The Principal Characteristics of Insulating Oil Are

e 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?




equipment.

What test limits should I look for from in-service oil?




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

desiredcharacteristics by careful selection of crude and refining technique, inhibited oils contain small

amounts of an additive called anantioxidant 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 oxidationresistance.

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: -

1. Storage of drums on their sides ('on the roll') with bungs at the three and nine o'clock positions. This

will maintain a head of oil against the bungs and prevent 'breathing'*.

2. A temperature-controlled/stable temperature environment. This would include protection from direct

solar radiation. This minimises the expansion and contraction of the oil due to temperature fluctuations,

and also prevents the formation of condensation beneath tab-seals.

3. Storage undercover on its own is of minor benefit as breathing and the formation of condensation can

still occur if the above requirements are not met. Thus, even if drums are stored undercover, they

should still be 'on the roll' and in a stable temperature. Undercover/indoor storage, in combination with

the above would be close to 'ideal' conditions. The addition of humidity control would also be a benefit.

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-

bottlefrom inside the transformer.

So, before considering the alternatives, when would we recommend replacing the oil?

1. If the oil is aged (oxidised),even if not to the point at which it would normally be replaced, it makes

economic sense to seize the opportunity to replace the oil and effect a reduction of water content at the

same time. The practicalities and potential outage costs will tend to favour doing the two operations

simultaneously.

2. If there is other work to be carried out on the equipment, for which the oil must be removed, there

may be a case for sending the oil for reclamation and replacing with reclaimed oil instead of committing

it to temporary storage. This work may include investigation and rectification of the means by which the

water entered the transformer (see note below).

3. If the transformer is a relatively small or low voltage unit the alternatives may not be deemed

appropriate. The important consideration here is, how much oil, and hence water, will remain in the

cellulose insulation when the oil is drained from the transformer? The lower the voltage at which a

transformer operates,the lower will be the ratio of mass of cellulose insulation to mass of oil. This will be

of the order of 200:1 for a 240MVA, 400/132kV transmission unit, compared to about 7:1 for a 1MVA,

11/0.433kV distribution transformer. This means that a relatively small proportion of the distribution

transformer's total water content will remain when the oil is drained, and an oil change may be regarded

as a cost effective solution.

In general, a 'wet' transformer can be dried to a very acceptable degree, on site. The equipment and

techniques for achieving this are well developed and if applied appropriately will give good results. 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. Some common reasons for

disappointing results are the failure to rectify the cause of the moisture ingress and the premature

cessation of processing when only the oil has been dried, but not the cellulose insulation.

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.







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?







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.

It is resistant to scratching, which can harbour contamination.

It is relatively easy to keep clean.

It is quite evident when it is not clean.

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.

The Principal Characteristics of Insulating Oil Are

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