What are the most important characteristics
of insulating oil and why?
The principal characteristics of insulating oil
are:
1. High Flash Point: To minimise risk of
the formation of a flammable mixture of oil vapour and air at
high operating temperatures.
2. Low Pour Point: To reduce risk of inefficient cooling
or of slow O.C.B. tripping due to high oil viscosity.
3. Low Viscosity: To ensure good impregnation of cellulose
insulation and free flowing circulation and heat transfer.
4. Low Density: To avoid ice crystals floating in oil
as dissolved water precipitates and freezes.
Note: The above physical properties will
be unlikely to alter significantly in service. Except by contamination
by other materials such as diesel, paraffin, lubricating oil
or other liquid insulants.
5. Good Chemical Stability: To resist oxidation in "hostile''
environment i.e. high temperature, oxygen, copper, iron, water
and other oxidation products.
Note: Oxidation is the natural process of
aging of a transformer oil. It will not only produce sludge,
which may reduce the cooling efficiency of the transformer but
can lead to tank corrosion by volatile acids. Even low levels
of oxidation of the transformer's oil will reduce the life of
its cellulose insulation. The only treatment is oil-change and
reclamation of old oil or in-situ regeneration.
6. Low Dissolved Water Content: To preserve electrical
properties, especially where other contaminants may be present,
and to avoid the precipitation of free water at extremely low
temperatures.
Note: It is important to understand the dynamic
relationship between oil and cellulose water content and temperature.
The electrical, chemical and physical integrity of cellulose
will be adversely affected by high water content. Prolonged
or repeated on-site processing via a high vacuum processing
plant will dry the oil and hence the paper.
7. Good Appearance: A "safety net" to exclude
any visible contaminants e.g. dirt, dust, sediment or water,
that may either be missed by the objective tests, or which would
degrade any test equipment into which they were deposited.
Note: Visible contaminants can usually be
removed by filtering.
8. High Breakdown Voltage: To ensure the oil's ability
to withstand a sustained high A.C. voltage without breakdown.
Note: Filtering/Dehydrating/Degassing (i.e.
treatment via a high vacuum processing plant) will remove the
contaminants, which cause a reduction in breakdown voltage.
9. Low Dielectric Dissipation Factor (DDF): To minimise
dielectric loss or leakage current. Sometimes referred to as
"power factor".
Note: If due (as is often the case) to soluble
contaminants, other than water, high dielectric dissipation
factor (or low resistivity) can only be treated by reclamation/regeneration
of the oil.
10. Low Gassing Tendency (i.e. Gas Absorbing): To minimise
gas evolution under electric stress.
What's so special about electrical
oil?
While, to a certain extent, the individual characteristics
of an insulating oil may be obtainable from other oils, it is
the combination of characteristics from the one oil that is
so special. Other low viscosity oils, for example, may, if clean
and dry, give a high breakdown voltage and possibly high resistivity
and low DDF, but this is unlikely to be achieved in combination
with low pour point, high flash point and excellent oxidation
resistance.
Of particular importance is that traditional insulating
oils achieve this combination of characteristics without the
use of additives. This has the benefit of making them readily
reclaimable.
How frequently should I sample the
oil in my equipment?
For advice on the monitoring and maintenance of
oil in service we recommend that you refer to the current edition
of IEC 60422 Supervision and maintenance guide for mineral
insulating oils in electrical equipment, or BS 5730 Monitoring
and maintenance guide for mineral insulating oils in electrical
equipment.
Which characteristics of in-service
oil should be monitored?
For advice on the monitoring and maintenance
of oil in service we recommend that you refer to the current
edition of IEC 60422 Supervision and maintenance guide for
mineral insulating oils in electrical equipment, or BS 5730
Monitoring and maintenance guide for mineral insulating oils
in electrical equipment.
What test limits should I look for
from in-service oil?
For advice on the monitoring and maintenance of
oil in service we recommend that you refer to the current edition
of IEC 60422 Supervision and maintenance guide for mineral
insulating oils in electrical equipment, or BS 5730 Monitoring
and maintenance guide for mineral insulating oils in electrical
equipment.
My transformers are only small but
they feed critical services, what oil tests can I do to monitor
their condition?
In many instances, the value of an item of oil-filled
plant will not be related to its capital cost, but to the importance
of the load it supplies. Hence even relatively small, low rating
and low cost plant can justify the performance of sophisticated
monitoring techniques such as dissolved gas analysis (DGA).
