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Insulating Oil used in Transformers, Voltage Regulators, etc.

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/7 tl-}/) Prepared by:

Sk. A. Rahman Ex-member, PDB ~cipal Elec. Engineer

And Head, Ccn TR BCL, Dhaka

It has got different trade names namely Wemco "C" Oil (Westinghouse), shell Diala Oil "B"

(Burmah Eastern), # 10 "C" Oil (GE), etc. The oil is a refined mineral oil obtained from the

fractional distillation of crude petroleum . It is free from moisture, inorganic acid, alkali, sulfur,

asph<tlt, tcr, vegetable or animal oils Having the undernoted advantages, the oil llas been

selected for use a5 insulating oil in different electrical equipment and facilities:

1. High dielectric strength.

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Low viscosity- provides for good heat tr:msfrt.

Freedom from inorganic acids, alkalis, ~md corrosive sulfor-prevcnts injury to insulation and

materials of -.:orLstruction.

Good resistance to emulsification. In case of moisture contamination, it quickly settles to

t11e bottom of the tank.

5. Freedom from sludging under normal operating conditions over Jong periods of time

accomplished by proper selections of creeds and refining methods.

6. Because of its low viscosity it analyses rapid settling of the arced products m circuit

breakers tap-changers, and other arcing contact apparatus. "

7. Low pouer point allows use under low temperature conditions.

./8. The high flash point allows higher operating temperatures with freedom from fire haz.ard.

Enemies of Transformer Oil:

In order to discuss the problems involved in the maintenance of insulating oil in service and the

required handling procedures, it is important to realize the factors which can cause damage to

insulating oil

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Oxidation - Oxidation is the most common cause of oil and and insulation deterioration .

Faulty gaskets and poor welds may pursuit a unit to breathe air. The oxygen in the air

will react with the oil to fonn organic acids, water and eventually oil sludge. Once

sludging has started a clay treatment is required to remove the deterioration products.

Acid in twn, attacks the winding insulation, and sludge deposits tend to decrease cooling.

Moisture in the transformer fluid tends to lower the di-electric strength of the fluid, which

combined with sludge will lower the flashover value of insulators and tenninal boards -. inside the tra..'lsformer tank.

Contamination-Moisture is the chief cause of oil contamination. The main source of

moisture is from breathing air into the transformer, the faulty gaskets or during

transformer ~-sembly or inspection with the manhole cover removed. Other solid

c9ntaminations in the oil can come from solid insulation, metal parts etc.

Excessive Temperature- Excessive heat will accelerate oxidation if any oxygen 1s

present and ha':tcn the decomposition of the oil. Heat also will degrade the~ other

insulating materials and thus g:::nerate carbon dioxide and water which further

deteriorates the oil.

Other Factors- Minor faults. such as corona discharges, spark0g and local overheating

under the oil level will produce combustible gases and carbon. Which can be harmful

to the operation of a transformer. It is desirable to identify these minor faults and correct

them when possible.

Test programs of insulating oil a.e normally as shown below:

General Tests l Complete Test5 include General plus ej Through i.

-------a. Acid Number I Neutralization a. Acid Number/ Neutrali:zation Ho.

b. Interfacial Tension b. Interfacial Tension

c. Di-electric breakdown c. Di-eJectric breakdown

d. Power Factor 50 Hz. at 25uC d. Power Factor 50 Hz at 25°C

• e. (;olour

f. Pour Point

g. Specific Gravity

h. Viscosity

i. Moisture Content.

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Sample of oil for testing purpose should be collected cautiously so that the same is not

contaminated during samplinihg . The oil should be kept stored in dry clean continer.

The container preferably be completely filled up and closed/ capped to prevent contact

with atmosphere .

The General Tests are described as under:

Di-electric Breakdown - ASTM-D877:- The dielectric breakdown voltage is an

importmt measurement of the electrical stress which an insulating liquid can withstand

without puncture I failure. It is measured by applying a voltage between two electrodes

under prescribed conditions for testing the liquid. It serves as an indication 1-1/ri(~.L-~ 1 contaminants,Particularly moisture and solid contaminants (conducting).

ASTM D- 877 specifies a test cup equipped with one-inch dia. Vertical disc electrodes

spaced 0.10 inch apart be used to obtain the standard ... dielectric test value.

Neutralization Number/ Acid Number, ASTJ\1-974.

This number is the number o( milligrams of K(OH) required to neutralize the acid content in one gra,_-n of oil under test . It measures the· ~cid content of oil. With service aged oil it can be used as an indication of the presence oJ contaminants. The acid Number is most important in indicating chemical change or deterioration of the oil. It may be stated here that some of the products of oxidation are acidic.

