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© 2017 AGC Refining & Filtration LLC
Transformer Oil, Part 2: Deterioration
of Mineral Transformer Oil

AGC REFINING & FILTRATION
TRANSFORMER OIL, PART 2: DETERIORATION OF MINERAL TRANSFORMER OIL 2
Contents
Deterioration of Transformer Oil 3
Oil Deterioration Inside Transformers 3
Gas Inside Transformers 4
Reconditioning of Used Transformer Oil 4
Fuller’s Earth Filtration 5
The Moisture Analyzer 5
Vacuum Distillation in Transformer Oil Purification 8
Vacuum Concepts 8
The Allen Oil Conditioner 10
The Allen Vacuum System 11

Deterioration of Transformer Oil
The Effect of Oxygen
Moisture contamination is one of the most obvious causes of deterioration in the insulating quality of
transformer oil. This contamination can be eliminated by purification. A less rapid, but more serious
characteristic deterioration is the formation of acids and sludge, which is caused by oxidation. Thus the
exclusion of oxygen is of prime importance. In open-breather transformers, the oxygen supply is almost
unlimited and oxidative deterioration is much faster than in sealed transformers.
Atmospheric oxygen is not the only source of oxygen available for the oxidation of insulating oil; water
also serves as a carrier of oxygen and leaky gaskets constitute a real hazard, causing both oxidation and
moisture contamination. The rate of oxidation also depends on the temperature of the oil; the higher the
temperature, and the faster the oxidative breakdown. This points to the importance of avoiding
overloading of transformers, especially in summer time. Oxidation results in the formation of acids in the
insulating oil which in turn, contributes to the formation of sludge.
Moisture in Oil
Water can be present in oil in three forms:
 in a dissolved form,
 as tiny droplets mixed with the oil (emulsion), and
 in a free state at the bottom of the tank.
Coalescence occurs when the tiny droplets combine to form larger drops, which sink to the bottom and
form a pool of free water.
The effect of moisture on the insulating properties of oil depends on the form in which the moisture exists.
A very small amount of free or emulsified water has a significant influence in reducing the dielectric
strength of oil, (see Table 1), whereas dissolved water has little or no effect on the dielectric strength.
The Effect of Temperature on Moisture
The amount of moisture that can be dissolved in oil increases rapidly as the oil temperature increases.
(see Table 2). Therefore, insulating oil purified at too high a temperature may lose a large percentage of
its dielectric strength on cooling, because the dissolved moisture is then becomes an emulsion.
Oil Deterioration Inside Transformers
Inside transformers, sludge sticks to the surfaces through which heat should be dissipated. The sludge
forms a blanket barrier to the flow of heat from the oil to the coolant and from the core and coils to the
cooled oil. If allowed to continue long enough, the sludge may even block the flow of oil through the
cooling ducts. As a result, the transformer insulation gets too hot and is damaged, particularly between
turns of the windings. Deterioration of the turn insulation may eventually lead to short circuits between
turns and the breakdown of the transformer. When oxidation progresses to the points where sludge is
being precipitated, the first step should be to remove the sludge from the transformer by a high-pressure
stream of oil and to either replace the contaminated oil or purify it with Fuller’s Earth to remove the acid
and sludge. Complete treatment of the oil is normally less costly than replacing it.
Absorption of Moisture by Insulating Materials
Solid paper (cellulose) insulation in transformers is very porous and absorbs much water. Some of the
water that is dissolved in the oil is absorbed by the insulation. Once the water is absorbed by the
insulation, it is difficult to remove. The most effective method for drying out the insulation in transformers
is with heat and vacuum. An Allen Oil Conditioner with a vacuum pump suitable for large capacities can
be used for this. When the vacuum is not available, the transformer insulation must be dried out by

