Transformer Oil 1

Transformer Oil

Petroleum oils have been used in electrical equipment since the latter part of the last century. Sebastian de Ferranti, who might be considered to have been the father of the transformer, recognised their benefits as long ago as 1891.

Their performance has been improved a little since then, both as a result of better refining techniques and in the way in which they are selected and used. They still represent a very important component of much electrical power plant.

Firstly, it is appropriate to look a little at the sources and production of oil.

All types of mineral oils are obtained from crude petroleum, which is said to have been formed from buried and decayed vegetable matter or by the action of water on metal carbides.

Crude petroleum is defined by American Society for Testing and Materials, ASTM. D288 as follows:

A naturally occurring mixture, consisting predominantly of hydrocarbons which is removed from the earth in liquid state or is capable of being removed.

Crude petroleum is commonly accompanied by varying quantities of extraneous substances such as water, inorganic matter and gas.

The removal of such extraneous substances alone does not change the status of the mixture as crude petroleum. If such removal appreciably affects the composition of the oil mixture then the resulting product is no longer crudepetroleum.

There are three main groups of hydrocarbon molecules. These are paraffins, napthenes and aromatics.

Each has a characteristic molecular structure, and no two crudes are exactly alike in the relative proportions of the hydrocarbon types or in the proportions and properties of the products to which they give rise.

Many classifications have been proposed for the various types of crudes, but the most generally accepted is that based on the main constituent of the distillation residue and consists of four descriptions: paraffinic, asphaltic, mixed or intermediate, and napthenic.

The world’s known supply of crude oil is made up of very approximately 7 per cent paraffinic, 18 per cent asphaltic (including 5 per cent napthenic) and 75 per cent mixed or intermediate.

Typical analyses of crude oils classified as naphthenic and paraffinic showing actual proportions of aromatics, naphthenes and paraffins present

PROPERTY REQUIREMNTS OF LIQUID DIELECTRICS  

EQUIPMENT ESSENTIAL PROPERTY DESIRABLE PROPERTY

TRANSFORMERS

Chemical stability towards Oxidation, Thermal Degradation and Hydrolysis Thermal characteristics Good thermal conductivity High specific eat fluids

Non Toxic Biodegradable Compatibility with transformer

construction materials- Mutual compatibility with other

transformer Fluids- Easy reconditioning- Easy to handle.

SWITCH GEARS Low carbon formation Good arc quenching Good Lubricating properties

-No deposition of solid or resinous materials while decomposition-Low Flammability.

COMPOSITION

• Paraffinic base crude predominantly paraffinic hydro carbon, high wax and bitumens.

• Naphthenic base crude predominantly naphthenic & aromatic, less wax, more bitumens.

• Mixed base crude intermediate between paraffin and naphthene. Both wax & bitumens.

Types of Transformer Oil

Generally there are two types of Transformer Oil used in transformer,

1. Paraffin based Transformer Oil2. Naphtha based Transformer Oil

Naphtha oil is more easily oxidized than Paraffin oil. But oxidation product i.e. sludge in the naphtha oil is more soluble than Paraffin oil. Thus sludge of naphtha based oil is not precipitated in bottom of the transformer. Hence it does not obstruct convection circulation of the oil, means it does not disturb the transformer cooling system.

But in the case of Paraffin oil although oxidation rate is lower than that of Naphtha oil but the oxidation product or sludge is insoluble and precipitated at bottom of the tank and obstruct the transformer cooling system. Although Paraffin based oil has above mentioned disadvantage but still in our country it is generally used because of its easy availability.

Another problem with paraffin based oil is its high pour point due to the wax content, but this does not effect its use due to warm climate condition of India.

