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Properties Of Transformer Oil

Properties Of Transformer Oil

Properties Of Transformer Oil

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Transformer Oil

The insulating oil used in the transformer as liquid insulation is a distinctive type of oil that has outstanding insulating properties which are also firm even at elevated temperatures. The mineral oil is generally used in oil-based power transformers to prevent the development of arcing and corona discharge and to throw away the heat developed inside the transformer (i.e. act as a coolant).

The purpose of the insulating oil is to protect the transformer inner core and windings which are completely submerged down in the insulating oil. Also, it is the responsibility of the enclosed transformer oil to avoid the reaction of oxygen with the paper insulation which is made of cellulose material and thereby extending the life span of the transformer. The transformer insulating oil will act as a blockade between the oxygen available in the atmosphere and the solid insulation (cellulose paper) to prevent their immediate contact and thereby keep down the oxidation reaction. The amount of Insulating mineral oil placed inside the transformer is commonly evaluated by a MOG (Magnetic Oil level Gauge).

The transformer insulating oil can be analysed to determine the information regarding the health status of the transformer and therefore It is named as the information carrier.


The potency of the transformer insulating oil can be determined by some selected Features of the insulating oil. The desired features (or parameters) of transformer insulating oil are:

Electrical characteristics:

Dielectric capability, definitive resistance, dielectric dissipation factor.

Chemical characteristics:

wetness content, acidity, sludge content.

Physical features:

Interfacial pressure, viscosity, Importance of flash point and pour point.

Electrical characteristics of Transformer Insulating Oil

Dielectric Properties of Transformer Oil 

– The dielectric capability of the transformer oil (dielectric strength of transformer oil) depends on the breakdown strength (BDV) of transformer oil. Breakdown strength voltage is measured as the voltage at which spark plugs between the two electrodes that are dipped in the oil, isolated by a particular distance. A significantly less value of BDV indicates the existence of a considerable amount of wetness and steering ions in the oil. The method of evaluation of BDV of the insulating mineral oil involves a mobile BDV computing kit which is usually placed near to the equipment. Inside the kit oil is made to be retained in a pot and within it, a set of electrodes are placed in it. Then the voltage is cautiously accessed between the two acting electrodes. The applied voltage is all set to be restrained and the oil is observed continually to find the value of the voltage at which flashing initiates fire between the electrodes. This is the value of voltage whereon the dielectric ability of transformer oil holding the electrodes gets defected. The process can be repeated for several times and measurement is taken from the same fragments of oil, and the mean final value of these readings is calculated. BDV is a primary factor of transformer oil, as it indicates the health of oil abruptly and it can be estimated at the onsite eventually without much effort. In the initial years of service, the oil would be dry and clean and gives better BDV when compared with the oil after been in service for years which have accountable water content and other carrying impurities.

Definitive Resistance of Transformer Oil –

The other demanding property of transformer oil is the specific resistance which can be determined by finding the direct resistance across the two adverse sides of one cm3 slab of oil. The representing unit of specific resistance is found as ohm-cm at a particular rundown temperature. The temperature of the transformer insulating oil considerably increases, when the resistivity of the insulating oil fallout sharply. Later having a long break when the transformer is put on charging the temperature of the oil will rise to ambient value but when the transformer has fully loaded the temperature of the oil shoots up rapidly leading to the overloaded condition. Hence the transformer oil must possess increased resistivity at normal room temperature and it should also possess a desirable value of resistance at higher temperatures as well. Therefore the specific resistance of the transformer oil should be estimated at the temperatures of minimum as well as maximum set values. The definitive specific value of resistance for transformer oil at a set value of 90oC is found as 35 × 1012 ohm-cm and at 27oC the value change as 1500 × 1012 ohm-cm.

Dielectric Dissipation in Transformer Oil –

Dielectric dissipation factor can be termed by another names called loss factor is also accepted as the tan delta of the insulating transformer oil. When the insulating materials are located in the middle of the active section and the base section of electrical equipment, aperture current will be developed which flows through it . The insulating material which is non-conductive is offensive in nature and the current flow and therefore the flow of current through the dielectric material admirably leads the dielectric voltage by 90o. Here voltage represents the instant voltage that occurs in the middle of phase and ground parts of the transformer. Though it is evident that no insulating material is purely dielectric but possibly have some conducting properties. Also, it to know that the tan-delta value of the insulating transformer oil increases with decrease in the resistivity of the oil also vice versa. So it is not considered beneficial to conduct resistivity test and also tan delta test of transformer insulating oil because they are interdependent and do not require separate tests. It can also be concluded simply that tanδ is indicative of the conducting nature of imperfect non-conductive materials like oil.

