In this paper GlobeCore presents the main operational principles of oil regeneration plants with Fuller’s Earth reactivation facility and demonstrate the economic advantages over replacement of used transformer oil with new oil.

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TRANSFORMER OIL REGENERATION
Stanislav Sukhar
GlobeCore
www.gcequipment.com
Email: info@gcequipment.com
Tel:+380532241211

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1.0 Introduction
Nearly 40 % of world’s transformers in 2010 were estimated to be more than 25 years
old. This figure is even higher for Europe, with 60 % of transformers reaching the end of
their life expectancy of 25 to 30 years. As with all aging equipment, failures become
more and more common. Data available from National Grid companies worldwide
indicate that the most probable cause for transformer failure is damage of its cellulose
insulation.
Ideal solution would be replacement of all old transformers, but this requires
considerable investments and will bring disruption of power supply to residential and
business customers. Moreover replacement of power transformer is complicated logistical
operation and as most of these transformers made on individual order, lead times may
be as long as one year. However life of most of transformers can be extended if timely
and correct maintenance procedures are applied.
Regeneration of transformer oil can extend average transformer life by 25 to 30 years.
As a transformer’s oil ages, it oxidizes and begins to break down giving rise to by-
products that produce sludge, which attacks the chemical bonds that hold the cellulose
insulation together. Oil regeneration will remove both the sludge and the sludge forming
by-products from both the oil and the cellulose insulation, thereby preventing the
production of sludge.
GlobeCore’s regeneration process utilizes the principle of sorbent reactivation, which
reactivates the Fullers Earth and enables it to be reused many times, unlike older
processes in which it could only be used once and then had to be disposed of, which was
time consuming, costly and also had environmental implications.
2.0 Ageing of Oil and Cellulosic Materials
Oil in a transformer serves as cooling medium and insulating material together with oil
immersed paper and board. Number of different ageing process affect oil and paper and
both these materials affect each other.
Transformer oil consists mostly of hydrocarbons (more than 99%), a mix that is similar
to conventional diesel. However, oil’s minor components, which constitute less than 1%
of the oil, have the greatest impact on its properties
Oxidation is the most common cause for oil decay. This is a complicated process mainly
because oil oxidation and paper degradation are connected with each other.
Transformer oils can be divided into two categories inhibited and uninhibited oils.
Inhibited oils contain synthetic antioxidants, while uninhibited oils relay on natural
sulphur antioxidants, which are usually present in less refined oils. Inhibited oils have a
long lifecycle, but inhibitor content shall be monitored throughout their service life to
keep it at sufficient level. This requires implementation of monitoring programs, and
sometimes the use of uninhibited oils is a preferable choice, as uninhibited oils will start
to oxidise early, but in gradual and foreseeable way.
Water, acids and sludge, which produced during oxidation process, are the most harmful
compounds for transformer insulation system. Sludge is a highly polar mixture of
components, which do not dissolve in oil and settles on transformer tank and insulation

