Common Quality Problems Of Transformer Oil

Functions of insulating oil in transformers:

1. Insulation: Transformer oil has much higher insulation strength than air. The insulating material is immersed in oil, which not only improves the insulation strength, but also protects it from moisture.

2. Heat dissipation: Transformer oil has a large specific heat and is often used as a coolant.

3. Arc extinguishing effect: Because the transformer oil has good thermal conductivity, and can decompose a large amount of gas under the high temperature of the arc, it can generate a large pressure, thereby improving the arc extinguishing performance of the medium and quickly extinguishing the arc.

Below we introduce to you 10 common quality problems of transformer oil:

01 Why should the density (or relative density) of transformer oil be controlled?

Density or relative density is related to the composition of oil and the amount of water present. For transformer oil, controlling its density also controls the amount of water in the oil in a sense, especially for preventing transformers working in cold areas from appearing ice floes during temporary outages in winter. If there is too much water in the transformer oil, ice crystals will adhere to the electrode when the temperature is low, but when the temperature rises, the ice crystals adhered to the electrode will melt, increase conductivity, and cause the danger of discharge. The density of this pair of insulating oils is generally required to be no more than 895kg/㎡ at 20°C.

02 The influence of kinematic viscosity on the use of transformer oil

In the transformer, transformer oil is used as a medium for insulating and transferring heat. It is required to select a suitable viscosity to ensure that the oil has an ideal cooling effect in long-term operation, and a reasonable low-temperature viscosity to ensure that the transformer can work safely when it is stopped and restarted. . Therefore, the United States ASTM 19487 transformer oil standard specifies the requirements of 0°C and 100°C kinematic viscosity, and the IEC 296 standard promulgated by the International Electrotechnical Commission also specifies 40°C, -15°C (or -30°C, -40°C) kinematic viscosity Requirements. Excessive viscosity affects heat transfer, and vice versa reduces work safety.

03 What is the freezing point and pour point, and the impact of this index on the performance of transformer oil?

The freezing point of transformer oil is the highest temperature at which the oil level does not move. The pour point is the lowest temperature at which transformer oil can flow. Transformer oil is a mixture of different hydrocarbons, and the freezing points of various hydrocarbons are also different. Therefore, when the temperature drops, it does not solidify immediately. It has to go through a thickening stage and gradually solidify in a wide temperature range. Therefore, the freezing point or pour point is only the approximate maximum temperature when the transformer oil loses fluidity. The freezing point and pour point reflect the low temperature of the oil to a certain extent. In the GB-7595 in-service transformer oil quality standard promulgated by China, it is stipulated that when additional switch oil is added with a pour point depressant, the freezing point test should be added and the temperature should be specified. In areas below -5°C, the freezing point of oil products is not higher than -10°C; in areas where the temperature is lower than -10°C, the freezing point of oil products is not higher than -25°C; in areas where the temperature is lower than -25°C, oil products The freezing point is not higher than -45°C. If it is used in a climate below the freezing point, the oil will lose its flow and the equipment will not be able to start and work.

04 What is the significance of flashpoint monitoring of transformer oil in operation?

Flash point is an important safety indicator during storage and use of transformer oil, especially for monitoring transformer oil in operation. Flash point is an indispensable item. The decrease in flash point indicates that there are volatile combustibles in the oil. These low-molecular hydrocarbons are often produced when the insulating oil is thermally cracked at high temperatures due to overheating caused by partial failure of electrical equipment. Therefore, electrical equipment can be discovered in time through flash point monitoring. Whether there is an overheating fault, for the newly charged equipment and the transformer oil after maintenance, measuring the flash point can find out whether there is light distillate oil mixed in. If the flash point is too low, it will cause a fire or even an explosion in electrical equipment. Therefore, there are strict flash point control indicators in the new oil standards for transformer oils in various countries. Generally, the closed flash point is not less than 140°C, and the open flash point is not less than 145°C. The flash point of "operating oil" is also strictly controlled, and the flash point drop value measured each time shall not be 5°C lower than the previous time.

05 The significance of determining the acid value for the use of transformer oil

The acid value indicates that the oil contains acidic substances, that is, the total value of organic acids and inorganic acids. Generally, the acid is expressed in mg of potassium hydroxide required to neutralize the acidic substances in 1g of transformer oil. For new transformer oil that has not been used Almost no acidic substances, its acid value is quite small, but oil products will inevitably come into contact with oxygen in the air after long-term storage, especially after the electrical equipment is charged into operation, and the oil products are easy to be aged, and the production is mainly low in the initial stage of oxidation. Molecular organic acids are further oxidized to produce polymer organic acids and acid products. The presence of the above-mentioned acidic substances in the excellent product will increase the conductivity of the oil, reduce the insulation performance of the oil, and may also cause corrosion to metals. . In the case of high operating temperature (above 80°C), the solid fiber paper insulation material is promoted to aging, thereby shortening the service life of the equipment.

