Transformer Oil Testing

A. Transformer oil testing items

(1) Freezing point

(2) Water content

(3) Interfacial tension

(4) Acid value

(5) Water-soluble acidity

(6) Breakdown voltage

(7) Flash point

(8) Volume resistivity

(9) Dielectric loss

(10) Chromatographic analysis

(11) Furfural content analysis in insulating oil

B. Transformer oil testing items and test significance

1 Appearance:

Check the appearance of the running oil, and you can find the presence of insoluble sludge, fiber and dirt in the oil. In routine tests, there should be a record of this item.

2 Color:

The new transformer oil is generally colorless or light yellow, and the color will gradually deepen during operation, but this trend of change is relatively slow under normal circumstances. If the color of the oil deepens rapidly, you should investigate whether the equipment is overloaded or overheated. If other relevant characteristic test items meet the requirements, the operation can be continued, but monitoring should be strengthened.

3 Moisture:

Moisture is one of the important reasons that affect the insulation aging of transformer equipment. The increase of water content in transformer oil and insulating materials directly leads to a decline in insulation performance and promotes oil aging, which affects the reliability and service life of equipment operation. Strict supervision of moisture is an essential test item to ensure the safe operation of equipment.

4 Acid value:

The acidic products contained in the oil will increase the conductivity of the oil and reduce the insulation performance of the oil. When the operating temperature is higher (such as above 80 ℃), it will also promote the aging of the solid fibrous insulating material and cause corrosion and shortening. Equipment service life. Since the acid value in the oil can reflect the aging of the oil, it is important to strengthen the supervision of the acid value for taking correct maintenance measures.

5 Oxidation stability:

The oxidation stability test of transformer oil is an important method to evaluate its service life. Due to the good oxidation stability of domestic oil and the addition of antioxidants, this project is usually tested on new oils, but for imported oils, especially oils that do not contain antioxidants, in addition to testing new oils, This test should also be carried out after several years of operation in order to take appropriate maintenance measures and extend the service life.

6 Breakdown voltage:

The breakdown voltage of transformer oil is a very important monitoring method to test the resistance of the transformer oil to the ultimate electrical stress. Normally, it mainly depends on the degree of contamination, but when the water in the oil is relatively high High or impurity particles have a greater impact on the breakdown voltage.

7 Dielectric loss factor:

Dielectric loss factor is very sensitive to the aging and pollution degree of transformer oil. The new oil contains few polar impurities, so the dielectric loss factor is also very small, generally only on the order of 0.01% to 0.1%: but when the oil is aging due to oxidation or overheating, or when other impurities are mixed, it is generated As the polar impurities and charged colloidal substances gradually increase, the dielectric loss factor will also increase. When the oil aging products are very small and cannot be detected by chemical methods, the dielectric loss factor can be clearly distinguished. Therefore, the determination of the dielectric loss factor is a common means of transformer oil inspection and supervision, and has special significance.

8 Interfacial tension:

The measurement of the interfacial tension between oil and water is an indirect and effective method to check the soluble polar impurities produced by aging in the oil. In the initial aging stage of oil, the change of interfacial tension is quite rapid, and in the middle of aging, the rate of change is also reduced. The generation of sludge has increased significantly. Therefore, this method can also make a reliable judgment on the trend of sludge generation.

9 Sludge:

This method is to check the sludge deposits that can settle out of the oil when n-heptane is added in the running oil while it is still in a dissolved or colloidal state. Due to the different solubility of sludge in new oil and aging oil, when the aging oil penetrates into the new oil, the sludge will precipitate out, and the deposition of sludge will affect the heat dissipation performance of the equipment, and it will also cause serious damage to solid insulating materials and metals. The corrosion of the sludge leads to a decrease in insulation performance and is more harmful. Therefore, when the ratio of oil is greater than 5%, the sludge precipitation test must be carried out.

10 Flash point:

The flash point is an essential item for the supervision of running oil. Lower flash point means that volatile combustible gases are produced in the oil; these combustible gases are often produced by the partial overheating of electrical equipment and the thermal cracking of insulating oil at high temperatures caused by arc discharge. The failure of the equipment can be found in time through the determination of the flash point. At the same time, for the newly filled equipment and the transformer oil after the overhaul, the flash point measurement can also prevent or discover whether the light distillate oil is mixed in, so as to ensure the safe operation of the equipment.

11 Gas component content in the oil:

The combustible gas in the oil is generally generated due to the partial overheating of the equipment or the decomposition of electric discharge. If the cause of flammable gas is not identified and eliminated in time, it is very dangerous to the safe operation of the equipment. Therefore, the use of gas chromatography to determine the gas components in the oil is very effective for eliminating the latent fault of the transformer. This item is an indispensable inspection content in transformer oil operation supervision.

12 Water-soluble acids:

Transformer oil is generally prone to generate low-molecular organic acids, such as formic acid, acetic acid, etc., in the primary stage of oxidation, because these acids have better water solubility. When the water-soluble acid content in the oil increases (i.e. When it contains water, it will corrode solid insulating materials and metals, reduce the insulation performance of electrical equipment, and shorten the service life of the equipment.

13 freezing point:

According to my country's climatic conditions, transformer oil is graded according to low temperature performance. For example, the three grades of 10, 25, and 45 refer to freezing points of -10, -25, and -45℃ respectively. Therefore, for the acceptance of new oil and the mixing of different grades of oil, the determination of the freezing point is necessary.

14 Volume resistivity:

The volume resistivity of transformer oil is the same as the dielectric loss factor, which can determine the degree of aging and pollution of transformer oil. Moisture in the oil, pollution impurities and acid products can all affect the reduction of resistivity.

C. Transformer Oil Treatment

If the result of transformer oil testing cannot meet the required standards, then transformer oil treatment should be performed. According to the condition of transformer oil, select the suitable method of transformer oil treatment, for example, by vacuum dehydration and degassing to remove moisture and gas, as well as impurities. However if the oil is deteriorated, contains much acids and oxides, the oil color becomes dark, then Transformer Oil Regeneration/transformer oil reclamation is required.

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