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system components, forming deposits. These sludge deposits prevent normal oil flow
within transformer, causing it to overheat. Acids attack copper parts of transformer
windings, this leads to corrosion and subsequent transformer failure.
Service life of transformer paper insulation is highly influenced by water and acids
content in insulation system. Paper itself is the main source of water in a transformer as
the amount of water created in oil is relatively small (if oil is purified regularly),
comparing to what is contributed from paper insulation degradation. Water content in
the oil, which can be easily measured with the use of moisture sensors, can be used to
determine water content in paper insulation, which is much harder to measure directly.
Cellulose, hemicellulose and lignin are main components of paper. However, the
degradation of cellulose has the most significant impact on papers strength. Changes in
paper’s physical properties is the main cause of concern, as it directly affects paper’s
ability to withstand short-circuit and vibration stresses. Excessive water content
influences dielectric properties, but can be reduced by application of maintenance
procedures. But changes in paper’s physical properties are impossible to reverse.
IEEE defines Transformer’s end of life as 75% loss in paper tensile strength (DP).
Beyond this point transformer may not reliably withstand the next surge load or short
circuit.
Since the life of the transformer is determined by the state of oil and cellulose insulation
action is required to preserve their properties.
3.0 Remedial Actions
There are number of maintenance procedures to address above problems. Decisions
should always to be made on a case-by-case basis and should use as much information
as possible, including any data from DGA oil tests and continuous gas-in-oil monitoring.
3.1 Oil purification
Purification is “a process that eliminates or reduces physical contamination by means of
physical processes (filtration, dehumidification, degasification, etc.)”. Typically this
means a combination of mechanical filtration and vacuum degassing. It can be done
both off-line and on-line. The effect is normally an efficient removal of particles larger
than 0.2 micron and removal of most of the dissolved water and gases in the oil. This is
usually sufficient to correct a low electrical breakdown voltage, caused by a combination
of particles and high water content, which is a fairly common condition.
In order to preserve the insulation capabilities of the oil it is often necessary to carry out
such a treatment periodically. However, purification is not a long-term solution for a wet
transformer, since only a small fraction of the total water content is removed. To obtain
a significant reduction of the water content in a transformer, a more powerful process,
namely Oil Regeneration, must be applied.
3.2 Oil Regeneration
The main principle of Oil Regeneration is to pump the oil through the sorbent (Fuller’s
Earth). During this process the oil undergoes “molecular filtration” in the micro porous
sorbent. By-products of oil ageing (burnt deposits, hydro-peroxides, ketones, carboxylic
acids, aldehydes, phenols, etc..) are removed and stay in the Fuller’s Earth granules.
Fuller’s Earth is naturally occurring clay that possesses no environmental danger and can

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be disposed of as a construction waste after its absorbing properties have been
exhausted.
GlobeCore regeneration plants are designed with multiple sorbent filter columns.
Columns can be either in filtration mode or sorbent reactivation mode. As oil processing
proceeds these will flip-flop, the switching taking place when the sorbent has
accumulated a certain amount of contaminants. This feature, managed by the SCADA
(Supervisory Control and Data Acquisition System) control system, allows the
regeneration of oil to proceed continuously. Reactivation of the sorbent is fully
automated and enables the plant to process oil using the same Fuller’s Earth up to 300
times which is typically equal to 2 or 3 years of operation before sorbent replacement is
required.
The final stage of oil regeneration is injection of inhibitor. Optimal content of inhibitor in
transformer oil is considered to be between 0.3 and 0.4 %. Inhibitor is dissolved in a
small portion of regenerated oil and pumped into main flow from inhibitor block.
GlobeCore Oil Regeneration plants treat any mineral transformer oil to comply with all
relevant international standards (please see table 1 below). Oil Regeneration provides a
better alternative to oil replacement as treated oil parameters in most cases surpass
those offered by new oil, as can be seen in table 1.
Table 1
Summary of typical oil parameters before and after regeneration treatment on a GlobeCore
multiple sorbent column CMM-R plant.
Parameter
Test
Methods
Standard Value
BS 148:2009
Before
regeneration
After
regeneration
Appearance Visual
Clear, free from sediments
and suspended matter Cloudy brown
Clear and
transparent
Acid number,
mg КОН/g
IEC 296;
ASTM D-664
≤ 0.03
0.63 0.01
Corrosive sulphur
ISO 5662;
ASTM D-1275
Non Corrosive
Present None
Moisture content,
ppm
IEC 733;
ASTM D-1533
≤ 20
170 5
Breakdown
voltage, kV
IEC 156;
ASTM D-1816
≥ 30
11 73
Dielectric loss
tangent at 90ºС
IEC 247;
ASTM D-924
≤ 0.005
4.0 0.001
Gas content, %
IEC 60599;
ASTM D-3612
---
12 0.1
Surface tension,
mН/m, at 25ºС
ISO 6295;
ASTM D-2285
---
22 45
Mechanical
contaminants size,
micron
Clear, free from sediments
and suspended matter 50 0.2
Oxidation stability:
mg КОН/g
IEC 1125А;
IEC 1125B;
IP-307
≤ 1.2
--- 0.2