The acid value of the unused transformer oil is generally 0.01mgKOH/g, and the acid value of the operating oil is controlled not to exceed 0.1mgKOH/g.

06 What is the significance of measuring the pH value of insulating oil during operation?

Generally, the unused (new) transformer oil contains almost no acidic substances, and its acid value is low, with a pH value in the range of 6-7. The pH value is mainly used to indicate the index of the water-soluble acid of the insulating oil. According to the oil analysis of China's on-site investigations, simulation tests and laboratory aging test results, the transformer oil generally has an acid value greater than 0.1mgKOH/g and the pH value is equal to or less than 4.0 when the transformer operating oil is more likely to precipitate sludge. On the contrary, the transformer oil can basically ensure the good and reliable operation of the transformer. When the acid value rises above 0.2mgKOH/g or the pH value is lower than 3.8, the oil quality deteriorates significantly and more sludge is produced. Therefore, the pH value of the operating oil should be greater than 4.2.

07 What is the breakdown voltage of insulating oil (BDV)?

Breakdown voltage is an index for evaluating the electrical performance of transformer oil. It can be used to determine the degree of water and other suspended solids pollution in the oil, as well as the inspection of the degree of drying and filtration of the oil before it is injected into the equipment. When a gradually increasing voltage is applied to the clean and dry oil, shooting electrons will be emitted at the negative end of the voltage. When the electrons have sufficient energy, the oil molecules can be micro-dissociated. Therefore, the entire dissociation process is strengthened as the voltage rises. When a certain voltage is reached, a large amount of conduction current will be generated to form an arc. This phenomenon is called breakdown, and the voltage during breakdown is called breakdown voltage. If there is water or solids in the oil, the breakdown voltage will be reduced. At this time, the conductivity of water and solids is greater than that of oil. The low breakdown voltage of the oil during operation is a signal of the dangerous work of the transformer. For transformer oil, the breakdown voltage specified in the domestic and foreign standards is generally 40-50kV, with a high of 60kV or even more.

08 What is permittivity?

Permittivity is also called permittivity (or relative permittivity), which measures the ratio of the capacitance between and around the two electrodes of a capacitor when only insulating oil is filled with the vacuum capacitance of the same plate. Different oil products have different permittivities. The permittivity usually changes with temperature and frequency. In actual use, the oil's permittivity changes with temperature and frequency as small as possible.

09 What is the dielectric loss factor of transformer oil?

The dielectric loss factor of the insulating oil is expressed by the tangent of the dielectric loss angle, and the dielectric loss angle is the phase angle and complementary angle between the applied AC voltage and the current passing through it.

Transformer oil exists as an insulating medium in transformers or similar equipment. Under the action of the changing electric field generated by the AC circuit, theoretically only a weak capacitive current can pass through the medium. The phase between it and the applied voltage is 90°C earlier, so it is a reactive current, which only affects the power factor of the device and does not cause power loss, but in fact there will be more or less in the oil that can make the internal charge imbalance or due to electric field The polar molecules produced by the action, they can act as conductors, thereby generating resistive conduction current (or leakage current). This current is in phase with the applied voltage, so it is an active current that causes power loss, which is called dielectric loss of insulating oil, which is expressed by the ratio of conduction current to capacitive current (delay), which is called dielectric loss factor. Dielectric loss factor is an important index to evaluate the electrical performance of transformer oil, especially the deterioration or contamination of oil will cause more obvious changes in the loss factor of the medium. There are few polar substances in new oil, so the dielectric loss factor is generally in the range of 0.0001 to 0.001. If the operating oil loss factor is too large, measures must be taken.

10 What does the change in the interfacial tension of transformer oil indicate?

(1). Interfacial tension means the tension between insulating oil and immiscible water;

(2). Oxidation products, sludge, etc. produced by oil products due to aging and deterioration after use have an impact on the interfacial tension. (3) The interfacial tension of unused transformer oil can generally reach 40-50mN/m. After the oil is aged, various organic acids (one COOH) and alcohol (one OH) and other polar substances are generated, which makes the oil The interfacial tension gradually decreases.

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PureTech Oil Filtration is a professional manufacturer of Transformer Oil Filtration Machines and oil regeneration plants (fuller's earth technology). We have been committed to providing customers with solutions and applicable equipment to help customers save transformer operating costs, improve the full utilization of limited resources, and achieve the purpose of environmental protection. If you have oil treatment problems , or are faced with the choice of changing transformer oil or filtering transformer oil, please contact our sales engineers, and we will provide you with the most suitable solution.

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