Interfacial Tension, ASTM D-971:

The interfacial tension between the insulating oil and water is a measure of the molecular attractive force between the unlike molecules anii is expressed in dynes per centimeter. This test provides a means of detecting soluable polar contaminants and products of deterioration. Soluble contaminants or oil deterioration products generally decrease the interfacial tension value.

--r ' , I r~ - .. ~ / Power Factor/ Loss tan delta orDissipation Factor:

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The power factor is the ratio of the power dissipited in the oil in watts to the porduct of the effective voltage and current in volt-ampere, when tested with a sinusoidal field under prescribed conditions. A high power factor value is an indication of the presence of contaminants or deterioration products, such as water, oxidation products, etc. ,

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Loss tan delta is the ratio of the power dissipated in the oil to the reactive power. When the insulation system of oil, rotating machines~ etc is energized with an AC volt., the p.f is equal to the cosine of the angle between the charging current and the applied voltage. Loss tan delta/dissipation factor may be taken as the p.f when the latter very low.

Relationship between charging current and ap.plied voltage is shown below:

I = Charging Current

8 = 90-8

E = Applied voltage

An oil power factor test is a very good barometer and is a required test for oil used in EHV equipment.

Good new oil has a power factor of O.OS percent or less at 20°C. Higher power factor indicates deterioration and/or contamination with moisture, carbon er other conducting matter, varnish, deterioration products, etc. Carbon or asphalt in oil can cause discoloration. Carbon in oil \:viii not necessarily increase the power factor of the oil unless moisture is alsc present. It is suggested that the following serve as guides for grading oil by power factor (p.f.) tests.

Oil having a p.f of less than 0.5% at 20°C is usually considered satisfactory for servi cc. · Oil having a power factor between 0.5 and 2% at 20°C should be considered as being it doubtful condition, and at least some type of investigation should be made. Oil having a power factor of over 2% at 20°C should be investigated and should be reconditio.ned or replaced.

The preceding guides may be elaborated on by saying that good new oil has a p.f of approximately 0.05% or less at 20°C and that the p.f. can gradually increase in service to a value as high as 0.5% at 20°C without, in most cares, indicating deterioration. When the p.f exceeds 0.5%, an investigation is indicated.

Other tests of comparatively less importance but significant in _the evaluation of insulating oil condition are also described below:

Color, ASTM D-1500 - The primary significance of color is to observe a rate of change from previous samples of oil from the same transformer or breaker. Noticeable darkening in short periods of time indicate either contamination or deterioration of the oil. A darkening color, with no significant change in neutralization value or viscosity, usually indicates contamination with foreign materials. The color of an insulating oil is determined by means of transmitted light and is expressed by a numerical value based ori comparison with a series of color standards.

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Pour Point, ASTM D-97 - The pour point is the temperature at which insulating oil will just flow under prescribed conditions. The pour point is useful in detennining the type of equipment in which a given oil can be used, but has little significance as far as contamination or deterioration are concerned.

Specific Gravity, ASTM D-1298 - The specific gravity of insulating oil is the ratio of the weights of equal volumes of oil and water at 15.56° C (60°F). The specific gravity may be pertinent in detennining the suitability for use in specific application.

Viscosity, ASTM - D88 - The viscosity of insulating oil is its resistance to uniformly continuous flow without turbulence, inertia or other forces. It is usually measured by timing the flow of a given quantity of oil under controlled conditions. A marked viscosity increase accompanied by an increasirig neutralization number and darkening color, could also indicate deterioration of the oil due to oxidation.

Moisture Content, D-1533 - Th presence of free water may be observed by visual examination in the form of separated droplets or as a cloud dispersed throughout the oil. Water in solution is nof!!Uilly determinedoy physical or chemicai means. It is measured in parts per million. Water in inv3:nably causes decreased dielectric strength of the oil. Figure 13, shows an automatic Karl Fischer titrating apparatus.

Flash Point, ASTM, D-92 - 'The flash point of insulating oil is the temperature to which the material must be heated in order to give off sufficient vapor to form a flammable mixture with air under tl11.~nditions of the test.

. "H~1M, l>-91- · Fire point:J'1ne Fire Point is the higher temp. than Flash Point, at which the oil vapours will continue to bum when ignited till the oil is exhausted.

/ Reclaiming And Reconditioning of Oil

Reconditioning

Reconditioning involves the removal of water and solid particles from the oil. 111is is done by using one of several available types of filters, centrifuges, and vacuum dehydrators.

