Oil Dissipation Factor (Tan-Delta) & Resistivity Test System

Oil suffers from electrical and mechanical stresses while a transformer is in operation. In addition there are contaminations caused due to chemical interactions with windings and other solid insulations, catalyzed by high operating temperature. As a result the original chemical properties of Transformer Oil changes gradually, rendering it ineffective for its intended purpose after many years. Hence this oil has to be periodically tested to ascertain its basic electrical properties, and make sure it is suitable for further use, or necessary filtration/regeneration has to be done. Testing of DC resistivity and Tan-delta/Dissipation-Factor of such insulating Oils is one of the most critical tests to determine the Oil condition. Power Electronical provides instruments to achieve this objective of accurately measuring the Power-Factor and Volume Resistivity of Insulating liquids

Specific Resistance and Dielectric Dissipation Factor of Transformer Oil

The specific resistance/resistivity of oil is a measure of DC resistance between two opposite sides of one cm3 block of oil. Its unit is ohm-cm at a 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 an overload 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 get measured at 27oC as well as 90oC. To measure this property of the transformer oil the equipment such as PE-AORDF-1[Automatic Oil Resistivity & Dissipation Factor (Tan-Delta) Test System] or PE‐ORDF‐2 [Manual Oil Resistivity & Dissipation Factor (Tan-Delta) Test System] are used.

Dielectric dissipation factor is also known as loss factor or tan delta of transformer oil. When an insulating materials is placed between live part and grounded part of an electrical equipment, leakage current will flow. As an insulating material is dielectric in nature the 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.

Loss Angle

Hence 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 plainly, the leakage current through insulation does have two component one capacitive or reactive, and another one is resistive or active. Again it is clear from the above diagram, the value of ′δ′ which is also known as loss angle.

If the loss angle is small, then the resistive component of the current IR is small which indicates a high resistive property of the insulating material. High resistive insulation is a 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. The high value of this tanδ is an indication of the presence of contaminants in transformer oil.

In one sentence it can be said that tanδ is a measure of the imperfection of dielectric nature of insulation materials like oil. Thus testing and measurement of it on periodic basis is very important, to do so equipment such as PE-AORDF-1[Automatic Oil Resistivity & Dissipation Factor (Tan-Delta) Test System] or PE‐ORDF‐2 [Manual Oil Resistivity & Dissipation Factor (Tan-Delta) Test System] are used.