron Widup
Sigma Six Solutions, Inc. ....................................................................................... John White
Southern New England Electrical Testing, LLC ................................................. David Asplund, Sr.
Southwest Energy Systems, LLC .......................................................................Robert Sheppard
Taurus Power & Controls, Inc. ............................................................................... Rob Bulfinch
Three-C Electrical Co., Inc. ................................................................................James Cialdea
Tidal Power Services, LLC ....................................................................................Monty Janak
Tony Demaria Electric, Inc. ............................................................................ Anthony Demaria
Trace Electrical Services & Testing, LLC ...................................................................Joseph Vasta
Utilities Instrumentation Service, Inc. ........................................................................Gary Walls
Utility Service Corporation .................................................................................. Alan Peterson
Western Electrical Services ......................................................................................Dan Hook

Page 112

Setting the Standard

About NETA
NETA (InterNational Electrical Testing Association) is an association of leading electrical testing companies;

visionaries, committed to advancing the industry’s standards for power system installation and maintenance

to ensure the highest level of reliability and safety.

NETA is an accredited standards developer for the American National Standards Institute (ANSI) and defines

the standards by which electrical equipment is deemed safe and reliable.

NETA is also the leading source of specifications, procedures, testing, and requirements, not only for

commissioning new equipment but for testing the reliability and performance of existing equipment.

QUALIFICATIONS OF THE TESTING ORGANIZATION
An independent overview is the only method of determining the long-term usage of electrical apparatus and

its suitability for the intended purpose. NETA Accredited Companies best support the interest of the owner, as

the objectivity and competency of the testing firm is as important as the competency of the individual technician.

NETA Accredited Companies are part of an independent, third-party electrical testing association dedicated to

setting world standards in electrical maintenance and acceptance testing.

Hiring a NETA Accredited Company assures the customer that:

• The NETA Technician has broad-based knowledge — this person is trained to inspect, test, maintain, and

calibrate all types of electrical equipment in all types of industries.

• NETA Technicians meet stringent educational and experience requirements in accordance with ANSI/NETA

Standard for Certification of Electrical Testing Technicians, (ANSI/NETA ETT).

• A registered Professional Engineer will review all engineering reports.

• All tests will be performed objectively, according to NETA specifications, using calibrated instruments traceable

to the National Institute of Science and Technology (NIST).

• The firm is a well-established, full-service electrical testing business.

CERTIFICATION
NETA Certified Technicians conduct the tests that ensure that electrical power equipment meets the ANSI/NETA

standards’ stringent specifications.

Certification of competency is particularly important in the electrical testing industry. Inherent in the

determination of the equipment’s serviceability is the prerequisite that individuals performing the tests be

capable of conducting the tests in a safe manner and with complete knowledge of the hazards involved. They

must also evaluate the test data and make an informed judgment on the continued serviceability, deterioration,

or nonserviceability of the specific equipment. NETA, a nationally-recognized certification agency, provides

recognition of four levels of competency within the electrical testing industry in accordance with ANSI/NETA

Standard for Certification of Electrical Testing Technicians, (ANSI/NETA ETT).

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