FIRST EDITION
March 2021
Understanding
Transformer Testing
A GUIDE TO DIAGNOSTIC TESTS
FOR OIL-FILLED TRANSFORMERS
Transformer oil testing gives you
insight into the true condition of
your high-voltage equipment so you
can make intelligent, cost-effective
transformer management decisions.
This guide covers a range
of tests and provides an
overview of what the
results may mean for
your equipment.
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TABLE OF CONTENTS
INTRODUCTION
P3
TESTS
P4
Dissolved Gas Analysis (DGA)
P4
Karl Fischer Moisture (KF)
P5
Liquid Power Factor (PF)
P6
Oxidation Inhibitor Content (DBPC)
P6
Dissolved Metals (ICP)
P7
Furanic Compounds (FUR)
P7
Particle Count and Filming Compounds (PC/FC)
P8
Neutralization Number
P9
Relative Density (Specific Gravity)
P10
Color
P10
Visual Examination
P11
Dielectric Breakdown Voltage (Disk Electrodes)
P11
Dielectric Breakdown Voltage (VDE Electrodes)
P12
Resistivity
P12
Flash Point/Fire Point
P13
Degree of Polymerization (DP)
P13
AGE Acid Scavenger (Perclene Fluid)
P14
Polychlorinated Biphenyls (PCB) Content
P14
Corrosive Sulfur
P15
Particle Count Distribution
P15
Interfacial Tension (IFT)
P16
FURTHER READING
P17
NEXT STEPS
P17
2
INTRODUCTION
Since the turn of the 20th century,
proven testing methods to collect data
transformers have been filled with mineral
from samples, and we leverage what we
insulating oil. The primary functions
know from scholarship, research, industry
of mineral oil are to act as a dielectric
standards, and experience to accurately
and insulating material, to provide heat
diagnose faults and to plan remediation.
transfer and disperse heat, and to act as a
This guide is an overview of these
barrier to protect the cellulose insulation
testing methods, offering a glance into
from the damaging effects of oxygen and
the benefits and limitations of each
moisture.
test. Similar tests are applicable in both
A secondary function of mineral oil in
mineral-oil-filled and alternative-fluid-filled
transformers—and the function that we
transformers, with alternatives including
hope to demystify with this guide—is to
askarel, natural and synthetic esters,
act as a diagnostic tool for evaluating the
silicone and Wecosol. Although there is
solid insulation inside the transformer.
some overlap, this guide will focus on
You may be familiar with the idea,
testing for mineral oil transformers.
“The life of the paper is the life of the
Thank you for taking the time to
transformer.” A well-used phrase in the
educate yourself on this topic, and for
electrical reliability industry, this statement
trusting SDMyers as your source of
means that the condition of the cellulose
information. We hope this guide sheds
insulation is, essentially, the condition of
some light onto the value of oil testing
the entire unit. If the cellulose insulation
and, most importantly, the value of
fails, so does the transformer.
diagnostic analysis as the most effective
As transformer maintenance
professionals, we look to the diagnostic
tool available for increasing the reliability
of your transformer.
function of mineral oil to learn as much
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as we can about a transformer. We use
- The SDMyers team
3
TESTS
DISSOLVED GAS
ANALYSIS (DGA)
ASTM D3612
Standard Test Method
for Analysis of Gases
Dissolved in Electrical
Insulating Oil by Gas
Chromatography
DETECTS
Presence and
concentration of
hydrogen, oxygen,
nitrogen, carbon
monoxide, carbon
dioxide, methane,
ethane, ethylene, and
acetylene.
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INDICATES
Fault conditions that
lead to production of
fault gases.
