The life of transformer is mainly determined by the life of its insulation system. The more direct parameter reflecting the mechanical strength of the insulating material in the transformer is the degree of polymerization of the insulating material. The aging process of insulation materials is also directly related to the operation status of transformers. For the typical insulation structure of a power transformer, in addition to the paper-clad insulation of the copper conductor of the winding and the insulating tube of the outgoing device, the insulation materials in the transformer also include the pressure plate, the angle ring, the insulating pad, the brace and the insulating paper cylinder.
The composition of insulation material in transformer is basically cellulose. From the molecular structure of cellulose, it can be seen that cellulose is a chain connected by one ring structure at the micro level. The degree of polymerization (DP value) of insulating material is the number of chain links of cellulose molecule. The greater the degree of polymerization, the longer the length of the single cellulose, and the higher the mechanical strength of the insulating material.
The aging mode of transformer insulation determined by CIGRE. It can be seen from this model that the transformer oil is oxidized under the action of temperature and oxygen, producing CO and CO2, and also acid (and sludge). Similarly, cellulose will also oxidize under the action of temperature and oxygen, and then decompose at high temperature, resulting in molecular chain breakage, reduced polymerization degree and reduced mechanical strength. At the same time, under the action of water and acid, hydrolysis occurs, and the depolymerization will also break the molecular chain and reduce the mechanical strength of solid insulating materials.
The decomposition and hydrolysis of solid insulating materials at high temperature will produce CO and CO2, as well as water, oxygen and acid, which in turn will continue to aggravate the aging and decomposition of solid insulating materials. In addition, furan-type compounds (furfural) will also be produced during the hydrolysis of solid insulating materials. Measuring the furfural content in oil can indirectly determine the aging degree of insulating materials.
The main factors affecting the life of solid insulation materials in transformers are temperature, moisture and oxygen. The solid insulation materials inside the transformer are mostly Class A insulation, except for the use of heat stable paper in some places. The definition of Class A insulation in IEC is that the mechanical strength of insulating materials will be reduced by less than 50% after 7 years of continuous operation at 105 ° C. This definition does not take into account the influence of moisture content of insulating materials. Research shows that moisture will accelerate the hydrolysis process of cellulose in insulating materials, accelerate the aging of insulating materials and reduce the life expectancy of transformers under the combined action of high temperature.
At present, large transformer companies mainly use kerosene vapor phase drying to dry the body, and have high-performance vacuum treatment equipment. The moisture content of the insulation material of the newly delivered transformer can be controlled within 0.5%. If such water content can be maintained, it can be found that the life expectancy of the transformer can reach 40 years at 90 ° C. However, when the water content in the insulation material reaches 4%, the expected life of the transformer will be only one year under the same temperature conditions.
Of course, the actual operating temperature of the transformer will change, and the moisture will also transfer between the transformer oil and insulating material with the change of temperature. It is difficult to predict the life of the transformer simply and quantitatively. When the degree of polymerization of insulating material DP=150~200, the binding force between fibers is obviously weakened, and the mechanical strength of insulating material remains about 20%~40%, which symbolizes the end of life.