In the purchase and use of transformer oil, it is important to pay attention to the transformer oil performance parameters to meet the requirements of the transformer and use conditions. This article will introduce you in detail through the three performance indicators of transformer oil: electrical, chemical and physical.
Electrical Performance Indicators Of Transformer Oil
1. Electrical Insulating Strength
The insulation strength of transformer oil is expressed by the breakdown voltage (BDV). The minimum breakdown voltage of transformer oil must be higher than transformer's requirements. The standard for testing breakdown voltage of transformer oil is ASTM D877. Breakdown voltage is an important indicator for new oil acceptance. It is also an important indicator that needs to be tested in use. If the transformer oil is mixed with water, impurities, transformer oil oxidation, will reduce the BDV.
2. Resistivity
Volume resistivity is an important electrochemical performance index of transformer oil. It reflects the dielectric properties of the oil. If it is not qualified, it will cause electrochemical corrosion of the oil system's speed control components
Especially in the servo valve, the lower the resistivity, the more serious electrochemical corrosion. Transformer oil resistivity testing standard is ASTM D1169. The oil volume resistivity with dielectric loss factor, can determine the degree of aging and pollution of transformer oil. Water, contaminated impurities and acids in the oil can reduce the resistivity.
3. Dielectric loss factor
Dielectric loss factor indicates the electric energy loss caused by electrolyte polarization and conductivity under the action of electric field. The dielectric loss factor of transformer oil should be as small as possible. Oil oxidation in use produces oxidation products, acidic substances, these contaminants will make the current direction change and transposition. This transposition not only loses electrical energy, but also makes the transformer temperature increase. Accelerate transformer and transformer oil aging and deterioration. The standard for testing the dielectric loss factor of transformer oil is ASTM D924-08. The dielectric loss factor can be clearly distinguished when the oxidation level of transformer oil is low. Therefore, the determination of the dielectric loss factor is an important indicator for transformer oil monitoring.
Chemical Performance Indicators Of Transformer Oil
1. Moisture Content
Transformer oil should avoid moisture. Moisture not only reduces the insulating properties of transformer oil, but also affects the insulating paper and coils. The insulating paper will absorb water, resulting in lower insulation performance, and will also shorten the service life.
When sampling transformer oil, pay attention to the temperature. When the transformer is not working at low temperature, the insulating paper will absorb the water in the oil. After the power is applied, the oil temperature rises and the solubility of water in the oil increases. The water absorbed by the insulating paper will be released into the oil, making the water content in the oil increase. When sampling transformer oil, it should be done when the oil temperature is close to the working temperature. The acid generated by the oxidation of transformer oil will also increase the solubility of water in the oil and will accelerate the deterioration of transformer oil.
A more accurate method for testing the moisture in transformer oil is the Karl Fischer Moisture Test (ASTM D1533).
2. Acid Value
The acid contained in transformer oil is expressed by the acid value. The presence of acids in transformer oil can cause these hazards.
1. making it easier for transformer oil to dissolve water and reduce the electrical properties of transformer oil.
2、Reducing the insulating capacity of insulating paper.
3、Accelerate the deterioration of oil quality.
4、Make the metal rust and corrosion.
The standard for testing the acid value of transformer oil is ASTM D664.
The acid value can reflect the aging of transformer oil and is an important monitoring indicator for transformer oil testing.
Physical Performance Indicators of transformer oil
1. Interfacial tension
In the process of oxidation (aging) of transformer oil, some polar oxidation by-products will be produced. The determination of the interfacial tension between oil and water can reflect these polar impurities contained in the oil. The interfacial tension of new oil is generally high, but as the oil oxidizes, or the transformer oil is contaminated and mixed with impurities, it will reduce the interfacial tension of the oil. The interfacial tension of transformer oil decreases significantly in the early stages of oxidation, and in the middle of aging, the rate of change of interfacial tension decreases, while sludge generation increases significantly. Therefore, the interfacial tension can also reflect the trend of sludge generation. The standard for interfacial tension testing of transformer oil is ASTM D971.
2. Flash point
Flash point is in the specified test conditions, the transformer oil surface volatilization of the oil gas occurred in the minimum temperature of flash combustion. The flash point of transformer oil should be as high as possible, generally should not be lower than 136 ℃. If the flash point is reduced it will increase the risk of fire.
Transformer in operation, due to internal short circuit fault caused by local overheating high temperature, so that the insulating oil thermal cracking, produce volatile combustible low molecular hydrocarbon compounds, may lead to in a short period of time flash point is reduced. By testing the flash point, faults in the transformer can be detected. It can also detect whether light distillate oil is mixed in, thus ensuring safe operation of the transformer.
3. Pour point
The pour point of transformer oil is the highest temperature of transformer oil when it stops flowing at low temperature. Therefore, the pour point is related to the low temperature performance. For the low temperature performance of transformer oil, the lower the pour point, the better. The pour point of transformer oil must be lower than the lowest temperature experienced at the time of use. If the lowest temperature experienced in use is lower than the pour point, then the transformer oil will stop flowing at low temperatures and convection will not occur. Paraffin-based oils have a higher pour point than cycloalkyl oils. The pour point is mainly related to the paraffin content, which is high. It will cause the pour point of oil to be higher.
4. Viscosity
The viscosity of transformer oil refers to the force inside the oil that prevents the oil from flowing. The greater the viscosity, the greater the resistance to flow that is prevented by the oil itself. Obviously, when the viscosity of transformer oil is larger, then the greater the resistance to convection is, which is not conducive to heat dissipation. Therefore, the viscosity of transformer oil should be small. Since a lower temperature will increase the viscosity, attention should also be paid to the low temperature viscosity of transformer oil.
