Why is transformer oil required to be filtered?
1. Acid value
The acid value indicates the amount of acidic substances present in oil, measured by the milligrams of potassium hydroxide required to neutralize the acids in 1 gram of oil. This value includes both organic and inorganic acids, but in most cases, oil does not contain inorganic acids. Therefore, the acid value essentially represents the content of organic acids in the oil. New oil primarily contains cyclohexanecarboxylic acid as its organic acid component. During storage and use, organic acids such as fatty acids are formed due to oxidation. For new oil, the acid value serves as an indicator of its refining degree, while for operational oil, it reflects the extent of oil degradation and is a key criterion for determining whether the oil can continue to be used.
2. Water-soluble acid
Its extreme limit is less than 4.2. There are two possible reasons for exceeding the limit: oil aging or oil contamination. For water-soluble acid exceeding the limit, the following countermeasures can be taken: increase the number of tests and compare with the acid value to identify the cause. Perform oil regeneration treatment, and if economically feasible, consider oil replacement.
3. Breakdown voltage
Its upper limit value is less than 20 kV. There are two possible reasons for exceeding the limit: excessive moisture content in the oil; contamination by impurity particles in the oil. The following measures can be taken to address the breakdown voltage exceeding the limit: identify the cause, perform vacuum oil filtration, use a plate and frame pressure filter, or replace the oil with new oil. Generally, the breakdown voltage of oil after vacuum filtration treatment should be greater than 50 kV.
4. Flash point
The upper limit value is below 130°C or a 5°C decrease from the previous test value. There are two possible causes for exceeding the limit: localized overheating faults in the equipment or incorrect oil replenishment. For flash points exceeding the limit, the following countermeasures can be taken: identify the cause and eliminate the fault; perform degassing treatment with a vacuum oil purifier or replace the oil.
5. External Appearance
If the oil appears opaque to the naked eye, contains visible impurities or suspended particles, or has an abnormally dark color, it indicates an abnormal oil appearance. Possible causes of abnormal appearance include the presence of moisture, fibers, carbon black, or other solid contaminants. For oil with abnormal appearance, the following measures should be taken: - If the oil appears hazy or milky, it indicates the presence of moisture, and the moisture content should be checked. - If carbon particles are found in the oil, the oil color turns black, or even emits a burnt odor, it may indicate internal arc or partial discharge faults. The cause should be investigated in conjunction with other tests, and faults should be addressed accordingly. - If the oil color darkens significantly, attention should be paid to whether oil aging is accelerating. This can be analyzed by combining oil acidity tests or by enhancing the monitoring of oil operating temperature.