What is the difference between Unused
and Reclaimed Insulating Oil?
The differences are of three main types, Technical,
Environmental and Economic.
Technical
The differences are very small and will not be of concern to
most users. Both oils should comply with IEC 60296 Specification
for unused mineral insulating oils for transformers and switchgear
(Note: this specification does neither expressly include nor
exclude reclaimed oil) or BS 148 Specification for unused
and reclaimed mineral insulating oils for transformers and switchgear.
All reclaimed insulating oil produced by EOS will fully meet
the characteristics required for unused oils complying with
the above standards, although, in a few areas the margin by
which it exceeds the requirement may not be as great as for
unused oil.
Environmental
There are three main environmental differences in the production
of unused and reclaimed oils. These are Conservation, Energy
Use, and Waste Minimisation.
Conservation
Because the world's crude oil reserves all originate from processes
occurring deep in the Earth's geological history, it is probably
true to say that, in human terms at least, "they have stopped
making it". Thus all oil is considered to be a finite natural
resource. When the last barrel has been extracted from the well,
there will be no more. It is therefore important that these
resources are used wisely, in order to make them last as long
as possible. For each barrel of insulating oil we recycle, we
reduce the need to extract that volume from the Earth.
Energy Use
The energy required for the exploration, extraction, shipment
and refining of unused oil is many times that involved in the
recovery and reclamation of used oil and the production of reclaimed
oil. This excess of energy usage contributes not only to the
financial cost of the product, and the further exploitation
of the Earth's resources, but also adds to Global Warming.
Waste Minimisation
By returning used insulating oil to EOS for recycling, our customers
are, in addition to contributing to sustainability, also ensuring
that what would otherwise be a waste disposal problem, is being
entrusted to a reputable company with fully documented and approved
procedures and audit trails, who is thus able to aid them in
meeting their Duty of Care.
Economic
Cost
Because the costs involved in the reclamation of a used oil
are lower than those for the base crude, its shipment and refining,
RTO is typically 30% less than the price of unused oil.
Stability
While the cost of unused oil is subject to the vagaries of the
international oil market and currency fluctuations, resulting
in sudden and unpredictable swings in prices for the finished
product, reclaimed oil is subject only to the effects of domestic
inflation.
Balance of Payments
All unused insulating oil supplied in the UK is imported. As
the UK does not have an electrical oils refinery, we must import
the finished product. This results in a substantial negative
contribution to the UK's balance of payments. By re-using the
oil already in the UK we can reduce the outward flow of foreign
exchange.
How many times can a used oil be
reclaimed?
This will depend on a number of factors, for example
the original constitution of the oil, how severely aged it is,
the reclamation method employed etc.
The mineral insulating oil conventionally used within
the UK for many years is of a type, which lends itself to repeated
reclamation. This is particularly true when used in plant operated
and maintained in accordance with standard UK practice. The
high standards of monitoring and maintenance generally employed
in the UK ensures that oil is not allowed to deteriorate beyond
the point at which it can be reclaimed, as to do so would be
sure to shorten the life of the equipment.
The diversity of the used oil returned to EOS means
that there tends to be a "sweetening" of the more
severely aged oils by oils that are less degraded or those that
have not previously been reclaimed. In other parts of the world,
where oil tends to be reclaimed either in-situ, within its equipment
(probably at an advanced stage of degradation), or from a limited
source with little input of less degraded oil, it is common
practice to recover the oils oxidation resistance by the use
of anti-oxidant additives. This has never been necessary with
insulating oil reclaimed by EOS.
What is the difference between uninhibited
and inhibited electrical oils?
While uninhibited electrical oils are "straight"
mineral oils, containing no additives but achieving their desired
characteristics by careful selection of crude and refining technique,
inhibited oils contain small amounts of an additive called an
antioxidant to inhibit oxidation thus increasing chemical stability.
Depending on the standard with which the oil complied
when originally supplied, the base, uninhibited oil, prior to
inhibition, may or may not possess an acceptable degree of oxidation
resistance.
Caution is strongly recommended in the monitoring
of the aging of inhibited oils. This is because the "end
of life characteristics" are different from those for a
conventional uninhibited oil. Although the service life is likely
to be extended by inhibition, once the inhibitor has been depleted
the rate of aging will tend to be greater than that for an uninhibited
oil.