Dissolved Gas Analysis (DGA)
The primary use of dissolved gas analysis
equipment. Appropriate operation of new
(DGA) is as a routine monitoring oil test
equipment may require an extremely low
for electrical equipment. Incipient fault
gas content in the newly installed oil—a
conditions—disruptions in the normal
typical specification value is 0.5% (5000
electrical and mechanical operation of
ppm) by volume of total gas dissolved
electrical equipment—cause the oil to
in the oil. There are several methods for
break down, generating combustible
running this (ASTM D831, D1827, D2945),
gases. The profile of those gases can be
but a complete DGA by method D3612
interpreted to diagnose whether fault
gives the most comprehensive result. Not
conditions exist, and how severe those
only does the test quantify the total gas
faults may be.
in ppm, but it also tells which gases are
DGA is also used to determine the
concentration of dissolved atmospheric
present and in what quantities.
Also performed on samples drawn
gases (oxygen, nitrogen, and carbon
during factory heat runs (and sometimes
dioxide) so that the operation of
during factory electrical testing), DGA
oil preservation systems such as
can monitor the integrity of newly
conservators, continuous nitrogen
manufactured equipment. Similarly, most
systems, and nitrogen blankets can be
installations of new, large transformers
evaluated.
require close monitoring by DGA during
DGA is used when new oil is placed in
a transformer, or on newly manufactured
the first days, weeks, and months of
operation.
4
KARL FISCHER
MOISTURE (KF)
ASTM D1533
Standard Test Method
for Water in Insulating
Liquids by Coulometric
Karl Fischer Titration
DETECTS
Presence and
concentration of water in
transformer oil.
INDICATES
Potential for further
degradation of cellulose
insulation.
Karl Fischer Moisture (KF)
Karl Fischer (KF) testing measures water
oil in ppm no longer tells the complete
content in transformer oil. Water content
story. More important values from an
is a chemical property of new oil related
operational and maintenance standpoint
to its purity. New oil leaves the refining
are the percent saturation of the oil and
process with very low water content, but
the percent moisture by dry weight of the
can pick up additional moisture during
solid insulation. These are calculated using
storage, transfer to delivery containers or
the moisture content of the oil in ppm
vehicles, transportation, and installation. A
and the temperature of the oil at the time
typical specification value for new oil, as
of sampling. A typical specification for
received from the supplier, is a maximum
percent moisture by dry weight for a new
of 25 ppm moisture.
unit, prior to energizing is 0.5%.
When new oil is installed in new
As an in-service oil test, moisture
equipment, it is typically processed
content is a critical parameter. Again, the
through filters, heat, and vacuum. A typical
critical values are the percent saturation
specification value for new oil leaving the
and the percent moisture by dry weight
processor to be filled into new equipment
calculated from the oil temperature and
is a maximum of 10 ppm moisture.
the moisture content in ppm reported by
Once the oil has been installed in the
the Karl Fischer Titration.
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equipment, the moisture content of the
5
LIQUID POWER
FACTOR (PF)
ASTM D924
Standard Test Method
for Dissipation Factor
(or Power Factor) and
Relative Permittivity
(Dielectric Constant)
DETECTS
The dielectric strength
of insulating oil.
INDICATES
Loss of dielectric
properties of liquid
insulation.
OXIDATION
INHIBITOR
CONTENT (DBPC)
ASTM D2668
Standard Test Method for
2,6-Ditertiary Butyl ParaCresol and 2,6-Ditertiary
Butyl Phenol in Insulating
Oil by Infrared Absorption
DETECTS
Presence and
concentration of
oxidation inhibitor.
Liquid Power Factor (PF)
Dissipation Factor, or Liquid Power Factor
a very low liquid power factor. Moisture,
(PF), is a measure of the dielectric losses
oxidation, and contamination all serve to
in an insulating liquid when used in an
increase the liquid power factor. For new
alternating current electric field.
oil as received from a supplier, typical
Dissipation factor and liquid power
specification values for liquid power factor
factor are not exactly equivalent, but they
are ≤ 0.050% when measured at 25° C and
vary by less than one part in a thousand
0.30% when measured at 100° C.
up to a value of approximately 5% for the
Liquid power factor is a particularly
liquid power factor. They are essentially
useful in-service tool for testing and
interchangeable for the values that are
monitoring oil because the test is sensitive
likely to be encountered in operating
to moisture, the oxidation of the oil, and
electrical equipment.
contamination from outside sources.