How to check dielectric strength of transformer oil – dielectric strength test of transformer oil
Oil testing consists of measuring the breakdown voltage and other chemical and physical properties of the oil, either using portable test equipment or in a laboratory. Proper testing extends the life of the transformer, which reduces the need to pay for replacement.

Chemical characteristics of Transformer Oil

Moisture Analysis in Transformer Oil – the presence of moisture subjects or water subjects in transformer insulating oil is deeply problematic since it may dangerously affect the quality of dielectric properties in the oil which later accounts for the failure of the transformer. The paper insulation ( cellulose paper) in the equipment will also suitably affected by the water content accumulated in the oil. The cellulose paper used as solid insulation for protecting the core and windings of the transformer absorbs the water in the oil and losses it’s insulating property to a considerable extent and reduces the life span of the transformer. Paper tends to absorb the moisture content as it is hygroscopic in nature. But when the transformer is sufficiently filled with the load the temperature of the oil hikes and the wetness content in the oil tends to get soluble in it. This further results in the release of water from the paper also. Thus it is very tedious to take a sample of the oil with the increased temperature. Next, when the atmospheric oxygen reacts with the transformer oil, acid compounds get accumulated in it which reaches the saturation limit of water in the oil. The formed acidic compounds start deteriorating the oil which later accounts for the failure of the transformer failure. Hence it is better to analyse the amount of moisture content in the oil and to necessitate the proper preventive measures to restore the quality of insulating oil.

Acidity of Transformer Oil – Transformer oil with acidic constituents turns harmful for the effective functioning of the power transformers. Over the years of service, the insulating oil which acts as dielectric to protect the core and windings of the transformer gets dissolved by acidic substances due to the various reactions it undergoes. These acidic substances formed throughout the operating condition of the equipment starts deteriorating the insulation property of both liquid and solid insulation materials. Hence it is necessary to test the contaminated insulating oil for its acidity and to remove the acidic constituents from the oil as it also accelerates the process of ageing through oxidation. Rusting of the iron materials by which the core is made also takes place. The acidity can be measured as oil present in one mg of KOH need to counterpoise the acid materials present in one gram of oil. This can be termed as neutralization value.

Transformer Oil: Physical characteristics

Interfacial stress in the Transformer Oil – Naturally there exists a tension between the water and oil surface by which means the energy between the water content and oil interface can be measured and to be known as Dyne/cm or mill-Newton/meter. This interfacial stress parameter can be highly appreciated in determining the decaying products that got deposited in the dielectric insulating oil. It is also found good for the insulating oil to have a high value of Interfacial tension which is influenced by the oxidation reaction that takes place in the oil.

Flash Point Value of Transformer Oil – The temperature of the dielectric transformer oil whereon it starts giving out a vapor that forms a mixture with air which is highly flammable. This flammable gas is dangerous as it can produce sparks when exposed to fire. This flashpoint parameter is considered important as it analyses the character of the oil is getting fire. Therefore it is obligatory for a better transformer insulating oil to have a responsibly higher flashpoint.

Pour Point Value of Transformer Oil – The pour point factor of the insulating mineral oil is the least point of temperature where the oil began to flow changing from the frozen state. It is essential to closely monitor the pour point of the transformer oil if the equipment is in service at places where the temperature usually below zero degrees. If the temperature of the oil decreases rapidly below the pour point the oil gets freeze and start cooling the parts of the transformer. It is desirable to choose a transformer oil that has excellent pour point. The pour point of Paraffin-based oil is high when compared with Naphtha based transformer oil, but it is suggested to use paraffin oil in hot weather places. Pour Point of transformer oil mainly assessed by the amount of lubricant content present in the oil. The Paraffin derived transformer mineral oil has a higher pour point as it has more wax content in it.

The viscosity index of Insulating Transformer Oil – The viscosity level of the dielectric transformer oil is the measure of the amount of resistance offered by the oil to its continued flow even in normal operating temperatures. The oil provides resistance to its flow through the parts of the transformer which increase temperature. An effective transformer oil must retain a low viscosity so that it proposes comparably a small resistance opposing the customary flow and allows the proper functions like transferring the heat produced in the transformer. It is also proposed that the viscosity index of the transformer oil should be less and it should vary gradually when the temperature of the oil decreases. Most of the available transformer oil tends to be more viscous when the temperature drops down.

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