Parameters of Transformer Oil

The parameters of Transformer Oil are categorized as,

1. Electrical Parameters – Dielectric Strength , Specific Resistance, Dielectric Dissipation Factor.

2. Chemical Parameter - Water Content, Acidity, Sludge Content.

3. Physical Parameters - Inter Facial Tension, Viscosity, Flash Point, Pour Point.

DensityDensity Of A Liquid At Temperature‘t’ Is The Mass of The Liquid Occupying Unit Volume at That Temperature

 Test Method: IS: 148 (Part 16) – 1977 Evaluating Methods:

•Hydrometer Method•Density Bottle Method

Density Range: 0.85 – 0.89 G / CcFacilitates Separation of Free Oil & Oil with Dissolved MoistureHigh Value Of Density – Poor Heat DissipationCharacteristics

Max. Limit As Per Is: 335 0.89 G / Cc

Viscosity of Transformer Oil

In few wards, Viscosity of Transformer Oil can be said that Viscosity is the resistance of flow, at normal condition. Obviously resistance to flow of transformer oil means obstruction of convection circulation of oil inside the transformer.

A good oil should have low viscosity so that it offers less resistance to the convectional flow of oil thereby not affecting the cooling of transformer. Low viscosity of transformer oil is essential, but it is equally important that, the viscosity of oil should increase as less as possible with decrease in temperature. Every liquid becomes more viscous if temperature decreases.

KINEMATIC VISCOSITY • Measure of Resistance to Gravity Flow of a

Liquid, The Pressure Head Being proportional to its Density UNIT: CENTISTOKES (Cst)

• Test Method – IS: 1448 (Part 25) – 1976• Time Required for Liquid to Flow x Viscosity =

K.V (Through a long Capillary)• VISCOSITY – TEMPERATURE RELATIONSHIP• Low Viscosity at Low Temperature • Efficient Heat Removal from Windings

Dielectric Strength of Transformer Oil is also known as Breakdown Voltage of transformer oil or BDV of transformer oil.

Break down voltage is measured by observing at what voltage, sparking starts between two electrodes immerged in the oil, separated by specific gap. low value of BDV indicates presence of moisture content and conducting substances in the oil.

For measuring BDV of transformer oil, portable BDV measuring kit is generally available at site. In this kit, oil is kept in a pot in which one pair of electrodes are fixed with a gap of 2.5 mm (in some kit it 4mm) between them. Now slowly rising voltage is applied between the electrodes. Rate of rise of voltage is generally controlled at 2KV/s and observe the voltage at which sparking starts between the electrodes. That means at which voltage Dielectric Strength of transformer oil between the electrodes has been broken down

ELECTRICAL STRENGTH(Breakdown voltage)

• Measure of resistance in kilovolts required to breakdown the oil across a specified gap under specified conditions.

• Measure of ability to withstand electrical stress • Indicated / detects the presence of free water, dirt, moisture,

cellulose fibre or conducting particles.(Except dissolved water, acids, sludge)

• The migration of wet solid matter of contaminants to the stressed

area between electrodes leads to formation of strings to bridge the gap

• Low value of BDV indicated presence of Contaminants / Moistures / Conducting particles.

DIELECTRIC STRENGTH

The dielectric strength of an insulating oil is a measure of the oils ability to withstand electrical stress without failure.

The test involves applying a ac voltage at a controlled rate to two electrodes immersed in the insulating fluid. The gap is a specified distance. When the current arcs across this gap the voltage recorded at that instant is the dielectric strength breakdown strength of the insulating liquid.

Contaminants such as water, sediment and conducting particles reduce the dielectric strength of an insulating oil. Combination of these tend to reduce the dielectric strength to a greater degree.

Clean dry oil has an inherently high dielectric strength but this does not necessarily indicates the absence of all contaminates, it may merely indicate that the amount of contaminants present between the electrodes is not large enough to affect the average breakdown voltage of the liquid.

Specific Resistance (Resistivity) of Transformer Oil

This is another important property of transformer oil. This is measure of DC resistance between two opposite sides of one cm3 block of oil. Its unit is taken as ohm-cm at specific temperature. With increase in temperature the resistivity of oil decreases rapidly. Just after charging a transformer after long shut down, the temperature of the oil will be at ambient temperature and during full load the temperature will be very high and may go up to 90oC at over load condition.

So resistivity of the insulating oil must be high at room temperature and also it should have good value at high temperature as well. That is why specific resistance or resistivity of transformer oil should be measured at 27oC as well as 90oC.

Minimum standard Specific Resistance of Transformer oil at 90oC is 35X 1012 ohm – cm and at 27oC it is 1500X1012 ohm – cm.

Specific Resistance (Resistivity)

Measure of Volume Resistance.

