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What is dielectric strength of insulating oil

What is dielectric strength of insulating oil


What is dielectric strength of insulating oil ? Why is the insulating oil breakdown?



Electrical insulation oil is one of the important raw materials for power transmission and transformation systems. It is mainly used in oil-filled electrical equipment such as transformers, capacitors, cables, and oil switches, and plays a role in insulation, heat dissipation and cooling.


Put the insulating oil sample in the specified oil cup. Under the specified test conditions, gradually increase the voltage of the oil test between the two electrodes. When the voltage rises to a certain value, the voltage of the oil test between the two electrodes almost suddenly drops to Zero, the current passing through the test oil increases suddenly, and it is often accompanied by the phenomenon of light emission, sound, smoke, and carbon particles dissociating in the oil. It is usually said that the oil is "breakdown", and the critical voltage at this time is called It is the "breakdown voltage", and the electric field strength (average breakdown voltage value per centimeter) at this time is called the "dielectric strength" of the insulating oil. It is used to express the ability of insulating oil to resist electric fields.

I.e. E = U/d

d ——Distance between electrodes, cm;dielectric strength of insulating oil

U ——The average breakdown voltage of oil test, kv;

E ——Dielectric strength of oil test, kV/cm.

The main form of insulation oil breakdown is electrical breakdown. There are always a certain amount of free ions in the oil. When pressurized, the ions on the electrode surface begin to move from one electrode to the other due to ionization. Before the ionization reaches the level of breakdown, the speed of ions increases with the increase in voltage, and the number of ions also increases rapidly. Under the action of a strong electric field, they move faster and increase in number, which will cause a sharp increase of current, leading to the breakdown of the insulating oil. This breakdown generally requires a higher voltage, so both the new insulating oil and the insulating oil in operation with good quality have a higher breakdown voltage.


Besides, the insulating oil will inevitably absorb moisture, pollution and deterioration, so that the oil contains impurities (especially polar impurities), which are polarized under the action of a strong electric field and arranged along the direction of the electric field. A conductive "small bridge" is formed between the electrodes, causing the insulating oil to be broken down. This is the so-called "small bridge" theory.


According to the above-mentioned mechanism of insulating oil breakdown, as well as practical experience, the main reason for insulating oil breakdown is: pollution of insulating oil caused by external impurities, which has little to do with the chemical composition of the insulating oil itself. It is not a standard for evaluating the quality of insulating oil, but a routine and necessary test. It is used to clarify the degree of contamination of insulating oil by water and other suspended substances, and whether it is appropriate to conduct vacuum drying and filtration tests before injecting into power transformers or circuit breakers and other equipment.


When the insulating oil is polluted by external impurities such as moisture, particulate matters, and the pollutants caused by insulating oil oxidation itself, such as decaying products, acids, etc. The dielectric strength may not meet the required value and standards, which needs to be improved by transformer oil treatment (Filtration, Vacuum degassing, and Regeneration). PureTech's DVOP Double-stage High Vacuum Transformer Oil Purifier is designed especially for power transformers above 110 KV level, it can quickly and effectively remove water, gases and mechanical impurities from insulating oil. The oil flow capacity ranges from 1800 liters/hour to 18000 liters/hour, and OEM service is accepted.

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