Liquid power factor is an electrical
Frequently, the pattern of increase for the
property of the oil. It relates both to the
25° C and 100° C values can be used to
function of the oil and to its purity. Highly
identify specific conditions of concern.
refined oil, free from contamination, has
Oxidation Inhibitor Content (DBPC)
This test measures the two compounds
percent, or Type II (inhibited), with a
used as added oxidation inhibitors and
maximum inhibitor content of 0.30 weight
reports the total content of the two
percent. An appropriate specification
compounds as total oxidation inhibitor.
range for acceptance of inhibited oil is
This is a test of the chemical properties
0.20 to 0.30 weight percent inhibitor.
of the oil. The test is performed on both
For in-service oil, inhibitor should
new oil—for acceptance testing—and as
be replenished if the inhibitor content
a maintenance and monitoring test on in-
decreases to below 0.1% by weight.
service oil.
Under normal circumstances, mineral
New oil is typically characterized as
oil dielectric fluid will not generally
being either Type I (uninhibited), with a
oxidize if the inhibitor content is properly
maximum inhibitor content of 0.08 weight
maintained.
INDICATES
Occurrences of
oxidation within
the insulation.
6
DISSOLVED
METALS (ICP)
ASTM D7151
Dissolved Metals by ICP,
Standard Test Method
for Determination of
Elements in Insulating Oils
by Inductively Coupled
Plasma Atomic Emission
Spectroscopy
DETECTS
Presence of copper, iron,
and aluminum
in transformer oil.
INDICATES
Production of metals
through arcing/sparking
and potential for
accelerated oxidation.
FURANIC
COMPOUNDS
(FUR)
ASTM D5837
Standard Test Method for
Furanic Compounds in
Electrical Insulating Liquids
by High Performance
Liquid Chromatography
(HPLC)
DETECTS
Furans in transformer oil.
Dissolved Metals (ICP)
Dissolved copper and other metals act
increasing due to aging or jumping due
as catalysts to promote oxidation. They
to fault conditions, the recommendation
also serve to elevate liquid power factor
for in-service oil is to baseline test units
to unacceptable levels. Dissolved metals
and then test every few years to evaluate
in sufficient quantity to promote aging of
gradual increases in metals.
the oil can be removed by reclamation.
If dissolved metals content is to be
Dissolved metals analysis is also useful
used to help identify a transformer
to help diagnose fault conditions such
fault, the abnormality will show up in
as severe overheating or arcing/sparking
the dissolved gas analysis, triggering
indicated by other tests such as dissolved
a recommendation to perform metals
gas analysis.
analysis. Standard procedure is to test
Dissolved metals analysis is sometimes
samples routinely for copper, iron, and
performed on new oil to evaluate whether
aluminum, although many others can be
refining or storage practices are resulting
run if conditions indicate a need.
in elevated dissolved metals levels
being introduced into the system. Since
dissolved metals levels are generally
measurable with brand new transformers,
once energized, and tend to decrease to
“none detected” levels before gradually
Furanic Compounds (FUR)
Analysis for furanic compounds
new oil should have a negligible furanic
(2-furaldehyde and several derivatives) in
compound content. New oil in a new
oil is a test of chemical properties. Furanic
transformer should be baseline tested and
compounds are typically only present in
should have less than 20 ppb (μg/kg) of
oil because of paper degradation. It is an
furanic compounds. Any increase in furanic
optional test of the composition of new oil.
compounds content, and particularly the
On rare occasions, furanic compounds
presence of any furanic compound other
may be present in new oil because
than 2-furaldehyde, is an indication that
of the refining process. Since furanic
the paper is being damaged by heat,
compounds analysis is a diagnostic test
moisture, electrical stress, or oxidation.
for degradation of the solid insulation,
INDICATES
Cellulose insulation
damage through heat,
moisture, electrical stress,
or oxidation.