The filters include va.rious kinds of cartridge type filters as well as tl1e common filter press. These filters are capable of removing water, carbon, particulate matter and sludge which is in suspension. Their ability to remove water is dependent upon the dryness of the filter. It is essential that steps be taken to make certain that the filter is dry before the filtration is begun. 1he best method to detennine when a filter in use has become saturated with water and lost its effettivf:ness is by checking the dielectric strength of the filtered oil or, y a continuous indication that water is present in outgoing oil. If the oil being processed contains a large amount of contamination, it is necessary to frequently change the filter.

The centrifuge is particularly useful when there are large quantities of water or other contamination present. It is very effective in removing water but cannot remove the contaminants as completely as a filter. Neither the centrifuge or the filter is designed to treat oil chemically.

In some situations it is advantageous to use a combination centrifuge and filter press. The oil is first passed through the centrifuge to remove most of the contamination then through the filter to remove any remaining contamination. This makes it possible to make use of the best qualities of both systems. A vacuum dehydrator provides an efficient method to remove water and also gas from insulating oil. The vacuilm dehydrator exposes the oil to high vacuum and heat for a short

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period of time. Usually some device, such as a nozzle or baffle plates, are included to provide a large oil smface to assist in removing the water and gas from the oil If solid material is present in the oil, a filter should be used in conjunction with the vacuum dehydrator.

Reclaiming

Reclaiming of insulating oil consists of the removal of deterioration products such as sludge and organic acids. The media used for a reclaiming process is fuller's earth, which is a natural occurring clay. The excellent absorbent properties of fuller's earth will remove acids and other contaminants. The fuller's earth treatment is made ~y percolation through a coarse clay. Percolation may be accomplished either by pressure or gravity flow. The two methods are the same in principal ·and commercial equipment is availabl~·. In the pressure percolator the oil is forced through the clay by a pump. It has a chamber to hold the clay and is so designed that the oil must pass over the clay. Pressure percolators are capable of processing large volumes in a short time and since the amount of clay is relatively small, it must be changed frequently. In gravity percolation the oil flows through a column of coarse clay by gravity. A typical system would have a tank of oil to be treated at a high level above a cylinder or tank filled with the clay; below the treating chamber is a tank to receive the filtered material. Once this process is started, it continues with little attention other than periodic sampling. In this system the flow is slow and the volume of clay is relatively large and needs to be changed less frequently than with the pressure process. In either of these treatments checks on the neutralization number of the oil serve to indicate whether the treatment is adequate and when a change of clay is required.

Another method of clay treatment uses a finely divided clay and a relatively high operating temperature. This method will make the most efficient use of the clay. Commercial apparatm is available.for this treatment. It consists of a heated mixing chamber where measured amounts of oil and clay are stirred as they arc heated until a desired temperature is reached. The oil-clay mixture then goes into a tank from which it is pumped through a spcciaUy designed filter to separate the oil and the clay material.

The choice of reclaiming method is based upon the most practical and economical considerations for a particular system and location. In any case, if reclamation is to be pcrfonne~ the oil should first be put through a filter to remove the free water to prevent wetting of the clay. The oil which has been treated should be put into a container designed to prevent the pida1p of moisture.

Gas Analysis of Transformer

A combustible gas check should be made every two year on transformers below 288 kV. This may be done with a portable combustible g~ analyzer and if the combustible gas is 0. 5% or above, a gas sample should be taken and sent to t11c laboratory for analysis.

Conservator type transformers and others which are completely filled or which have a diaphraf:,'111 over the oil have no gas space can not be checked with a portable unit but in cases where gas is accumulated in the Buchholz Chamber. Using appropriate air tight containers, oil samples should be taken from these units and sent to the laboratory to have the gas extracted from the oil and analysed for different gases (combustible/non-combustible).

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Portable Gas Mete~'s

There are available on the world market several type; of portable combustible gas meters. On such meter is operated by filling a rubber bladder (supplied with the instrument) with a gas sample from the transformer. The rubber bladder is then attached to the inlet of the instrument and the gas passes through a venture where it is mixed with equal parts of air before entering the measwing portion of the instrument.

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Tes ting service

One type service involves analyzing the gas from the gas space as well as tests on an oil sample. These samples are taken in stainless steel sample containers.

Another type service involves extracting gas from an· oil sample and analyzing the gas sample and making tests on oil sample.

Interpretation

In the inten>ntation of gas analysis information of any transfonner, three considerations are very important:· (1) the total percent of combustible gas, (2) the percent of each component, and (3) the rate of increase in combustible gas content. Field testing of gas in the gas space of a transformer yields the total combustible gas present. The following is a list of ranges and suggested action:

1. 0-1.0% Combustible Gas - This is a very low percentage which requires no further action.