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Key:
C1-Cn – Fuller’s Earth Column (number of columns depends on the capacity of the equipment), IT –
Intermediate Tank, DT – Safety Tank (Oil Trap), NS – Noise Silencer, CF – Charcoal Filter, F – Filter, CC –
Control Cabinet, ST – Oil Storage Tank, АF – Air Filter, PS1-PS3 – Pressure Transducer, VP1 – Vacuum
Pump, P1 – P2 Oil Pumps,LS1 – LS4 – Level Sensors, LS5 – Leak Sensor, DM – Differential Pressure
Gauge,V1 – V9 – Pneumatic Ball Valves, MV1 – MV9 – Manual Control Valve
Figure 1 - Regeneration Flow Diagram
4.0 Regeneration Plant Design and Operation
GlobeCore CMM-R plant comprises degassing and regeneration units. Its design allows
for stationary as well as mobile operation when mounted on a roadworthy trailer. The
flow diagram of the regeneration plant is shown in Figure 1. Plants vary in the numbers
of Fullers Earth columns. GlobeCore plants are available with 6, 12 or 24 columns as
required for different flow rates.
The CMM-R plant is fully controlled via a SCADA system from a Laptop, positioned within
the operators compartment, and a touchscreen on the control panel. Valve operation is
effected via solenoid controlled compressed air. Included on the control system is a
manual override mode. During operation, degassing and regeneration units shall be
connected to a transformer. Oil is drawn into the degassing unit where it is heated to
operational temperature and filtered. Oil is then pumped into the regeneration unit and
back to the degasser for the removal of moisture and dissolved gasses. Figure 2 depicts
the schematic arrangement of the CMM-R plant.
The Plant’s design includes Fuller’s Earth reactivation in situ for continuous reuse of
Fuller’s Earth. Fuller’s Earth regeneration facility will allow oil processing to be carried
out using one set of columns whilst the other set is reactivating. Alternatively oil
processing or reactivation could be carried out in all columns.

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Figure 2 - CMM – R Diagrammatic Connection Configuration
5.0 Applications
Oil Regeneration Plants are designed to carry out regeneration on energized and de-
energized transformers as well as on a tank to tank basis.
5.1 Regeneration of Oil in Transformer
Oil shall be taken from the transformer and tested before regeneration begins. The
results will show the duration (number of passes) required to regenerate the oil.
Dissolved gas results will indicate whether a transformer may be treated in its energized
or de-energized state, depending upon the level of combustible gases found present.
Usually the regeneration plant is used on 50 % to 80% of its production capacity while
treating energized transformers to ensure that laminar movement of oil is maintained
within the transformer oil tank. With average oil characteristics it takes typically 8 to 10
passes to regenerate oil to “like new” condition. When treating de-energized
transformers the plant can be used on 100% of its production capacity as laminar flow
does not need to be considered
5.2 Desludging of Paper Insulation
Regeneration plants are capable of removing sludge deposits from transformer windings
and insulation. This is achieved by increasing circulating oil temperature to the aniline
point, which is usually around 80 ºC. Dissolved deposits of sludge are removed by the
sorbent before oil is returned to the transformer. On-line oil regeneration therefore does
not only restore oil quality to “like new” conditions but also clears the transformers of
contaminants that could not be removed by simple oil replacement.
This process will be more efficient on energized transformers as winding vibration and
generated heat will also help to remove sludge. With average oil parameters it typically
takes 10-20 passes to desludge an energized transformer and up to 50 passes on a de-
energized transformer.
5.3 Tank to tank Regeneration
Oil that has been collected in a storage tank can be regenerated by direct connection to
a regeneration plant. Before commencing regeneration, the oil needs to be left for a
period of time until all free water and suspended solids has had time to settle out. Once
this has been completed the oil is pumped through the Regeneration plant and is treated
in a single pass to “like new” conditions to comply with all relevant standards.