7
PARTICLE COUNT
AND FILMING
COMPOUNDS
(PC/FC)
Particle Count and Filming Compounds
(PC/FC)
ASTM D6786
Standard Test Method
for Particle Count Using
Automatic Optical
Particle Counters
where moving parts are in contact
This testing should be done on equipment
same time to help identify possible faults.
The microscopic ferrography part of
with insulating oil, such as in Load Tap
the testing starts with the generation
Changers (LTCs). Following Particle
of a ferrogram. A ferrogram is a special
Count analysis (see page 15), microscopic
microscope slide that is designed to
ferrography is used to visually identify the
trap particles from the oil. During the
particles seen in the oil. Ferrous metals
generation of the ferrogram, the slide sits
DETECTS
Presence of particulates.
can be differentiated from non-ferrous
over a magnetic field that will cause the
types. Arcing spheres can be identified, in
ferrous particles to align with the magnetic
contrast to filming compounds (polymeric
field. As the oil flows over the slide, the
INDICATES
Possible damage from
moving parts, such as a
Load Tap Changer.
varnish that can form on electrical
particles are also distributed by size. The
contacts).
slide is then flushed with a fixer and dried.
This test can help identify misaligned
Once dried, the slide is examined under
moving parts. Cutting and shearing wear
a microscope by a trained technician and
can be identified and large arcing spheres
the particles are identified. Identification
can be a sign of misaligned contacts or
of the size, shape, and composition of the
shorts, depending on if they are copper
particles is vital for proper interpretation
or iron. It is strongly recommended that a
of the conditions in the unit being tested.
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dissolved gas analysis be performed at the
8
NEUTRALIZATION
NUMBER
ASTM D974
Standard Test Method for
Acid and Base Number by
Color-Indicator Titration
DETECTS
Relative amounts
of acid/impurities.
INDICATES
The extent of
oil oxidation.
Neutralization Number
Neutralization Number—either Acid
oil oxidation. Many of the oxidation
Number or Base Number—is frequently
products that are formed in oil as it ages
determined for many different petroleum
react with potassium hydroxide and are
products using ASTM D974. For
measured as a group by the acid number
transformer oil, the concern is with the
determination.
acid number. Impurities in new oil and in
ASTM D974 is a manual method of
in-service oil will react with the reagent
titration—adding the potassium hydroxide
used in acid number determination
solution to an oil sample which has
(potassium hydroxide—KOH). The relative
had a color indicator added to it. The
amount of these impurities is quantified
color indicator changes color when free
as the acid number, which is sometimes
potassium hydroxide is present. If there
referred to as “total acid number.” Acid
is no color change, the added potassium
number is reported as milligrams of
hydroxide reagent is reacting with aging
potassium hydroxide per gram of sample
compounds in the oil. When a color
(mg KOH/g).
change is noted, the sample size, volume
Acid number is a chemical property of
KOH added are used to calculate the acid
new oils have very little in the way of
number of the oil specimen.
impurities in them and a correspondingly
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of added KOH, and concentration of the
the oil, related to its purity. Highly refined
Some laboratories have modified ASTM
low acid number. A typical new oil
D974 to allow use of an automatic titrator.
specification is a maximum acid number
In this case, electrodes that measure pH
of 0.015 mg KOH/g. Acid number is
are used instead of the color indicator.
also a useful in-service oil test because
This can be an acceptable method, but
it is a direct measure of the extent of
instrument calibration is complex.
9
RELATIVE DENSITY
(SPECIFIC GRAVITY)
ASTM D1298
Standard Practice for
Density, Relative Density
(Specific Gravity), or API
Gravity of Crude Petroleum
and Liquid Petroleum
Products by Hydrometer
Method
DETECTS
The density of the
insulating liquid.
