The aging of transformer insulating paper is mainly due to the degradation of cellulose in insulating paper. Through the research of X-ray on cellulose, it is found that there is a part of regular molecular arrangement within cellulose molecules, and the arrangement between molecules is very neat, which is called crystalline area. There are also some areas with irregular arrangement and different molecular spacing, which is called non crystalline area.
Generally speaking, the degradation generally occurs in the crystallization free zone, because the molecular bond in the crystallization zone is relatively stable, and the intermolecular force is very large, so the impact is relatively small; but the force between the molecules in the non crystallization zone is weak, and the degradation is more obvious under the influence of environmental, chemical and other factors.
The first factor affecting the aging of oil paper insulation of transformer: temperature
Temperature is the main factor affecting the aging of the insulation system of the transformer. The thermal stability of the insulation paper is relatively poor. During the operation of the transformer, when the temperature exceeds 100 degrees, the cellulose will automatically decompose, and decompose water and a small amount of CO and CO2. As the operation time increases, the aging degree of the insulation paper will become more and more serious. Most of the mechanical properties and electricity of the transformer The gas performance is invalid, and cellulose itself has low aging resistance and stability, which is also the biggest defect of insulation paper.
During the operation of transformer, the heat will also rise, and the thermal degradation will cause the cellulose molecular bond to break. The thermal stability of C-O bond is much weaker than that of C-H bond in insulating oil. Even in normal temperature environment, C-O bond is easy to break.
The heat generated by the operation of the transformer will cause the temperature of the insulating paper to rise. In the 1930s, foreign experts once proposed the rule of ten degrees. They found that the service life of the transformer will be reduced by half when the operating temperature of the transformer increases by ten degrees. During the operation of transformer, there are energy losses in the core and winding, which on the one hand affect the efficiency of transformer operation; on the other hand, the loss is converted into heat energy, causing the heating of transformer. When there is heat generated inside the transformer, the temperature of the transformer will rise. When the temperature of the transformer is higher than the temperature of the cooling medium, it begins to emit heat to the cooling medium. The speed of heat dissipation depends on the temperature difference between the transformer and the cooling medium and the heat dissipation capacity of the transformer.
Under certain conditions, the larger the temperature difference, the faster the heat dissipation. Therefore, under a certain load, the temperature of the transformer will gradually increase, until the heat released is equal to the heat generated, the temperature will not increase, and maintain a certain appreciation of the temperature of the cooling medium. The temperature rise of transformer has a very important impact on its operation, the most important is the impact on insulation materials. If the temperature is too high, the insulating material will be damaged and lose its insulating ability, or its effective service life will be shortened. In addition, if the temperature is increased, the resistance of the winding will be increased, so the copper consumption will be increased. Therefore, the temperature of each part of the transformer at rated load must not exceed the value allowed by the insulating material.
To reduce the temperature rise, we can start from two aspects: on the one hand, we should try our best to improve the efficiency and reduce the loss of the transformer in the design and manufacture; on the other hand, we should improve the heat dissipation capacity of the transformer to make it more effective in cooling. The heat dissipation capacity is related to the structural arrangement of the transformer and the cooling method adopted. Therefore, how to strengthen the cooling and make the heat better emitted is an important issue. It should be pointed out that the problem of temperature rise and cooling must be considered comprehensively. On the one hand, the lower the temperature rise, the better. This is because a certain insulating material can run at a certain temperature rise. Too low temperature rise indicates that the effective material is not fully utilized, or that unnecessary and too strong cooling measures are adopted, which are not economical.
In addition, if the insulation material that can withstand higher temperature is used, the transformer can be operated at higher temperature and more effective materials can be saved, but the cost of the insulation material may be higher. On the other hand, the factors that determine the total loss that can be allowed sometimes are not the limits of temperature rise, but the requirements for transformer efficiency. The latter is determined by the viewpoint of power economy. At this time, temperature rise is not the key issue. Therefore, the issues of temperature rise and cooling must be considered from the perspective of saving key materials and comprehensive economy.
The second factor affecting the aging of oil paper insulation of transformer: electric field
Under the action of electric field, the insulation paper of transformer will be aged, which is one of the main factors affecting the aging of transformer insulation system. Generally speaking, the aging degree of oil paper insulation structure of transformer depends on the accumulation degree of partial discharge.
In the process of production and manufacture of transformer, pressure immersion process is adopted, but there are still some wrinkles and bubbles in the internal insulation structure or some dead corners of transformer. Under the action of partial discharge, these bubbles will collapse, resulting in many free electrons. Under the action of electric field, these electrons will impact the stability of C-H, C = C with certain kinetic energy The structure of organic compounds is destroyed. In addition, the electric field will accelerate the degradation of insulating oil, thus forming acid characteristics, which will deposit on the surface of insulating paper, accelerating the aging speed of transformer insulation system.
The third factor affecting the aging of oil paper insulation of transformer: moisture
Moisture is the key factor to determine the aging of transformer insulation system, and the main reason is the transformer.
The fourth factor affecting the aging of oil paper insulation of transformer: oxygen
The effect of oxygen on the insulation aging of oil paper is similar to that of moisture, which plays a key role. The cellulose component in the insulating paper is easy to oxidize. The experimental research shows that when the moisture content in the insulating paper reaches 0.3%, the degradation rate of cellulose is increased by about three times by oxygen. In the transformer, except for the residual oxygen inside the body and a small amount of oxygen infiltrated from the outside, the pressure is changed The aging products of the oil also contain oxidants which can not be ignored. Higher temperature can accelerate the oxidation reaction of cellulose.
The fifth factor affecting the aging of oil paper insulation of transformer: other factors
In addition to the above several main factors that affect the aging of oil paper insulation of transformer, there are also factors such as physical stress, light, microorganism and so on, which still play a significant role in the aging of transformer insulation system.