circulating hot dry oil. This oil should then be cooled and dried. Since the dielectric strength of insulation
is reduced by moisture, it is important that the insulation not be allowed to absorb moisture in the first
place.
Absorption of Nitrogen by Oil
Special precautions should be taken in operating nitrogen-blanketed transformers, to avoid bubbling of
the oil due to release of dissolved nitrogen when the pressure drops. Experience has shown that the
automatic gas-pressure regulating system should be adjusted to limit the nitrogen pressure range from
plus 3.4 to plus 21 kPa (plus 0.5 to plus 3 lb/in2) gauge to avoid formation of these bubbles and
subsequent problems due to corona deterioration.
The chemical decomposition of materials inside a transformer generates combustible gases. Degradation
by excessive heating or electrical discharges is common. The severity of gassing depends upon the
nature of the problem, which can range from low-level corona or overheating, to total insulating failure.
Early detection is important because it allows for corrective measures such as purification to take place.
In general, the kinds of gases generated depend on the type of insulation that is being degraded and the
temperature in the transformer.
Gas Inside Transformers
Faults involving overheating of cellulose insulation generate mainly carbon monoxide and carbon dioxide.
At low temperatures CO2 predominates, with increasing amounts of CO as the temperature rises. Under
normal operating conditions, there is continuous production of CO2 and CO in a ratio of about 3:1 and
relatively large amounts of these gases will be found in a normally operating transformer. Very high levels
of both gases with CO approaching or exceeding CO2 could signal a localized fault involving cellulose
insulation.
At the relatively low temperature and energy dissipation of partial discharges, the only gas produced is
H2. Low temperature and localized overheating produces CH4 (methane) and C2H6 (ethane) and some
hydrogen. As the temperature increases, ethylene becomes the predominant gas. At very high
temperatures of an arc, acetylene and hydrogen predominate.
Reconditioning of Used Transformer Oil
A variety of methods have been employed in the industry, to reclaim used transformer oils, each one with
varying degrees of success.
Centrifuges
A means of separating free and suspended contaminants such as carbon, water, sludge, etc. from oil is
the continuous centrifuge. The centrifuge cannot remove dissolved water from oil and the oil leaving the
centrifuge may be saturated at the temperature of operation and could contain more dissolved water than
when it entered the centrifuge. Also, the centrifuge is often an expensive and maintenance-intensive
piece of equipment.
Coalescers
Coalescers are used to remove free water from oil. Fiberglass elements trap small water particles, then
increasing differential pressure across the filter media forces the water particles together, and the large
water drops are extruded at the outer surface of the element. Large water drops are retained within a
water repellent separator screen and collect by gravity at the bottom of the filter while dry oil passes
through the separator screen. This method is similar to centrifuging in its limitations and performance and
any particulate matter in the oil will clog a coalescer and render it useless.
Vacuum Dehydrators

The vacuum dehydrator like the Allen Oil Conditioner is efficient in reducing the water content of
insulating oil to 10 to 5 wppm total water. In this equipment, oil is exposed to heat and a vacuum for a
short interval of time. In addition to water, a vacuum dehydrator will degas the oil and remove volatile
acids. Vacuum dehydrators are frequently used to evacuate and fill new transformers or dry out
transformers prior to re-introducing reclaimed oil. They will not affect any additives in the oil.
Fuller’s Earth Filtration
Fuller’s Earth is a natural product composed of calcined opaline clay.
Its major role is that of an absorbent in the following applications:
 acid adsorption from insulating oil and
 removing discoloration from oil.
Fuller’s Earth treatment typically follows the removal of moisture, solids, and gases from transformer oil.
Transformer oil is oxidized under the influence of temperature, oxygen, and moisture. This results in the
formation of acids, which is evident in the increase in the neutralization number (the neutralization
number is measured by the number of milligrams of KOH needed to neutralize one gram of oil).
Increased acidity damages the paper insulation in the transformer and an increase in total acid number
(TAN) is often accompanied as well by a decrease in dielectric strength.
Filtration through a Fuller’s Earth polishing filter as a final step before returning the purified oil to the
transformer has proven cost-effective in controlling acidity. It is possible to maintain a neutralization
number of 0.025 mg KOH/gm for extended periods of time with the same charge of earth.
Much of the original color is also restored.
Fuller’s Earth can be supplied in cartridges for easy installation and removal or in bulk in 50 lb. Bags.
Note: It is important to always start with low acid content of new oil of less than 0.025 mg KOH/g, to attain
the maximum life span of the transformer. It is important to never allow the acidity to exceed 0.01 mg
KOH/g.
If the acidity is allowed to rise beyond this value, certain metal salts could leach out of the Fuller’s Earth
and cause deposits and scum to be generated in the oil.
Once acidity is allowed to rise too far, even Fuller’s Earth filtration cannot restore the oil to acceptable
levels. Far more expensive ion-exchange filtrations is claimed to recover the oil, with often rather mixed
results.
The Moisture Analyzer
This analyzer provides a continuous, real-time and reliable measurement of the water content in various
liquids, such as transformer oil, lubrication oil, kerosene, jet fuel, diesel fuel, refrigeration oil, etc.
Advantages
The system:
 can be installed on the oil-out line of an Oil Conditioner or Hydroscav,
 provides instantaneous results in parts per million (ppm) of moisture,
 monitors the oil conditioning process continuously,
 allows for predictive maintenance,
 can be used in transformer oil, to estimate the water content of paper insulation,