Resistivoity = Dc Electic Eield / Current Densitiy

Factors Affecting Resistivity•Conducting Impurities•Products like Acid and Sludge•Free Ions and Ions forming particles•Moisture

Testing Standard—IS6103

Limits: New oil 1500 E 12 -Cm @ 27 c

35 E 12 -Cm @ 90 c

Service oil : 0.1 E 12 -Cm @ 90 c

Dielectric Dissipation Factor of tan delta of Transformer oil

Dielectric Dissipation Factor is also known as loss factor or tan delta of transformer oil. When a insulating materials is placed between live part and grounded part of an electrical equipment, leakage current will flow. As insulating material is dielectric in nature the electric current through the insulation ideally leads the voltage by 90o. Here voltage means the instantaneous voltage between live part and ground of the equipment. But in reality no insulating materials are perfect dielectric in nature.

Hence electric current through the insulator will lead the voltage with an angle little bit shorter than 90o. Tangent of the angle by which it is short of 90o is called Dielectric Dissipation Factor or simply tan delta of transformer oil.

More clearly, the leakage current through an insulation does have two component one is capacitive or reactive and other one is resistive or active.

It is desirable to have loss angle as small as possible. So we should try to keep the value of tan δ as small as possible. High value of this tan δ is an indication of presence of contaminants in transformer oil.

Hence there is a clear relationship between tanδ and resistivity of insulating oil. If resistivity of the insulating oil is decreased, the value of tan-delta increases and vice verse.

In one sentence it can be said that, tanδ is measure of imperfection of dielectric nature of insulation materials like oil.

Dielectric dissipation factor(Power factor / tan delta / loss angle)

• Quality control test– Measure of Dielectric losses in oil

• Reveals presence of contaminants picked by oil – derived due to oxidation.– Poor manufacture & bad use of equipment

• High value of DDF leads to increased1. Heating leading to thermal runway2. Corrosion & oxidation3. Water solubility and emulsifying power

• Low value of DDF – ensures freedom from moisture, polar compounds, soluble impurities and minimum energy loss

• Low Tan Delta – increased gassing Resistivity & Decreases hydroscopicity

• Limits New oil @ 90º C 0.002Oil in service @ 90º C

0.2 For > 145KV class1.0 For < 145KV class

Water Content in Transformer Oil

Moisture or Water Content in Transformer Oil is highly undesirable as it affects adversely the dielectric properties of oil. The water content in oil also affects the paper insulation of the core and winding of transformer. Paper is highly hygroscopic in nature. Paper absorbs maximum amount of water from oil which affects paper insulation property as well as reduced its life. But in loaded transformer, oil becomes hotter, hence the solubility of water in oil increases as a result the paper releases water and increase the water content in transformer oil.

Thus the temperature of the oil at the time of taking sample for test is very important. During oxidation acid are formed in the oil the acids give rise the solubility of water in the oil. Acid coupled with water further decompose the oil forming more acid and water. This rate of degradation of oil increases. The water content in oil is measured as pm(parts per million unit).

Water content in oil is allowed up to 50 ppm as recommended by IS – 335(1993). The accurate measurement of water content at such low levels requires very sophisticated instrument like Coulometric Karl Fisher Titrator .

Acidity of Transformer Oil

Acidity of transformer oil, is harmful property. If oil becomes acidic, water content in the oil becomes more soluble to the the oil. Acidity of oil deteriorates the insulation property of paper insulation of winding. Acidity accelerates thee oxidation process in the oil.

Acid also includes rusting of iron in presence of moisture. The acidity of transformer oil is measure of its acidic constituents of contaminants. Acidity of oil is express in mg of KOH required to neutralize the acid present in a gram of oil. This is also known as neutralization number.

ACIDITY OR NEUTRALISATION NUMBER(NN)

Acids in the oil originate from oil decomposition/oxidation products. Acids can also come from external sources such as atmospheric contamination. These organic acids are detrimental to the insulation system and can induce corrosion inside the transformer when water is present.