INDICATES
Oil contamination.
Relative Density (Specific Gravity)
Relative density (more commonly,
as transformer dielectric liquids typically
specific gravity) is a direct comparison
have specific gravity within a relatively
of the density (mass per volume) of an
narrow range.
insulating liquid to water. Water has a
Specific gravity is a new oil test that is
specific gravity of 1.000; transformer oil is
also used for in-service oil. Specific gravity
lighter than water, so the specific gravity
of oil should not change because of
is less than one. A typical specification for
aging. Significant changes while in-service
new oil is 0.84 to 0.91. This is a test of a
are an indication that the oil has been
physical property that relates to the oil’s
contaminated.
composition and function. Specific gravity
Other dielectric fluids have different
directly affects heat transfer. Specific
ranges for relative density (specific
gravity of oil is affected by the length and
gravity). The test is used for all types of
structure of the hydrocarbons in the oil.
new and in-service insulating liquids.
Mixtures of hydrocarbons that perform
COLOR
Color
ASTM D1500
Standard Test Method
for ASTM Color.
Color is a physical property of the oil.
there can be relatively lightly colored
As a new oil test, a very low color is an
oils that are unacceptable for continued
indication of a highly refined oil and is a
use and there are some darker oils that
relative measure of the purity of the oil.
continue to provide acceptable service.
A typical new oil value for color is less
Maintenance decisions are rarely
DETECTS
The darkening of
transformer fluid.
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INDICATES
The need to more
closely evaluate other
oxidation test results.
than 0.5 on the ASTM scale. As in-service
made based strictly on color, but an
oil ages and is oxidized, color typically
unacceptable color may indicate the need
increases, and the oil darkens, visibly.
to more closely evaluate other test results
Care must be taken in interpreting
that apply to oxidation of the oil.
color results for in-service oil because
10
VISUAL
EXAMINATION
ASTM D1524
Standard Test Method
for Visual Examination
of Used Electrical
Insulating Oils.
Visual Examination
Visual examination (D1524) on either
oil, is some variation of “clear and bright”
new oil or in-service oil is a pass-fail test
meaning no evidence of suspended
that looks for any foreign conditions or
particles, cloudiness, turbidity, sediment,
material in the oil sample. The “passing
or any condition resulting from any
grade,” whether for new oil or in-service
contamination by solids or free water.
DETECTS
Cloudiness or
particulates in the oil.
INDICATES
Possible contamination
of transformer fluid.
DIELECTRIC
BREAKDOWN
VOLTAGE (DISK
ELECTRODES)
Dielectric Breakdown Voltage
(Disk Electrodes)
Dielectric Breakdown Voltage is an
inches (approximately 2.54 mm) apart.
ASTM D877
Standard Test
Method for Dielectric
Breakdown Voltage
Using Disk Electrodes
electrical property of new oil. The
D877 has limited use to measure water
measurement of Dielectric Breakdown
contamination in oil because it is not
Voltage has application both to the
sensitive to moisture at saturation below
function of new oil and to its purity.
about 60%. It is sensitive to contamination
DETECTS
Electrical property
of new oil.
INDICATES
High moisture levels
and contamination in
transformer oil.
D877 use has been reexamined by
presence of particles in addition to high
(IEEE has virtually eliminated it as a test
moisture levels. It does not do a good job
for transformer oil in the draft revision of
of detecting oxidation decay products.
the acceptance and maintenance guide
For new oil as received from a supplier,
for mineral oil), but the test is still useful
a typical specification value is a minimum
enough to justify its inclusion in both a
of 30 kV. For evaluating in-service oil,
program for accepting new oil and for
low values indicate contamination by
evaluating oil in service.
very high moisture levels, contamination
The D877 method has two flat disk
electrodes with sharp edges spaced 0.10
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by some other materials and to the
many of the standards organizations
from outside sources, or presence of
conductive particles.
