Insulating oil in an electrical power transformer is commonly known as Transformer Oil. It is normally obtained by fractional distillation and subsequent treatment of crude petroleum. That is why this oil is also known as Mineral Insulating Oil. Transformer Oil serves mainly two purposes one it is liquid insulation in electrical power transformer and two it dissipates heat of the transformer i.e. acts as coolant. In addition to these, this oil serves other two purposes, it helps to preserve the core and winding as these are fully immersed inside oil and another important purpose of this oil is, it prevents direct contact of atmospheric oxygen with cellulose made paper insulation of windings, which is susceptible to oxidation.
1. Electrical Parameters – Dielectric Strength , Specific Resistance, Dielectric Dissipation Factor.
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.
Dielectric Strength of Transformer Oil
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 strants between two electrods 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.
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.
That is why specific resistance or resistivity of transformer oil should be measured at 27oC as well as 90oC.
Transformer winding resistance measurement
Transformer winding resistance measurement is carried out to calculate the I2R losses and to calculate winding temperature at the end of a temperature rise test. It is carried out as a type test as well as routine test. It is also done at site to ensure healthiness of a transformer that is to check loose connections, broken strands of conductor, high contact resistance in tap changers, high voltage leads and bushings etc.
There are different methods for measuring of transformer winding, likewise
♣ Current voltage method of measurement of winding resistance.
♣ Bridge method of measurement of winding resistance.
♠ Kelvin bridge method of Measuring Winding Resistance.
♣ Current voltage method of measurement of winding resistance.
♣ Bridge method of measurement of winding resistance.
♠ Kelvin bridge method of Measuring Winding Resistance.
Transformer winding resistance measurement is carried out as a type test, routine test and also as a field test.
In the factory, it helps in determining the following :
(a) Calculation of the I2R losses in transformer
(b) Calculation of winding temperature at the end of temperature rise test of transformer.
(c) As a benchmark for assessing possible damages in the field.
In the factory, it helps in determining the following :
(a) Calculation of the I2R losses in transformer
(b) Calculation of winding temperature at the end of temperature rise test of transformer.
(c) As a benchmark for assessing possible damages in the field.
Current Voltage Method of measurement of winding resistance
The transformer winding resistances can be measured by current voltage method. In this method of measurement of winding resistance, the test current is injected to the winding and corresponding voltage drop across the winding is measured.
By applying simple Ohm's law i.e. Rx = V ⁄ I, one can easily determine the value of resistance.
Dielectric Dissipation Fact
Bridge Method of measurement of winding resistance
The main principle of bridge method is based on comparing an unknown resistance with a known resistance.
When electric currents flowing through the arms of bridge circuit become balanced, the reading of galvanometer shows zero deflection that means at balanced condition no electric current will flow through the galvanometer. Very small value of resistance ( in milli - ohms range) can be accurately measured by Kelvin Bridge method whereas for higher value Wheatstone bridge method of resistance measurement is applied. In bridge method of measurement of winding resistance, the errors is minimized.
Loss factor or 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 currentthrough 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 simplytan 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.
Again it is clear from above diagram, value of ′δ′ which is also known as loss angle,
is smaller, means resistive component of the current IR is smaller which indicates high resistive property of the insulating material. High resistive insulation is good insulator. Hence it is desirable to have loss angle as small as possible. So we should try to keep the value of tanδ as small as possible. High value of this tanδ 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. So both resistivity test and tan delta test of transformer oil are not normally required for same piece of insulator or insulating oil.
In one sentence it can be said that, tanδ is measure of imperfection of dielectric nature of insulation materials like oil.
is smaller, means resistive component of the current IR is smaller which indicates high resistive property of the insulating material. High resistive insulation is good insulator. Hence it is desirable to have loss angle as small as possible. So we should try to keep the value of tanδ as small as possible. High value of this tanδ 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. So both resistivity test and tan delta test of transformer oil are not normally required for same piece of insulator or insulating oil.
In one sentence it can be said that, tanδ is measure of imperfection of dielectric nature of insulation materials like oil.
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