An increase in the acidity is an indication of the rate of deterioration of the oil with SLUDGE as the inevitable by-product of an acid situation which is neglected.The acidity of oil in a transformer should never be allowed to exceed 0.25mg KOH/g oil. This is the CRITICAL ACID NUMBER and deterioration increases rapidly once this level is exceed

Flash Point of Transformer Oil

Flash point of transformer oil is the temperature at which oil gives enough vapours to produce a flammable mixture with air. This mixture gives momentary flash on application of flame under standard condition.

Flash point is important because it specifies the chances of fire hazard in the transformer. So it is desirable to have very high flash point of transformer oil. In general it is more than 140o(>10o).

Pour Point of Transformer Oil

It is the minimum temperature at which oil just start to flow under standard test condition. Pour Point of Transformer Oil is an important property mainly at the places where climate is extremely cold. If the oil temperature falls bellow the pour point, transformer oil stops convection flowing and obstruct cooling in transformer.

Paraffin based oil has higher value of pour point, compared to Naphtha based oil, but in India like country, it does not effect the use of Paraffin oil due tits warm climate condition. Pour Point of transformer oil mainly depends upon wax content in the oil. As Paraffin based oil has more wax content, it has higher pour point.

INTERFACIAL TENSION Measure of molecular attractive force between

their unlike molecules at the interface of oil and water UNIT = n/m Test Method – IS: 6104 – 1971

The force required to lift a planner ring of platinum

from the oil-water interface into oil is measured

INTERFACIAL TENSION(IFT)

The Interfacial Tension (IFT) measures the tension at the interface between two liquid (oil and water) which do not mix and is expressed in dyne/cm.

The test is sensitive to the presence of oil decay products and soluble polar contaminants from solid insulating materials.

Good oil will have an interfacial tension of between 40 and 50 dynes/cm. Oil oxidation products lower the interfacial tension and have an affinity for both water (hydrophilic) and oil. This affinity for both substances lowers the IFT. The greater the concentration of contaminants, the lower the IFT, with a badly deteriorated oil having an IFT of 15 dynes/cm or less.

Polar Impurities and IFT Value

The transformer oil during higher temperature of operation reacts with atmospheric oxygen and the metallic parts acts as catalyst thus forming organic peroxides and small quantity of Aldehydes and Ketones.

As the oxidation proceeds, larger quantity of organic acids and other compounds of polar nature are releases into the oil.

The term “Polar” is used to describe a chemical bond, chemical group or molecules that are electrically unsymmetrical.

When a polar compound exceed s certain level, polymerization takes place

These polymers insoluble in oil and hydrophilic in nature are called SLUDGE.

When the concentration of polar impurities such as acids, aldehydes increases in the oil , the IFT value of oil is lowered.

A sharp decline in IFT value is observed just prior to formation of sludge.

Thus by conducting IFT test, the sludge formation can be easily predicted and corrective measures can be taken.

Mechanism of Quick field Test for IFT

When the oxidized oil is placed on paper, the polar compound present in it Are strongly absorbed , leaving behind the oil to be weakly absorbed.On placing the polar containment indicator on the centre of oil spot , the non polar oil molecules and as well as indicator spreads.

The displacement and the time taken for displacement depends upon theConcentration of polar compound present in oil. If the concentration is appreciable The polar indicator front moves more irregularly and slowly as it spreads through location of least resistance first. Due to this the shape of the indicator spot becomes irregular.

Degree of irregularity increases with the concentration of polar contaminants in oil and hence it is related to IFT of oil.

Place a drop of oil Oil spreads Place a drop of Polar indicator 45 dynes/cm

35 dynes/cm 25 dynes/cm 20 dynes/cm 15 dynes/cm

IFT QUICK FIELD TEST

Chart for rapidly determining the acid content of insulating oils

0.00 0.05 0.10 0.20 0.30 0.40 0.50 0.60 0.80 1.00

Nutralization Number (mgKOH/g)

Quick Field Test for ACIDITY

Take 1.1ml of transformer to be tested(approximately 1gram) in a test tube

Add 1ml of solution of 0.0085 N Sodium Carbonate and shake well

Add 5 drops of universal indicator

The resulting mixture develops a colour depending on the ph value.

Compare the colour with the colour chart given and find out the acidity value

IFT-NN Relationship

Studies have shown that a definite relationship exists between acid number(NN) and Interfacial Tension(IFT). An increase in NN should normally be followed by a drop in IFT. The IFT test is a powerful tool for determining how an insulating oil has performed and how much life is left in the oil before maintenance is required to prevent sludge.

