Requirements for Purification Adsorbent in Transformer Oil Operation
Due to the high requirements on the quality of transformer oil, strictly speaking, it is difficult to judge whether the quality of the oil is qualified without a simplified test, but sometimes the unqualified oil can also be initially judged from the appearance.
(1) The color of the oil. The new oil is generally light yellow, and the color becomes darker after oxidation. If the new oil is dark and dark, it is unqualified. The color of the oil darkens rapidly during operation, indicating that the oil quality has deteriorated.
(2) Transparency. The new oil is clear and fluoresces purple in a glass bottle. Loss of fluorescence or loss of clarity indicates mechanical admixture and free carbon.
(3) Smell. The new oil has a slight kerosene smell. If there is any other smell, it means that the oil quality has deteriorated. If there is a burnt smell, it means that the oil is overheated; if there is a sour smell, it means that the oil is seriously aged;
3 Operation requirements of transformer oil
During the operation of transformer oil, if it contacts with the air, it will get damp. This will unfairly reduce the breakdown voltage, increase the dielectric loss, and enhance the catalytic effect on metal corrosion, causing the formation of sediment in the oil. It is generally believed that the deteriorated oil is more susceptible to moisture than new oil, and the deterioration rate is 2-4 times faster. If it is found that most of the silica gel changes from blue to red or purple, it means that the desiccant has failed, and the dried silica gel should be replaced.
Since transformer oil is in operation, it will gradually age due to the influence of air, sunlight, humidity and temperature, so it is necessary to take active measures to keep it in good condition as much as possible to prolong its service life. At present, the following measures are generally adopted in actual production.
– Oxygenation protection
Inert nitrogen is used to fill the upper space of the transformer oil conservator, so that the oil is always in contact with the nitrogen, so that the transformer oil and the air are isolated, preventing premature aging of the oil.
Oil purifier for second injection
The oil purifier is a metal container filled with an adsorbent (acid-removing silica gel or activated alumina). When the transformer oil flows through the adsorbent, the water in the oil, free acid and oxides that accelerate the aging of insulating oil are all absorbed by the adsorbent. Absorbed, the oil is continuously regenerated, and the oil quality can remain in a qualified state for a long time. According to the different ways of oil circulation in the oil purifier, it can be divided into two categories. Thermocirculation oil purifier and forced circulation oil purifier
(1) Selection and dosage of adsorbent
The adsorbent should have the characteristics of porosity, strong activity, and strong acid absorption, so as to absorb moisture, peroxides, organic acids, carbonyls and other substances harmful to transformers and oil in oil. Generally, there are only two kinds of acid-removing silica gel (coarse and fine-pored silica gel) and activated alumina that are used with better and cheaper results. They are spherical and rod-shaped irregular porous solid substances, and their performance has the following requirements:
①Water absorption>20%
②Acid absorption>5mg (KOH)/g (silica gel);
③ Moisture content <0.5%;
④ The outer shape is milky white.
Therefore, the adsorbent should be dried and sieved before being added to the oil purifier.
The amount of adsorbent is determined according to the total mass of oil. When using acid-removing silica gel, it is about 0.75% to 1.25% of the total mass of the oil. Smaller-capacity transformers use large numbers; large-capacity transformers use small numbers. When activated alumina is used, it is about 0.5% of the total mass of oil.
(2) Management of the oil purifier during operation
In order to know the failure time of the adsorbent in time, it is necessary to carry out regular sampling tests on the oil in the oil purifier. The usual method is to close the oil purifier, isolate it for a day and night, and then take oil from the transformer body and the oil purifier at the same time. In the same way, the acid value is measured. If it is close to failure, the acid value of the two is almost the same; if it has not failed, the acid value in the oil purifier is much lower than that in the transformer body.
Antioxidant added to three oils
Adding antioxidants to the oil can prevent and delay the deterioration of the oil. At present, the commonly added antioxidant is 2.6-di-tert-butyl-p-methylphenol. It is based on the interaction of antioxidant molecules and free radicals to interrupt the chain of oxidized guacamole to inhibit the oxidation of oil.
The amount of oxidant added is about 0.3% to 0.5% of the total mass of the oil. However, antioxidants must be added to the oil when the oil is in the initial oxidation stage, or before this stage, in order to effectively prevent the oxidation of the oil. When the oil has been deeply oxidized, adding antioxidants has little effect. However, at present, antioxidants have been added to the new oil when it leaves the factory.
As the transformer runs for a long time, the oxidant in the oil is gradually consumed, and the content of antioxidants gradually decreases, so the oxidative capacity gradually weakens. Only when the concentration of antioxidants in the oil is kept in a certain value range, such as about 0.3%, can the oxidation of oil removal be achieved. Therefore, it is necessary to regularly sample the oil in the transformer body to determine the concentration of antioxidants. Once its concentration is found to drop below a certain limit, it must be added in time to maintain its antioxidant capacity.