11
DIELECTRIC
BREAKDOWN
VOLTAGE (VDE
ELECTRODES)
Dielectric Breakdown Voltage
(VDE Electrodes)
Dielectric Breakdown Voltage is an
for new oil as received from a supplier
ASTM D1816
Standard Test Method
Dielectric Breakdown
Voltage Using VDE
Electrodes
electrical property of new oil. The
are a minimum of 20 kV for a 1 mm gap
measurement of Dielectric Breakdown
and a minimum of 35 kV for a 2 mm gap.
Voltage has application both to the
For new oil installed in new equipment
function of new oil and to its purity. The
and for in-service oil, the acceptable
D1816 method has been used by many
values depend on the voltage class of the
standards organizations to replace the
equipment. A difficulty with the method
DETECTS
Electrical property
of new oil.
D877 method as both a new oil test and
is that it is also sensitive to dissolved
as an in-service oil test because the VDE
gases, which may not present any sort of
electrodes more closely resemble the
operational problem at levels that affect
geometry of conductors inside operating
the test. So, while an acceptable D1816
INDICATES
Presence of a
fault condition.
electrical equipment and because the test
value can be interpreted as an indication
is much more sensitive to moisture and to
of normal operation, a questionable or
cellulose particles.
unacceptable value may not automatically
There are two possible gap settings
be interpreted as a definite sign that
for the electrodes: 1 mm (approximately
something is wrong; further investigation
0.04 inches) and 2 mm (approximately
is needed.
0.08 inches). Typical specification values
RESISTIVITY
Resistivity
ASTM D1169
Standard Test Method
for Specific Resistance
Dissipation Factor, or Liquid Power Factor,
This is an electrical property of the
is a measure of the dielectric losses in an
oil, related to its function. High resistivity
insulating liquid when used in an alternating
reflects a low content of charge carrying
(Resistivity)
current electric field.
contaminants. In the U.S., it is most often
DETECTS
The dielectric strength
of insulating oil.
measured in the same device, but with a
power factor. There are in-service standards
direct current instead of alternating current.
for resistivity that are widely used overseas.
Specific Resistance (Resistivity) can be
run on new oil and is not as widely used as
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INDICATES
Charge-carrying
contaminants.
12
FLASH POINT/
FIRE POINT
ASTM D92
Standard Test Method
for Flash and Fire
Points
DETECTS
Electrical property of
new oil.
Flash Point/Fire Point
Fire point and flash point are physical
within the transformer environment.
properties of the oil. Because the
When standard design practices began to
fire point and flash point are directly
incorporate higher hot spot temperatures,
affected by the molecular weight and
the specification limit for flash point of
type of hydrocarbons in the oil, their
the oil had to be increased to maintain a
measurement (particularly that of flash
margin of safety. Further, the fire point
point) is an indication of the composition
of the liquid is frequently an important
of the oil.
consideration in design of fire suppression
These tests also test the oil’s function.
Transformer oil must operate safely
equipment and facilities associated with
liquid-filled electrical equipment.
INDICATES
The composition of
the oil, specifically
the presence of
hydrocarbons.
DEGREE OF
POLYMERIZATION
(DP)
D4243
Standard Test Method for
Measurement of Average
Viscometric Degree of
Polymerization of New
and Aged Electrical
Papers and Boards, ASTM
DETECTS
Average number of
glucose monomers that
make up a cellulose chain
in paper insulation.
INDICATES
Weakening of
cellulose insulation.
Degree of Polymerization (DP)
Paper insulation is made of cellulose,
a level of 200, it is brittle and is at the
which is composed of chains of glucose
end of its useful life as defined in C57.91,
monomers. The degree of polymerization
the IEEE Guide for Loading Mineral-Oil-
(DP) test determines the average number
Immersed Transformers. At this level, the
of glucose monomers that make up such a
electrical and mechanical strength of the
cellulose chain in the paper sample.
transformer is severely compromised.