The IFT provided an excellent back up test for the NN. IFT not accompanied by a corresponding increase in NN indicates polar contamination which have not come from normal oxidation.

Although a low IFT with a low NN is an unusual situation , it does occur because of contamination such as solid insulation materials, compounds from leaky pot heads or bushings, or from a source outside the transformer

The magnitude of the IFT is inversely related to the concentration of the hydrophilic degradation products from the deterioration of the oil. Since the hydrophilic materials are usually highly polar and thus not very soluble in the non-polar oil, the presence of these species can result in sludge formation. These materials that remain dissolved in the oil can affect the desired electrical properties of the oil. They will reduce the dielectric strength and increase the dissipation factor of the oil.

Sludge build-up can also affect the heat transfer characteristics of the oil by slowing or perhaps even blocking circulation of the oil. There is usually an inverse relationship between the neutralization number of an oil and its IFT.

As an oil sample undergoes oxidative degradation, its neutralization number will increase while its IFT value will decrease. It should also be recognized that a decrease in the IFT does not imply that the acidity must also be high, since there are other non-acidic contaminants that could be present in the oil that are hydrophilic and will lower the IFT but not raise the acidity.

QUALITY INDEX SYSTEM

Dividing the Interfacial Tension (IFT) by the Neutralisation Number (NN) provides a numerical value that is an excellentmeans of evaluating oil condition.

This value is known as the Oil Quality Index (OQIN) or Myers Index Number (MIN).

A new oil, for example has an OQIN of 1500.

OQIN = IFT 1500=45.0(typical new oil)/NN 0.03(typical new oil)

Typical pattern of increase in NN as a function of time is exhibited by curve for transformer oil operated at 60°C. The exponential rise in NN at the critical point results from the catalytic action of acids and the depletion of the oxidationinhibitors. Heat is the greatest accelerator of oil deterioration; deterioration is most marked above 60°C. Beyond 60°C, the rate of deterioration approximately doubles for each 10°C increase.

OXIDATION STABILITY(GLASS TUBE METHOD)

25g OF OIL IN A GALSS TUBE + 9.7Sq.cm COPPER WIRE - - - HEATED AT 100° C FOR 164 HOURS QHILE OXYGEN IS BUBBLED AT THE RATE OF 1 Lt / HOUR.

AFTER TEST OIL IS DILUTED WITH HEPTANE SLUDGE CONTENT & ACIDITY ARE DETERMINED.

SPECIFICATIONS:FOR NEW OILS: 1. TOTAL ACIDITY, mg KOH / g - < 0.42. TOTAL SLUDGE, % BY Wt. - < 0.1

OXIDATION STABILITY(GLASS TUBE METHOD)

SIGNIFICANCE:

DEVELOPMENT OF ACIDITY & SLUDGE DURING STORAGE OF OIL, AND ITS SERVICE LIFE IS MINIMISED IF THE OIL PASSES THIS TEST.

THE QUALITY OF OIL THEN WOULD BE SUITABLE.

CORROSIVE SULPHUR

DETECTION OF SULPHUR COMPUNDS IN OIL BY DISCOLOURATION OF SOPPER SURFACE IN CONTACT WITH OIL AT ELEVATED TEMPERATURE

METHOD:

25g OF OIL + 6 x 25mm POLISHED Cu STRIP BENT 60 - - - NITROGEN BUBBLED FOR 1 MIN - - - HEATED 140º C FOR 19 HOURS

COLOUR OF COPPER AFTER TEST: 1. Orange red, multi-coloured - - - non-corrosive 2. Black, gray or brown - - - corrosive New insulating oil is required to be non-corrosive Presence of sulphur compounds in oil - - Degrade metal windings in equipments

OXIDATIVE AGEING BY OPEN BEAKER METHOD

ANOTHER METHOD TO DETERMINE THE QUALITY OF NEW INSULATING OIL WITH RESPECT TO OXIDATIVE DEGRADATION

METHOD:

300 ml OF OIL + 15 Sq.cm OF POLISH COPPER WITH HEATED AT 115º C FOR 96 HOURS

After cooling the properties determined along with the specified limits are: I. DDF at 90º c - < 0.2Ii. Resistivity @ 90º c - > 0.2 x 10

@ 27º c - > 2.5 x 10Iii. Acidity - < 0.05 mg KOH/ gIv. Sludge content - < 0.05 % BY wt.

