Why is "vacuum" the crucial first step in transformer oiling? -- In-depth Analysis of the Principle of vacuum degassing and dehumidification
During the installation or major overhaul of transformers, oiling may seem simple, but in fact, it involves many tricks. A "golden rule" strictly followed by the industry is that before oil injection, the transformer body must undergo high vacuum treatment. This is by no means an unnecessary step; rather, it is the lifeline that ensures the stable operation of the transformer for decades. This article will delve deeply into the principle of vacuum degassing and dehumidification, revealing why "vacuum" is the irreplaceable and crucial first step in all of this.
When a huge transformer stands in the substation, its "blood" - insulating oil - is about to be injected. Before injection, technicians will spend a considerable amount of time using a professional vacuum pump to evacuate the interior of the transformer oil tank to a state that is infinitely close to vacuum. Many customers will ask: Why go to so much trouble? Isn't it more time-saving to inject oil directly?
Under normal pressure, the core (iron core and windings) of a transformer and the inner walls of the oil tank may seem dry, but in fact, they are hiding two major enemies:
Moisture
Insulation killer: The insulation strength of transformer oil will drop sharply due to a trace amount of water. Moisture can reduce the breakdown voltage of oil. Under high-voltage electric fields, it is very likely to cause partial discharge and even insulation breakdown, leading to catastrophic failures.
Aging catalyst: Moisture will undergo hydrolysis reaction with insulating paper (cellulose), accelerating the aging and embrittlement of paper fibers, causing them to lose insulation and mechanical strength, and significantly shortening the service life of transformers.
The culprit of rust: Moisture can cause the metal components inside the transformer (such as the core and clamps) to rust, affecting the performance of the magnetic circuit and generating impurities to contaminate the oil quality.
Gases (such as oxygen)
Oxidation accelerator: Oxygen is the main culprit for the aging of insulating oil and insulating paper. It will cause the oil to produce acidic substances and sludge (sludge), which will adhere to the windings and heat sinks, affecting heat dissipation and insulation.
Discharge accomplice: Gases dissolved in oil may precipitate under the influence of electric field and temperature changes, forming bubbles. The dielectric constant of bubbles is much lower than that of oil, which can easily cause partial discharge and threaten insulation safety.
Ii. Vacuum Degassing and Dehumidification: Unveiling the Scientific Principles
So, how can we get rid of these stubborn "saboteurs"? The answer is: Create a vacuum environment and use physical laws to make them "leave on their own without being driven away".
Lower the boiling point to make the water "boil" and evaporate
We all know that water boils at 100℃. However, in high-altitude areas, due to low air pressure, the water boils before it reaches 100℃. Vacuum oil injection precisely utilizes this principle.
By vacuuming the transformer oil tank, we have greatly reduced the internal pressure. When the air pressure drops to a certain level, the ambient temperature (even at room temperature) is much higher than the "boiling point" of water at that low pressure. At this point, the liquid water remaining deep in the insulating paper and in the gaps of the iron core will rapidly vaporize and turn into water vapor. Once they turn into gaseous state, they can be continuously pumped out of the oil tank by powerful vacuum pumps.
This process is like boiling water on the Qinghai-Xizang Plateau, except that the "low-pressure environment" we create is much more extreme than that on the plateau.
2. Dissolved gases have nowhere to hide
According to Henry's Law, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid surface. When we evacuate the air, the partial pressure of the air above the oil surface (including oxygen and nitrogen) almost drops to zero.
This makes:
For components that have already been immersed in oil, the gases dissolved in the oil will precipitate rapidly.
For the cavities that have not yet been oiled, all the gases in the corners are directly drawn away.
The vacuum environment is like an invisible giant hand, completely "pulling out" the gases deeply hidden in the pores of insulating materials and dissolved in the residual oil film.
Iii. The Serious Consequences of Ignoring the vacuum Step: Risks and Costs
If vacuum treatment is skipped or simplified to save time, it is no different from planting a "time bomb" for the transformer:
Inherent insulation performance deficiency: Partial discharge may occur in the early stage of operation, doubling the risk of breakdown.
Accelerated aging and sharp reduction in lifespan: A transformer with a designed lifespan of 40 years may need major repairs or replacements after only 10 years of operation due to severe insulation aging.
Soaring operating costs: Oil products need to be frequently filtered and degassed, significantly increasing maintenance costs.
Safety hazard: The risk of sudden failure increases, threatening the stability of the power grid.
"Vacuuming", this seemingly simple step, is an unshakable cornerstone in the oil injection process of transformers. Through ingenious physical principles, it fundamentally eliminates potential threats that affect the lifespan and safety of transformers, making it the most efficient and economical investment to ensure the long-term, reliable and economic operation of your power assets.