2. J.0-2.0% Combustible Gas - This is a low perccnl1ge of combustible gas which permits continued operatio~ but should be monitored on a monthly basis to establish the rate of combustible gas formation; if it continues to increase, a laboratory analysis should be made. Then if the combustible gases continue to increase, another laboratory test should be made to determin<-1 the combustible gases present and have the results analyzed by reference to the "Guidelines for gas Analysis Interpretations" stated below.

3. 2.0-5.0% Combustible Gas - Tbis is an intermediate quantity of combustible gas. A laboratory analysis should be made; and the results reviewed by reference to the "Guidelines for gas Analysis Interprdations". The unit should be evacuated, purged with nitrogrn and monitored every t\VO weeks to determine the rate of gas formation. lf the rate of gas formation L'> significant and a fault is indicated, the unit should he inspected and the fault corrected.

4. Above 5. 0% Combustible Gas - This is large quantity of combustihle gases. A laboratory analysis should be made and the resultc; studied by reference to the "Guidelines for Analysis Interpretations". The unit should be evacuated and purged with nitrogen and monitored daily to detennine the rate of gas formation. If the rate of gas formation is so high that the gas level cannot be monitored b.elow 5% \eve~ the transformer should be removed from service and the fault correcrcd.

Guidelines for Gas Analysis Interpretations

When a laboratory gas composition analysis is made, the above percentages apply to the quantities of combilstible gas which is present. Additional evaluations can be made as to a probable location and type of fault which caused the gas. 'The following guides apply to this type of interpretation.

1 Nitrogen plus 5% or less oxygen.

2 Nitrogen plus more than 5% oxygen.

1 Nitrogen, carbon dioxide and/ or carbon monoxide.

4. Nitrogen and hydrogen.

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Normal operation of transfonner

Check for tightness of transformer.

Transformer overloaded or operating hot, causing some cellulose breakdown. Check operating conditions.

Corona discharge, eletrolysis of water or rusting.

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5. Nitrogen, hydrogen, carbon dioxide, and carbon monoxide.

6 Nitrogen, hydrogen, methane with small amounts of ethane and ethylene.

7. Nitrogen with high hydrogen, methane, ethylene and acetylene.

8. Same as (8) except dioxide and carbon monoxide present.

Corona discharge involving cellulose or severe overloading of transformer.

Sparking or low level corona discharge causing some breakdown of oil.

High energy are causmg rapid deterioration of oil Brazed connections or tum-to-turn shorts are examples.

Same as (8) except arcing in combination with cellulose.

NOTE: In each case there may be other facts or variables present that may alter the interpretation.

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The detection of low energy inc1p1enr faults in transformers is an important protective maintenance technique to enable users to forestall gradual damage to transformers which can result in complete power o~tages. These faults in most cases cannot be detected electrically; however, they do result in gradual deterioration of solid and liquid insulation in the area of the fault. This deterioration can be caused by a defective joint, a poor connection, improperly placed materials, etc. and will generated gases, all of which are soluble in the insulating oil to varying degrees. As the oil becomes saturated with each gas the excess rises to the top of the oil. In the case of uniL.s complttely fil)ed with oil the gases fro;.n pocke~s. Some equipment utilize relays !c indicate these gas pochts. In the case of units with an inert gas space above the oil, the gas~s escape into the gas space. Analysis of a sample either of the gas in the gas space er the gas dissolved in the oil provides an excellent method for the early detection of faults.

"Transformer Gas Composition Analysis" provides for taking samples from the gas space of transformers for analysis.: An equipment for obtaining and analyzing to detcnnine the dissolved gas is known a Dissolved Gas Analyzer (DGA). 111is tcchruque can be used for all types of transformers but is particularly recommended for those units that are completely filled with oil such as those which use conservators or a diaphragm over the oil. The oil sample is tested in the laboratory where the dissolved gases are removed by allowing the oil to flow into an evacuated chamber as a thin film so that the oil is thoroughly exposed to the vacuwn, allowing the gases to volatilize. The system is returned to atmospheric pressure and the gases which have been drawn off are measured. From the volume of oil dq;~issed in the chamber and !h~ volume of reka~~d gas, content may be calculated. The released gases are then analyzed using a gas chromatograpn to detennine their composition. This analysis offers a means by which the type and sometimes the location of the fault may be determined, thus indicating the corrcc tivc action required.

Sampling

Accw-ate sampling is extremely important in order to determine composition of the gases which are present and the percentage of each, and to assure that the oil sample is representative. Careless sampling may lead to erroneous conclusions based on the analysis of the test data. In taking samples from the transformer, it is essential that the prescribed procedures be followed in order to obtain a representative oil sample.

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