TABLE 1: APPLICATION AND INTERPRETATION OF TESTS ON OIL IN TRANSFORMERS AS PER IS: 1866 – 1983

Sl. No. Characteristic

Equipment voltage

Permissible limit, satisfactory for use

Action if outside permissible limit

1 Specific resistance at 90° c ohm-cm, min

All voltages 0.1 * e12

Recondition it the value of dielectric dissipation

factor permits, reclaim if not.

2 Dielectric dissipation factor at 90° c, max

> 145 kv< 145 kv

0.21.0 Reclaim

Sl. No Characteristic Equipment

voltagePermissible limit,

satisfactory for useAction if outside permissible limit

3Neutralization value, mg / KOH / g

All voltages 0.5 Reclaim

4 Precipitate sludge All voltages No perceptible

sludge Reclaim

5 Flash point All voltages

Decrease in flash point 15°c (max) of

the initial value, minimum value

125° c

Reclaim after checking causes.

Sl. No Characteristic Equipment

voltagePermissible limit,

satisfactory for useAction if outside permissible limit

6 Interfacial tension at 27°c, n/m, min All voltages 0.015 Reclaim.

7 Electric strength

145 kv + above

< 145 kv> 72.5 kv< 72.5 kv

50kv min40 kv min40 kv min30 kv min

ReconditionReconditionReconditionRecondition

8 Water content ppm

> 145 kv< 145 kv

25 ppm35 ppm

ReconditionRecondition

LIMITING VALUES IS: 1866 – 1983

Test Equ. Voltage Method Limit

Electric strength > 145 kv< 145 > 72.5 kv< 72.5 kv

Is: 6792

504030

Water content ppm, max

> 145 kv< 145 kv

Is: 335

2535

Specific resistance @ 90, 10 E12 ohm, cm All voltage Is: 61 0.1

Tan delta @90, max > 145 kv< 145 kv Is: 6262 0.2

1.0

ACIDITY mg KOH / g, Max ALL VOLTAGE IS: 144 0.5

IFT, N/m. Min ALL VOLTAGE IS: 6104 0.015

FLASH POINT Deg C, Min ALL VOLTAGE IS: 1448

Max Dec 15 125

SEDIMENT AND / OR PRECIPITABLE SLUDGE ALL VOLTAGE IS: 1666 NIL

FOR TRANSFORMERS IN SERVICE

Properties Condition Remedies

Breakdown voltage LowFiltration under vacuum + temperature

Moisture High

Dielectric dissipation factor High

Filtration as above if the cause is moisture or other

Volume resistivity Low

IFT Low Reclamation

GENERAL

NV HIGH

FP LOW INVESTIGATE THE CAUSE

SLUDGE % HIGH DE-SLUDGING

SUGGESTED LIMITS FOR IN SERVICE OILS GROUP I BY VOLTAGE CLASS, BASED ON IS: 1866-1983

LIMITS _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

VOLTAGE CLASSPROPERTIES

145 KV & 72.5 KV BELOWABOVE & < 145KV 72.5 KV

ELECTRIC STRENGTH ≥ 50 ≥ 40 ≥ 30(MIN) (MIN) (MIN)

NEUTRALISATION VALUE ≤ 0.1 ≤ 0.1 ≤ 0.1(mgKOH/g) (MAX) (MAX) (MAX)INTERFACIAL TENSION N/m ≥ 0.015 ≥ 0.015 ≥ 0.015

(MIN) (MIN) (MIN)DIELECTRIC DISSIPATION ≤ 0.2 ≤ 1.0 ≤ 1.0FACTOR AT 90° C (MAX) (MAX) (MAX)WATER CONTENT, ppm ≤ 25 ≤ 35 ≤ 35

(MAX) (MAX) (MAX)SPECIFIC RESISTANCE Ohm-cm 0.1x1012 ≥ 0.1x1012 ≥ 0.1x1012