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Classification Oil Property
Acidity
(mg KOH/g)
Colour
BDV
(kV/2,5mm)
DDF
at 90°C
IFT
(mN/m)
Before
Regeneration
0,157 4,1 68 0,0812 23,5
After
Regeneration
0,009 1,5 80 0,0033 46,0
Follow-up
(Approx 6-9 months)
0,014 1,6 74 0,0063 42,3
IEC 60422
Std Specification
0,03 2,0 55-60 0,010-0,015 35
Key: DDF - Dielectric Dissipation Factor BDV - Breakdown Voltage IFT - Interfacial Tension.
Table 2 – Averaged Data from 3 Mobile Regeneration units
6.0 Historic data
The use of Fuller’s Earth for regeneration of transformer oil dates back to 1965 when it
was first used in the USA and the UK.
In 1990, Fluidex Ltd, a South African engineering company, developed modern mobile
regeneration plants utilizing automated reactivation processes. Since then they’ve been
used by leading maintenance companies including ABB, SD Myers and General Electric
for regeneration of oil in thousands of transformers worldwide.
In 2009, with the Oil Regeneration Technology transfer from Fluidex to GlobeCore, Oil
Regeneration Plants were redesigned and improved to make them yet more reliable and
environmentally friendly.
Table 2 below shows averaged data from more than 200 transformers treated by ABB in
Norway and Sweden with 3 mobile regeneration units. This table includes data for
unused mineral insulating oil (from IEC 60422) to enable the effectiveness of
regeneration to be seen.
7.0 Summary
Regeneration has several attractive features as compared to oil replacement. Although
safety regulations require to de-energise transformer while oil regeneration plant is
being connected and disconnected, it is recommended to carry out regeneration on
energized transformer. This means disruption in power supply to customers is minimal.
From a technical point of view, recirculation of oil within a transformer removes traces of
old oil from paper insulation, In the case of oil replacement, there will be a significant
trace of the old oil in the solid insulation that will contaminate the new oil. Transformer’s
paper insulation acts just like the paper in a car’s oil filter – it filters oil decay products
out of the oil and holds them. These products will dissolve in a new oil shortly after oil
replacement. Oil Regeneration Plant will remove products of oil and paper degradations
extracted from cellulose insulation during treatment.

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To summarize the effects of on-line regeneration with modern technology, we conclude:-
• Regeneration restores the oil’s properties to close to those of new oil.
• The oxidation stability of the regenerated oil is good when the treatment is
combined with inhibitor injection.
• The changes in properties after treatment (due to recontamination from degraded
products in the solid insulation) are small.
• From an environmental point of view, the advantages compared to oil exchange or
traditional regeneration technologies are indisputable. The most important reasons
are that oil is not a renewable resource and the new technology causes much less
waste of oil and sorbent than traditional methods.
• Thousands of transformers successfully treated worldwide.
8.0 References
1. 1
“Life Extension of Power Transformers, Oil Regeneration, on Site Drying and
onsite Repair” PdMSA 2010 Conference, Mats Dahlund, Paul Koestinger, Per
Meyer, Pravin Khanna.
2. 2
“Ageing Of Cellulose In Mineral-Oil Insulated Transformers”, CIGRE Broschure #
323, Task Force D1.01.10
3. 3
IEC 60422, 3rd Edition
4. 4
“Aging Study and Lifetime Estimation of Transformer Mineral Oil” Mohammad R.
Meshkatoddini, Shahid Abbaspour American J. of Engineering and Applied
Sciences 1 (4): 384-388, 2008, ISSN 1941-7020