When a voltage is applied to the insulating oil, as the voltage increases, the current through the oil increases sharply, causing it to completely lose its inherent insulating properties and become a conductor. This phenomenon is called breakdown of the insulating oil. The critical voltage value at which the insulating oil breaks down is called the breakdown voltage. The electric field strength at this time, called the dielectric strength of the oil, indicates the ability of the insulating oil to resist the electric field. The relationship between the breakdown voltage U (kV) and the dielectric strength E (kV/cm) is: E=U/d“
d”=The distance between the electrodes (cm).
Pure insulating oils have different breakdown mechanisms than insulating oils which usually contain impurities.
The breakdown of the former is caused by the liberation, which can be explained by the mechanism of gas dielectric breakdown, that is, under high electric field strength, oil molecules collide and become free ions and electrons, thereby forming electron collapse. The electron collapses toward the anode, and the accumulated positive charge collects near the cathode, eventually forming a channel with high conductance, resulting in breakdown of the insulating oil.
Generally, insulating oils always contain more or less impurities, and in this case, impurities are the main cause of breakdown of insulating oil. The dielectric coefficient ε of water droplets, fibers and other mechanical impurities in oil is much larger than that of oil (ε=7 of fiber, ε=80 of water, and ε≈2.3 of transformer oil), so under the action of electric field, impurities It will be attracted to a region where the electric field strength is large, and an impurity "small bridge" is formed between the electrodes, thereby reducing the breakdown strength of the oil. If there are enough impurities, it can also constitute a "small bridge" that penetrates the gap of the electrode, and a large leakage current flows through it, causing it to generate heat, and partially boiling and vaporizing the oil and water. As a result, the breakdown is along the "air bridge." And it happened.
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