There are two methods for the life evaluation of transformers. The first one is the statistical method. It performs the juice on the running transformer and then evaluates the life. This method is cheaper, but it is difficult for the newly developed transformer. The life data is obtained before leaving the factory; the second is the whole machine accelerated life test method, which uses the transformer test sample or the simulated test piece to carry out the accelerated life test to evaluate the life. This method is mature, but the cost is high, and it is difficult to describe the element. Intrinsic Mechanism of Device Damage This paper proposes a new method based on the determination of the degree of polymerization of transformer insulation paper, to evaluate the life of the transformer, to derive a new calculation formula, to design and implement the paper aging test, and to establish a new life assessment. system. It provides a life assessment of the newly developed transformer before leaving the factory, and the cost of the evaluation is only equivalent to the % of the second method.
1 Transformer aging mechanism From the perspective of reliability, the transformer is a system consisting of main components such as iron core, high voltage winding low voltage winding, main insulating insulating oil, fuel tank, casing, auxiliary equipment, etc. The weaker main components are insulating paper and Insulating oil. According to the paper, the insulating paper is the weakest component. Therefore, by studying the aging mechanism of the insulating paper, it is equivalent to the research on the aging mechanism of the transformer, and then the transformer life evaluation. The main component of the insulating paper is cellulose, which is composed of many When the cells are connected in series, the average number of the above-mentioned cells contained in one chain is called the degree of polymerization of the insulating paper (degreeofpolymerization, which is represented by Dp in the text), and the process of aging the insulating paper is that the molecular chain is shortened by a long chain. The process, that is, the process of Dp value decline, which is mainly caused by three factors of water and oxidation temperature. The more water in the water paper, the faster the Dp value decreases. The aging rate of the paper is directly proportional to the initial water content in the paper. The water content and the water content of the insulating paper are substantially proportional to the equilibrium, and the ratio is temperature dependent. Since the moisture content in the paper is difficult to measure, and the moisture content in the oil is easy to measure, we will calculate the moisture content in the paper by the moisture content in the oil. The reaction process between oxygen oxide and insulating paper is a rather complicated process, which is not only slow with oxidation flow. Relatedly, it also depends on the reaction with the oil. For the case where the oxygen content in the sealed transformer is extremely small, the process of paper oxidation can be neglected.
temperature. Dakin found that any chemical quantity or even some physical quantity changes obey the chemical reaction law when the chemical reaction mechanism is constant. The relationship between any chemical reaction and heat is subject to the following formula: where K0 is the chemical reaction rate; Absolute temperature; A, B is a constant, B is a substantially constant constant during the aging process of the transformer insulation material, and B is about 15 according to the experimental measurement.
2 Mathematical model of aging of insulating paper To establish the quantitative relationship between Dp value and life, this paper adopts the process of aging of insulating paper, which is the process of chain breakage between basic unit and basic unit in the fiber chain of insulating paper. The key inside the basic unit is not During the aging process of the broken insulation paper, the total length of the fiber chain remains unchanged, and the number of chains continues to increase.
The bond cleavage of long-chain molecules and the bond cleavage of short-chain molecules have the same degree of difficulty at the same temperature, that is, having the same possible enthalpy. This is the most critical assumption, which ensures that the Dp value decreases exponentially with time. The aging rate of the insulating paper at any time is proportional to the initial water content in the paper. This is an experimental law. Based on the above basic assumptions, we formulate the formula. When t is set, the total length of the chain is L, and the number of chains is No. In a short time At The number of chains corresponding to the decomposition is KtNAt, so there is: the formula for calculating the life of the transformer (see equation (8)) c. The aging process of the insulating paper is the degradation of the polymerization degree, the aging of the insulating paper and the temperature, oxygen and The water is closely related. The degree of polymerization decreases exponentially in the case of little oxygen and water. At the same time, the Dp value is dispersive and the Dp value follows a normal distribution.
The solution viscosity of the insulating paper after the fast life can be calculated as the DP value of the insulating paper. The estimated life of the SBS9 type transformer under the 2C ambient temperature and full load condition is 22.28 years. The probability of life expectancy exceeding 19.15 years is 90%. The new life assessment method proposed in this paper can provide the manufacturer with the expected life of the transformer before leaving the factory. The test cost is only 1% of the whole machine accelerated life method. The result can also be It is applied to the power supply bureau to estimate the residual life of the transformer in operation to achieve regular replacement of the transformer and improve the reliability of the power supply.
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