 provides measurement of relative saturation of water in oil at the recorded temperature,
 operates with the sensor directly immersed in the oil,
 is compact in size,
 saves time by providing immediate, dependable, and accurate results, and
 does not require sampling.
Figure 1: A Sample Moisture Analyzer
Relative Saturation
Besides showing the results as a concentration in wppm, relative saturation and temperature can also be
displayed.
Relative saturation provides useful information as it is related to the type of oil used. Certain oils can
tolerate higher moisture levels before inducing damaging effects. For example, in transformer oil, a 100%
saturation will always indicate a low dielectric breakdown voltage, regardless of its concentration in
wppm. (See also the white paper entitled Water Activity)
How it Works
The sensor that is placed directly in the oil stream, measures capacitance of a thin polymer film. The
capacitance changes proportionally with the change in relative saturation (RS) of water in the oil. The
relative saturation, expressed in units of percent, is the concentration of water in the oil, relative to the
solubility or concentration of water that the oil can hold at the measured temperature.
The analyzer can convert the measured RS to a concentration value (ppm by weight), which is displayed.
The conversion is preset for the type of fluid.

Figure 2: Water-in-Oil Analyzer
Measurements in mm
Table 1: The Influence of Moisture on the Dielectric Strength of Transformer Oil
Table 2: The Influence of Temperature on the Amount of Moisture Dissolved in Transformer Oil

Vacuum Distillation in Transformer Oil Purification
Distillation Column Design
In the purification of contaminated oil, vacuum is used to disturb an equilibrium condition that exists in the
contaminated oil at normal operating conditions. This disturbance is the removal of dissolved or dispersed
gases in the oil. Gases dissolve in liquids in amounts proportional to their partial pressure, and removal is
dependent on the molecular density of the environment above the liquid surface.
Distillation is the main process used to separate a liquid stream from multi-component contamination into
individual components of higher purity.
The Allen design of the vacuum distillation vessel is based on a principle of achieving optimum mass
transfer through the creation of a phase change over a large interfacial surface area. This is done by
spreading the oil in a thin layer over a very large surface area created by layers of stainless steel Raschig
Rings. This facilitates separation of contaminants by thermal conversion from a liquid to a vapor phase,
through the interstitial spaces between the rings.
The main design criteria are therefore:
1. maximizing the number of theoretical stages per height of section or column,
2. minimizing the pressure drop per theoretical stage, and
3. maximizing the operational range.
In the ring column, oil flows downward through the spaces in the rings, coating the ring surfaces in a thin
laminar layer of oil. The heat and vacuum lower the boiling point of the contaminants, which flow upward
as a vapor through the layer of multiple rings.
Because the vacuum vessel is preceded by a solids filter, the vacuum vessel does not require opening
and cleaning. The ring column is guaranteed for five years.
Vacuum Concepts
Vacuum and Suction
A common misunderstanding of a vacuum pump is that it “sucks’ gas from a chamber. There is no such
force as “suction” involved in vacuum.
Molecules are in constant motion, propelled by random collisions. When a molecule, as a result of these
random collisions, enters the pumping mechanism of the pump, it is then and only then that it is removed
from the chamber. The pump does not reach out and “grab” the molecules and suck them in. The pump is
like a fish with its mouth open, waiting for the little fishes to wander in where it can then grab them. So,
the first principle of vacuum technology is: vacuum does not suck!
Pumping Speed
Pumping speed is defined as the ratio of the throughput of a given gas to the partial pressure of that gas
at a specific point near the inlet port of the pump. In other words, it is the volume of gas (at any pressure)
that is removed from the system by the pump, per unit time.
Thus, pumping speed is a measure of the pump’s capacity to remove gas from the system, measured in
liters/second, cubic feet minute or cubic meters/hour. Most pumps have a broad pressure range over
which the pumping speed is almost constant.
Vacuum Pump Size
The size of the vacuum pump required for vacuum distillation is determined by the following parameters:

 the quantity of the contaminants,
 the boiling point of the dissolved gases,
 the oil temperature at the pump inlet,
 the degree of vacuum required at the pump inlet, and
 the pumping speed required.
Table 3: Pressure Ranges Associated with Typical Vacuum Pump Types
Table 4: Effect of Vacuum on Oil Conditioner Water Removal Performance

Example 1
The Allen Oil Conditioner
Operation
The Allen High Vacuum Oil Conditioner has six (6) modes of operation. They are:
 filtering of oil only,
 filtered initial fill,
 oil conditioning (purification),
 transformer drying,
 transferring oil only, and
 draining
Filtering of Oil Only
This mode of operation is used for filtering transformer oil to remove solids only by pumping the oil
through the pre- and/or post filter system. Several combinations of elements in various micron sizes are
available for this purpose. Normal filtering technique requires a minimum of two passes of the fluid
through the filter elements, to insure that all solids of the selected particle size distribution range have
been removed.
Filtered Initial Fill
The Oil Conditioner is initially filled before startup or after the system has been drained, by switching on
the inlet pump. While in this mode, the outlet pump motor, heaters, and vacuum system motors are not
energized.
Oil Purification or “Conditioning”
This mode of operation is used for re-conditioning transformer oil. The oil is pumped by the inlet pump (a
positive displacement rotary gear pump), from the transformer or reservoir via the inlet manifold, through
a basket strainer and the pre-filter vessel. From there it passes through several heaters into the vacuum
vessel where it flows over the distillation trays in a thin film, while subjected to high vacuum. Moisture and
gases are removed there. Optional Fuller’s Earth and Post Filtration will follow before discharge of the
clean oil.
Transformer Drying
This mode is used to remove remaining moisture and gases from the transformer insulation by
maintaining a deep vacuum on the transformer for as long as is required. This is made possible by the
heavy-duty, two-stage vacuum system that is the heart of the Allen Oil Conditioner.
Transformers are generally dried by maintaining a deep vacuum such as 10-3 Torr or lower for several
hours, depending on the size of the transformer.
Transferring Oil
This mode is used for transferring insulating oil from one location to another. This may occur while a
different batch of oil is being purified and circulated through the Oil Conditioner. The transfer pump is
sized for a high flow rate and cannot be used in series with the inlet pump.