How long can I store drummed insulating
oil?
British Standard BS 148:1998 includes the following
statement: "The limits for water content, breakdown
voltage, and dielectric dissipation factor for oil packed in
drums shall apply only to oils delivered within Great Britain
in drums of 200 litres nominal capacity and to tests carried
out within two weeks of leaving the manufacturer" And
it adds the note: "There is a tendency for water absorption
to occur and for the electrical characteristics to deteriorate
in oil stored in drums."
It is important to understand that there is not
a 'shelf-life' as such, as deterioration in storage is far more
dependent on the method of storage than the duration
of storage. The simplest advice is always to store in as close
as possible to ideal conditions, and to keep periods of storage
as short as possible. Kept under 'ideal' conditions drummed
insulating oil may be expected to maintain its specified properties
for an almost indefinite period, given a 100% secure drum.
We would not, however, recommend, even under ideal
conditions, storing drummed oil for any more than 6 months,
and would suggest a limit of 3 months be aimed for. No matter
how long or short the period of storage prior to use, and whatever
the storage conditions, all insulating oil should be tested
for compliance with, in particular, safety-critical properties
relevant to the type of equipment in which it is to be used.
How should I store drummed
insulating oil?
'Ideal' conditions consist of a hierarchy of requirements
to minimise the risk of deterioration, caused primarily by the
ingress of moisture via the process known as 'breathing' (see
note below). These are: -
Kept under the above 'ideal' conditions drummed
insulating oil may be expected to maintain its specified properties
for an almost indefinite period, given a 100% secure drum. We
would not, however recommend, even under ideal conditions, storing
drummed oil for any more than 6 months, and would suggest a
limit of 3 months be aimed for.
No matter how long or short the period of storage
prior to use, and whatever the storage conditions, all insulating
oil should be tested for compliance with, in particular, safety-critical
properties relevant to the type of equipment in which it is
to be used.
*'Breathing' occurs when the oil in a drum,
warmed by conduction from the ambient air or by solar radiation,
expands and raises the air pressure in the headspace within
the drum. If this pressure increases sufficiently, it will force
air out past the bung seal. Upon cooling the oil contracts and
produces a partial vacuum in the headspace, this can result
in moist air (or water) being drawn in past the bung seal. This
has been known to lead to substantial quantities of water being
discovered inside an unopened drum where inappropriate storage
methods have been employed.
I have a transformer that
is showing a high water content in the oil, should I change
the oil?
In most cases the answer is probably
"no", as there are other, more effective, ways of
treating high water content, which are discussed below. It is
important to remember that, although we generally measure the
water content of the oil, this is merely a matter of practical
convenience, as what we are most interested in is the water
content of the transformer, and, more specifically, that of
the cellulose insulation. The sample of oil, the water content
of which is measured, is, in effect, a message-in-a-bottle
from inside the transformer.
So, before considering the alternatives, when would
we recommend replacing the oil?
I have a transformer that is showing
a low breakdown voltage of the oil, should I change the oil?
Probably not, if low breakdown voltage is the only
unsatisfactory parameter. Low breakdown voltage is caused by
contamination of the oil by debris or free water. At very high
levels, dissolved water may also have an impact. This is especially
so when combined with other contaminants such as fibres. Both
water and solid contaminants may be removed by onsite, in-situ
processing via a high vacuum mobile oil treatment plant. EOS
operates the UK's largest fleet of mobile electrical oils processing
units (MPUs) and can advise
you on the best method of treatment to use in your particular
circumstances.
How should I take
a sample of oil from a transformer?
The purpose of sampling is to produce an accurate representation
of the body of the oil, ensuring any contaminants present are
found, while excluding those from external sources. It should
be remembered that all contact with air, sampling equipment
etc. could have an influence on the sample. Ideally, all external
influences, such as airborne contamination, rain, dust etc.
should be totally excluded. In some cases, where sampling
under poor conditions is necessary, this may mean the erection
of a small tent or makeshift cover. A 500ml sample will be sufficient
to carry out all routine, PCB and DGA tests.
If there is not a dedicated sample point provided, fit a sampling
adapter to a drain valve or filter valve. Ensure that the
exterior of the valve and adapter is clean before fitting. Open
the valve and establish a controllable & directional flow
into a bucket. Use a plastic hose kept especially for the purpose.
