Electrical engineers should be familiar with what is transformer oil, transformer oil or insulation oil is widely used in electrical equipments, but what is the main purpose of transformer oil ? Let us to discuss it today.
(1) Transformer oil plays the role of cooling, heat dissipation and insulation in the transformer.
When the transformer is running, the current passes through the coil, and the heat (Q) generated by the thermal effect is:
If the heat cannot be discharged in time, the temperature of the coil and the iron core will increase accordingly. When the temperature is too high, it can damage the insulating material of the coil or burn the coil. Oil in it, can play a role in expelling heat. The cooling process of oil to transformer core and wire group is accomplished by liquid conduction and convection.
After the oil close to the iron core and the coil absorbs a lot of heat generated in the iron core and the coil, the oil temperature rises, the density decreases, it rises upward, flows to the top, the tank wall and enters the transformer radiator from the upper part, passing through the tank wall and the transformer radiator. The radiator dissipates heat to the outside; after cooling, the density of the oil increases, flows downward, enters the bottom of the transformer, and flows to the position of the original hot oil to supplement the oil flow. At the same time, the cooled oil cools the iron core and the winding, the oil is heated again, and the convection is recirculated, so that the temperature tends to be uniform and the heat dissipates quickly. Transformers, transformers, bushings and other electrical equipment must operate in a medium with high dielectric strength, otherwise breakdown will occur, and transformer oil is a medium with "insulating" properties, and its dielectric strength is 5 times greater than that of air. In addition to the insulating effect, the oil also has a certain protective effect on the insulating materials in the equipment. If the transformer oil in electrical equipment such as transformers is insufficient or the oil level is too low, breakdown may occur due to insufficient dielectric strength.
(2) The circuit breaker oil mainly plays the role of extinguishing the arc in the oil circuit breaker. It also plays an insulating role in the past multi-oil circuit breakers.
When the oil circuit breaker cuts off the current, the internal static and moving contacts generate an arc at the moment of separation, and the arc temperature can be as high as several thousand degrees. The higher the voltage or the higher the current, the stronger the arc. If the arc is not extinguished in time, the equipment may be burned; if the arc cannot be extinguished, the power system may oscillate, causing overvoltage and causing power system accidents.
Generally speaking, the arc temperature is as high as 2700~7200℃, or even higher. At this high temperature, the oil quickly evaporates into oil and gas, and part of the oil and gas is decomposed into gas. Therefore, bubbles are formed around the arc, and 40% of the bubbles are oil and gas, and 60% are decomposed gases. The gas contains more than 70% hydrogen, more than 18% acetylene, and 5% to 10% methane. Hydrogen has high thermal conductivity and minimal density of all gases. Its thermal conductivity is about seven times that of air, and its density is only 7% of that of air. Hydrogen is an active gas, which can quickly cool the arc column, and its existence is beneficial to the extinguishing of the arc. Secondly, the oil vapor and gas heated by the arc are bound to expand, but they are hindered by the surrounding oil and the tank wall, so the pressure of the bubbles increases. In a general oil circuit breaker, it can reach more than 5 atmospheres. The convective heat dissipation effect of the gas is strengthened, and with the different structures of the arc-extinguishing chamber, the gas with pressure passes through the arc-blowing path of the arc-extinguishing chamber with different structures to force the arc to blow air, so that the length of the arc column becomes thinner, so the temperature of the arc is reduced. Lowering is easy to extinguish, so insulating oil is the medium for extinguishing the arc.
The oil in the oil-less circuit breaker is only used to extinguish the arc, and the insulation of the live part mainly relies on porcelain bottles or organic insulating materials. The oil in the multi-oil switch not only extinguishes the arc, but also undertakes the insulation of conductors and contacts to the ground.
In order to ensure the role of oil in the oil circuit breaker, the oil circuit breaker must also maintain a certain oil level, and the oil level is not allowed to be too high or too low. If the oil level is too high, the buffer space in the circuit breaker box will be correspondingly reduced. If the buffer space is too small, when the fault current is interrupted, the arc light generated will vaporize the surrounding insulating oil, resulting in strong pressure and oil injection will occur. If the oil level is too low, when the fault current is interrupted, the arc light may rush out of the oil surface, and the free gas will be mixed into the air, resulting in combustion and explosion. At the same time, the oil level is too low, which also exposes the insulation to the air, which is easy to cause moisture. The oil level standard is implemented in accordance with the manufacturer's regulations.
In the end, do you know the purpose of transformer oil filtration ? And why used transformer oil needs scheduled filtration and purification ? Why need transformer oil needs to be filtered and dehydrated before injecting to transformer ?
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