Draining
This mode of operation is used to completely drain the Allen Oil Conditioner, which is recommended
between batches of different types of oil.
The Allen Vacuum System
The heart of the Allen Oil Conditioner is the specially designed two-stage, high-vacuum system. It
consists of a first-stage vacuum booster and a second-stage vacuum pump. The water-cooled vacuum
booster is a high speed, non-lubricated gas “accelerator” that moves a high volume of gas/vapor that is
compressed in the inter-stage manifold, between the booster and the second-stage vacuum pump. The
vacuum pump sees the same volume of gas/vapor as it did without the booster, but at a much higher
operating pressure than before. The vacuum booster provides high-speed acceleration of gas removal by
means of a non-lubricated, rotary lobed, positive displacement pump. A high volume of gas/vapor at a
relatively low pressure is pumped via the high speed lobes to the backing vacuum pump that handles the
gas/vapor at a greatly reduced volume and higher pressure. This follows the pressure-volume relationship
of gases, where each one is inversely proportional to the other.
The second-stage vacuum pump is a heavy-duty, air-cooled rotary piston type, consisting of two rotary
pistons pumping in parallel. The pump attains a low ultimate pressure of less than 10-3 Torr. The rugged
and simple design ensures dependable service under the most severe applications. The pistons are
attached to cams that are mounted eccentrically to the main bore of the cylinders.
At the start of the cycle, the volume between the piston and the cylinder increases as the shaft rotates the
piston-cam assembly. Gas is drawn in through a channel in the piston, until its volume is at its maximum.
At that point the pocket is sealed from the inlet as the inlet channel closes off. Lubricating oil helps seal
the clearances.
The shaft then further rotates the piston-cam assembly in a way that compresses the sealed-off gas
against the pump cylinder and the discharge valve. The discharge valve opens when the gas pressure is
slightly above atmospheric. The gas and lubricating oil are then forced out and the cycle repeats itself.
The advantages of a rotary piston type pump are:
 minimum vibration,
 rugged design promoting long life, and
 can handle small particulates.
An upstream accumulator traps any condensable vapors, functioning as an efficient vacuum-enhancing
device, thereby optimizing the vacuum system.
Table 5: Typical Specifications for High-Vacuum Oil Conditioner System for Transformer Oil Purification.
Capacity
Up to 3,000 gallons per hour
Note: Larger capacities can be manufactured but are less cost-
effective.
Number of Passes
1-5
Note: Purification in a single pass can be designed by adjusting the
degree of vacuum and process temperature.

Degree of Filtration
5 micron pre-filtration
0.5 micron final filtration
Note: Other micron sizes can be provided.
Dirty Oil
Dielectric Strength: 20 kV (typical)
Moisture Content: 50–100 wppm (typical)
Gas Content: 0.25% by volume (typical)
After One Pass
Dielectric Strength: 60 kV
Moisture Content: 5 wppm or less
Gas Content: 0.05% by volume
After 3–5 Passes
Dielectric Strength: 80 kV or higher
Moisture Content: 2–3 wppm or less
Gas Content: 0.01% by volume or less
A high vacuum system consists of an Accumulator, a 1st stage Booster Pump and a 2nd stage High-
Vacuum Pump.
Transformer evacuation and filling under vacuum is possible.
Maximum moisture content to
be processed
1,000 wppm
Maximum Operating
Temperature
200 °F
All vessels ASME code design.
With A Fuller’s earth Filter
System
Starting Neutralization Number: 0.3–0.5 mg KOH/gm oil
Ending Neutralization Number: < 0.01 mg KOH/gm oil
Electrical
Programmable logic controller
Touch-pad interface
Ethernet tie-in capacity with control room
Control Panel & JBs: IP55 with internal cooling for desert conditions
Suitable for high ambient temperature (55 °C)
Auxiliary Terminal Boxes: IP65
All components U.S.A. National Electrical Code

Figure 3: Schematic of a Two-Stage High Vacuum System
Figure 4: Flow Diagram of a Typical Allen Oil Conditioner

Figure 5: An Allen Transformer Oil Conditioner
In operation at Sho-Me Power Co., Marshfield, Missouri, USA
Capacity 3000 gallons per hour, Two-stage Ultra-High-Vacuum, Single-Pass Purification
Figure 6: An Allen Trailer-Mounted Transformer Oil Conditioner
NW Electric Power Coop, Cameron, MO. USA.

High Capacity, Two-stage, High Vacuum, Single-pass Purification
Figure 7: An Allen High-Vacuum Transformer Oil Conditioner
Stillwater Electric Co. , Oklahoma, USA
Capacity 1800 gph, Solids Pre- and Post Filtration and Fuller’s Earth Treatment
Figure 8: An Allen Trailer-Mounted Oil Conditioner
Saudi Aramco Mobil Refinery, Yanbu, Saudi Arabia
Includes Refrigerated Condensing of Vapor Phase

Figure 9
Figure 10

www.AGCInternational.com
3045 East Elm Street
Springfield, Missouri 65802, USA
Toll Free: +1 800 865 3208
Phone: +1 417 865 2844

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