Clean the interior of the valve, sample adapter and hose by
flushing with oil, allow 2-3 litres to flow to 'waste' - do
not adjust the flow rate from now on. Record the oil temperature
as the flushing oil runs into the bucket. Do not put the thermometer
into the sample bottle. Rinse the sample bottle 2 or 3 times
by half filling it and vigorously shaking it. Continue shaking
while discarding oil to prevent any contaminants adhering to
the side of the bottle. Repeat and inspect for cleanliness.
Fill the bottle gently by directing the flow down the side to
reduce aeration until the bottle is full and overflowing. The
overflowing will help to release any air bubbles from the bottle.
Run some oil over the cap to rinse it. Leave a 0.5-1cm air gap
and finger tighten the cap. Be careful not to over-tighten.
Close the sample valve. Gently invert the sample bottle and
inspect for visible solid contamination and/or free water. If
contamination is found, the sample should be discarded and the
above procedure repeated to ensure that the first was representative.
Complete the sample data sheet or label in full.
How should
I take a sample of oil from a Switch Tank?
Although it is normal practice to remove oil from switchgear
for maintenance and replace with oil of a known and certified
quality standard, samples, either "as found" or "as
left" are often taken for diagnostic or statistical purposes.
The purpose of sampling is to produce an accurate
representation of the body of the oil, ensuring any contaminants
present are found, while excluding those from external sources.
It should be remembered that all contact with air, sampling
equipment etc. could have an influence on the sample. Ideally,
all external influences, such as airborne contamination, rain,
dust etc. should be totally excluded. In some
cases, where sampling under poor conditions is necessary, this
may mean the erection of a small tent or makeshift cover. A
500ml sample will be sufficient to carry out all routine, PCB
and DGA tests.
Clean the switch lid, paying particular attention
to the external rim of the gasket seat, a favourite home of
insects. Remove the switch lid and carry out a visual inspection.
Using a torch if necessary, check as much of the base of the
tank as possible, especially in corners and directly beneath
any access covers, sight glasses or operating mechanism bushes.
Record the nature and location of any contamination found. Remove
your glass sampling thief
from its protective case and inspect and wipe the outside of
it. Insert the thief into the oil short of the tank bottom,
place a finger or thumb over the top end and withdraw. The thief
is now about half full of oil. This is used to rinse the inside
of the thief by rocking it in a "see-saw" motion while
rolling it to ensure total rinsing of the internal surface.
This should be done over a large drip-tray. Repeat and examine.
Using the same technique as above, withdraw half a thief of
oil and place it in the sample bottle. Rest the thief in the
switch tank where it will be safe and clean. The sample bottle
is then capped and rinsed by vigorously shaking it. Continue
shaking while discarding oil to prevent any contaminants adhering
to the side of the bottle. Repeat and examine.
To take a sample from the bottom of the switch,
which is where any contaminants, if present, are most likely
to be found, insert the thief into the switch ensuring that
the finger or thumb covering the top hole is not removed until
the thief reaches the bottom of the tank. Uncover the hole allowing
oil from the bottom to enter the thief. While this is happening,
the thief should carefully be moved across the bottom of the
tank as much as possible. When the oil level in the thief is
the same as that in the switch, uncap the sample bottle and
cover the top hole of the thief and withdraw it from the switch
lifting it vertically. Hold the bottle over a bucket or drip
tray, and, without placing the end of the thief in the top of
the bottle, uncover the top hole and aim the oil into the sample
bottle such that it flows down it's side. Repeat, taking successive
dips from different parts of the switch, until the bottle is
full and overflowing. The overflowing will help to release any
air bubbles from the bottle. Pour the surplus oil from the bottle
over the cap to rinse. Leave a 0.5-1cm air gap and finger
tighten the cap. Be careful not to over-tighten. Gently
invert the sample bottle and inspect for visible solid contamination
and/or free water. If contamination is found, the sample should
be discarded and the above procedure repeated to ensure that
the first was representative. Complete the sample data sheet
or label in full.
How should
I take a sample of oil from a Drum?
The purpose of sampling is to produce an accurate representation
of the body of the oil, ensuring any contaminants present are
found, while excluding those from external sources. It should
be remembered that all contact with air, sampling equipment
etc. could have an influence on the sample. Ideally, all external
influences, such as airborne contamination, rain, dust etc.
should be totally excluded. In some cases, where
sampling under poor conditions is necessary, this may mean the
erection of a small tent or makeshift cover. A 500ml sample
will be sufficient to carry out all routine, PCB and DGA tests.
