Transformer Oil, Part 2: Deterioration of
Mineral Transformer Oil
An Allen White Paper
Copyright © 2009 Allen Filters, Inc.
All rights reserved.
Allen Filters, Inc.
P.O. Box 747
Springfield, Missouri 65801
United States
Toll Free: +1 800 865 3208
Phone: +1 417 865 2844
www.allenfiltersinc.com
Contents
5
Deterioration of Transformer Oil
5
The Effect of Oxygen
5
Moisture in Oil
5
The Effect of Temperature on Moisture
5
Oil Deterioration Inside Transformers
6
Absorption of Moisture by Insulating Materials
6
Absorption of Nitrogen by Oil
6
Gas Inside Transformers
6
Reconditioning of Used Transformer Oil
6
Centrifuges
6
Coalescers
7
Vacuum Dehydrators
7
Fuller’s Earth Filtration
7
The Moisture Analyzer
8
Advantages
8
Relative Saturation
8
How it Works
10 Vacuum Distillation in Transformer Oil Purification
10 Distillation Column Design
11 Vacuum Concepts
11 Vacuum and Suction
11 Pumping Speed
11 Vacuum Pump Size
12 The Allen Oil Conditioner
12 Operation
13 Filtering of Oil Only
13 Filtered Initial Fill
13 Oil Purification or “Conditioning”
13 Transformer Drying
13 Transferring Oil
13 Draining
13 The Allen Vacuum System
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:
yy
in a dissolved form,
yy
as tiny droplets mixed with the oil (emulsion), and
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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, an 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.
5
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
6
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:
yy
acid adsorption from insulating oil and
yy
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.
7
Advantages
The system:
yy
can be installed on the oil-out line of an Oil Conditioner or Hydroscav,
yy
provides instantaneous results in parts per million (ppm) of moisture,
yy
monitors the oil conditioning process continuously,
yy
allows for predictive maintenance,
yy
can be used in transformer oil, to estimate the water content of paper insulation,
yy
provides measurement of relative saturation of water in oil at the recorded temperature,
yy
operates with the sensor directly immersed in the oil,
yy
is compact in size,
yy
saves time by providing immediate, dependable, and accurate results, and
yy
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.
8
Figure 2: Water-in-Oil Analyzer
Measurements in mm
Table 1: The Influence of Moisture on the Dielectric Strength of Transformer Oil
DIELECTRIC STRENGTH (KV)
[2.54 mm (0.1 in) gap between 25.4 mm (1.0 in) disks]
35
30
25
20
15
10
5
0
0
10
20
30
40
50
60
WATER (PARTS PER MILLION BY VOLUME)
9
WATER SOLUBILITY (PARTS PER MILLION BY VOLUME)
Table 2: The Influence of Temperature on the Amount of Moisture Dissolved in Transformer Oil
240
200
160
120
80
40
0
0
10
20
30
40
50
TEMPERATURE (°C)
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.
10
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:
yy
the quantity of the contaminants,
yy
the boiling point of the dissolved gases,
yy
the oil temperature at the pump inlet,
yy
the degree of vacuum required at the pump inlet, and
yy
the pumping speed required.
Table 3: Pressure Ranges Associated with Typical Vacuum Pump Types
10-4
Allen Oil Conditioner Sustained Operating Vacuum
10-3
TORR
10-2
10-1
100
101
102
103
Two-Stage
Oil Sealed
Rotary
Pump
Single-Stage
Oil Sealed
Rotary
Pump
Water
Aspirator
Reciprocating Piston
Pump
11
Table 4: Effect of Vacuum on Oil Conditioner Water Removal Performance
120 °F
140 °F
160 °F
Final Water
Content
in Oil
(wppm)
180 °F
150
100
50
0
0 mm Hg
(30” Hg)
50 mm Hg
(27.9” Hg)
100 mm Hg
(26” Hg)
Vacuum
Example 1
Operating at 26” Hg and 160 °F, the lowest water content achievable is about 95 wppm.
The Allen Oil Conditioner
Operation
The Allen High Vacuum Oil Conditioner has six (6) modes of operation. They are:
yy
filtering of oil only,
yy
filtered initial fill,
yy
oil conditioning (purification),
12
yy
transformer drying,
yy
transferring oil only, and
yy
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 preand/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 firststage vacuum booster and a second-stage vacuum pump. The water-cooled vacuum booster is a high speed, nonlubricated 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 highspeed 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
13
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:
yy
minimum vibration,
yy
rugged design promoting long life, and
yy
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
14
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
15
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
16
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
17
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
18
Figure 9
Figure 10
19