The test is performed by first dissolving
This test is not performed very often,
a measured amount of the de-oiled paper
because of the difficulty in obtaining a
sample in a particular solvent. Then the
paper sample from a working transformer.
viscosity of the solution is measured, as
In addition, sampling the paper insulation
is the viscosity of the blank solvent. The
would weaken the transformer at the
DP of the paper is calculated from the
point of sampling. Therefore, on the rare
viscosity of the dissolved paper solution,
occasion that this test is performed, the
versus that of the blank solvent.
sample is often from a lead, where a repair
The paper in a new transformer is
mechanically strong (has high tensile
can more easily be made.
In other instances, failed units can be
strength) and has a DP value of between
sampled to aid in diagnosing the cause of
800 and 1200. As the paper ages, it
the failure. Much more typically, however,
breaks down because of heat, moisture,
information about the state of aging of
oxygen, and acids. As this breakdown
the paper insulation is obtained from
occurs, the chains become successively
performing furanic compounds analysis on
shorter, and thus the DP declines, resulting
the insulating fluid.
in weakened paper. When the DP reaches
13
AGE ACID
SCAVENGER
SDMyers, LLC, In-house
Test Method.
DETECTS
Presence and
concentration of allyl
glycidyl ether (AGE).
INDICATES
Levels of AGE additive.
AGE Acid Scavenger (Perclene Fluid)
AGE is an abbreviation for a chemical
equipment perchloroethylene insulating
whose full name is allyl glycidyl ether.
liquids addressed the prospect of
(The abbreviation is usually pronounced
fluid breakdown and acid formation
by pronouncing the individual letters
differently. (Wecosol by Westinghouse
A-G-E, rather than like the word “age”.)
is the most frequently encountered
AGE is a chemical that is added to some
original equipment perchloroethylene
perchloroethylene-based insulating
insulating liquid.) Therefore, the AGE test
liquids. The purpose of the AGE additive
is appropriate for samples from former
is to act as an acid scavenger when these
Askarel-filled transformers that were
liquids were used as retrofill fluids for
retrofilled with a perchloroethylene-based
Askarel transformers.
liquid (most frequently Perclene).
As perchloroethylene fluid ages, it
The AGE test determines the amount
breaks down and forms hydrochloric acid.
of the AGE additive that is in the fluid. The
The AGE additive acts to neutralize this
amount of sample needed for the test is
acid, so that the acid does not react with
small, about 5 ml. Test results are reported
the metals in the transformer.
in custom letter format, giving the AGE
Note that AGE additive applies
to perchloroethylene-based retrofill
concentration in ppm, together with
recommendations.
fluids only. Manufacturers of original
POLYCHLORINATED
BIPHENYLS (PCB)
CONTENT
ASTM D4059
Standard Test Method
for Analysis of
Polychlorinated Biphenyls
in Insulating Liquids by
Gas Chromatography.
Polychlorinated Biphenyls (PCB) Content
Polychlorinated biphenyls (PCBs) content
ND is less than this value. In-service
in insulating liquids is a test of a chemical
equipment and equipment for disposal
property. It is done primarily for purposes
is frequently regulated according to the
of complying with various environmental
insulating fluid’s PCBs content. Of the
and regulatory requirements governing
many PCBs analysis methods, ASTM
management of PCBs. New oil should be
D4059 is the most useful for measuring
“none detected” for PCBs—the detection
PCB content in electrical equipment in the
limit is typically specified as 2 ppm, so
United States.
DETECTS
Presence of PCBs.
INDICATES
Compliance/
noncompliance with
environmental regulations.
14
CORROSIVE
SULFUR
ASTM D1275B
Standard Test Method for
Corrosive Sulfur.
DETECTS
Sulfur and sulfur
containing compounds.
INDICATES
Corrosion.