Allow the oil in the drum to acclimatise by achieving
equilibrium with the ambient temperature. Clean the area around
the bung. Remove the tab-seal and the bung using a drum
key. Inspect the bung seal for signs of damage or contamination.
Remove your glass sampling
thief from its protective case and inspect and wipe the
outside of it. Insert the thief into the oil about half way,
place a finger or thumb over the top end and withdraw. The thief
is now half full of oil. This is used to rinse the inside of
the thief by rocking it in a "see-saw" motion while
rolling it to ensure total rinsing of the internal surface.
This should be done over a large drip-tray. Repeat and examine.
Using the same technique as above, withdraw half a thief of
oil and place it in the sample bottle. Rest the thief in the
oil drum where it will be safe and clean. The sample bottle
is then capped and rinsed by vigorously shaking it. Continue
shaking while discarding oil to prevent any contaminants adhering
to the side of the bottle. Repeat and examine.
To take a sample from the bottom of the drum, which
is where any contaminants, if present, are most likely to be
found, insert the thief into the drum ensuring that the finger
or thumb covering the top hole is not removed until the thief
reaches the bottom of the drum. Then uncover the hole allowing
oil from the bottom of the drum to enter the thief. While this
is happening, the thief should be moved across the bottom of
the drum and around the bottom seam. When the oil level in the
thief is the same as that in the drum, uncap the sample bottle
and cover the top hole of the thief and withdraw it from the
drum lifting it vertically. Hold the bottle over a bucket or
drip tray, and, without placing the end of the thief in the
top of the bottle, uncover the top hole and aim the oil into
the sample bottle such that it flows down it's side. Repeat
until the bottle is full and overflowing. The overflowing will
help to release any air bubbles from the bottle. Pour the surplus
oil from the bottle over the cap to rinse. Leave a 0.5-1cm air
gap and finger tighten the cap. Be careful not
to over-tighten. Replace bung to drum. Gently invert the sample
bottle and inspect for visible solid contamination and/or free
water. If contamination is found, the sample should be discarded
and the above procedure repeated to ensure that the first was
representative. Complete the sample data sheet or label in full.
Why should I use a glass sampling
thief?
There are three reasons for favouring glass for
sampling thieves.
1. It is resistant to scratching, which can harbour contamination.
2. It is relatively easy to keep clean.
3. It is quite evident when it is not clean.
Other Products/Sampling
Thief
How does the electrical oil reclamation
service work?
The EOS electrical oil reclamation service is a
'laundering' service. The customer supplies EOS with his used
oil and EOS reclaim it to BS148-standard. The customer is then
able to call off, for return, the same volume of reclaimed insulating
oil as used oil supplied, less loss in treatment.
Is all used oil acceptable for
reclamation?
In general, all mineral insulating oil, originally
complying with BS148 and produced from naphthenic feedstock,
will be reclaimable. We do, however, have strict QA procedures,
which are designed to identify any oil unsuitable due to exceptionally
severe aging or cross-contamination with other materials.
Can I return used oil in any container
I have available?
Most definitely not. In order to minimize the risk
of contamination of used oil our quality assurance procedures
require that drums for collection of used oil are of a suitable
type (either supplied originally containing electrical oil or
as empties specifically for the return of UTO) and that drums
are in a sound condition. EOS can advise on suitable containers.
Can I buy reclaimed insulating
oil even though I can't supply any used oil?
EOS does usually have stocks of reclaimed oil produced
from oil inherited from oil changes where an alternative product
has been used for refilling or plant has been scrapped, thus
we are able to supply reclaimed insulating oil to customers
without their own raw material. A small additional charge is
made for the supply of this.
I have a surplus of reclaimed insulating
oil; can I use it to fill my new transformers?
There is no technical reason why reclaimed insulating
oil may not be utilised in new transformers as their initial
filling. EOS will be pleased to supply your reclaimed insulating
oil, held by us, to any transformer manufacturer within the
British Isles. Beyond this, it is a matter for agreement between
the purchaser of the transformer and its manufacturer. Not all
transformer factories have the means to segregate reclaimed
and unused oil, so this may be an impediment. EOS can advise
on how best to pursue this option.