PARTICLE COUNT
DISTRIBUTION
ASTM D6786
Standard Test Method
for Particle Count Using
Automatic Optical
Particle Counters.
Corrosive Sulfur
Corrosive sulfur is a chemical property
of the oil, and the test for it is either pass
the corrosive sulfur test.
There has been some use of the test
or fail. While sulfur content relates to
for in-service oils. Where corrosion of
the composition of the oil, the presence
conductor or unusual deposits of hard,
or absence of corrosive sulfur in new oil
black material have been noted, the test
is more appropriately considered to be
is run to determine whether corrosive
a test for purity—properly refined new
sulfur from the oil is responsible. There
transformer oil should pass the corrosive
have been some suggestions that new oil
sulfur test.
which passes a corrosive sulfur test may
Free, elemental sulfur and some sulfur
develop a corrosive sulfur content while
containing compounds in new oil will react
in-service as certain compounds in the oil
with metals in the transformer, particularly
which contain tightly bound sulfur (these
copper and silver, which leads to corrosion
do not react in the corrosive sulfur test)
of conductor, connections, and soldered
may change chemically under thermal
or braised joints. This is foremost a new
or electrical stress, allowing reformed
oil test—virtually all specifications in use
corrosive sulfur compounds to react
in North America require that new oil pass
with metals.
Particle Count Distribution
Several interpretations of particle size
count in insulating liquids used overseas.
and particle size distribution analysis
In optical methods, an instrument is used
have been applied to electrical insulating
to count and to determine the size of
liquids. The optical method (ASTM D6786)
particles in an insulating liquid that block
is similar to methods used for lubricating
out light.
oils and to standard methods for particle
DETECTS
Size and quantity
of particles in
transformer oil.
SDMyers.com
INDICATES
Degradation of
wear parts.
15
INTERFACIAL
TENSION (IFT)
ASTM D971
Standard Test Method
for Interfacial Tension
of Oil Against Water by
the Ring Method
DETECTS
Dissolved polar
contaminants and
oxidation products.
Interfacial Tension (IFT)
The Interfacial Tension (IFT) test detects
The oil side of the interface becomes
small amounts of dissolved polar
more polar (more water-like), causing
contaminants and products of oxidation in
the interfacial tension to decrease at the
electrical insulating oils.
interface. This test provides an indication
The test is done by measuring the
of the sludge precursors in the oil long
surface tension at the interface between
before any sludge precipitates from the oil.
the oil sample and distilled water. The
Acids formed by oil oxidation also have a
water and the polar contaminants in the
large effect on the IFT value, and acids are
oil are attracted to each other and meet
necessary for the formation of sludge.
at the interface between the oil and water.
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INDICATES
Precursors for
formation of sludge.
16
FURTHER READING
Information on the testing standards for oil-filled transformers listed in this guide
are available online. If you need more information on specific testing standards,
testing processes, or the testing, diagnostic, and analytical services provided by
SDMyers, please call a Transformer Specialist on 330.630.7000 or reach out to
us HERE.
ASTM International
Formerly the American Standards for Testing and Materials, ASTM International now
operates globally, publishing more than 12,000 standards for materials, products,
systems, and processes.
(Fees and registration required)
www.astm.org
The Transformer Maintenance Guide
This guide details everything regarding transformers, from purchase to maintenance
to disposal. Thirty-nine years of extensive research is captured on 435 pages in this
hardbound edition.
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WHERE TO BUY
NEXT STEPS
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like Transformer Dashboard® that can help you understand what to do with your
test results at the click of a button, speak to an expert at SDMyers.
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17
Understanding Transformer Testing:
A Guide to Diagnostic Tests
for Oil-Filled Transformers
by SDMyers, LLC
Published by
Good Place Holdings
4835 Darrow Road
Stow, 44244 USA
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© 2021 SDMyers, LLC
All rights reserved. No portion of this book may be reproduced in
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