In order to maintain the dimensions of the equipment at an acceptable level, modern transformers have been designed with a more compact insulation that significantly increases the level of thermal and electrical stresses required to insulate the components between them during operation. Large-scale power transformers of 110kV and above mainly adopt oil-paper insulation structure. The main insulating materials are insulating oil and insulating paper and cardboard.
When the internal fault of the transformer involves solid insulation, regardless of the nature of the fault, it is generally considered to be quite severe. Because once the insulation properties of the solid material are destroyed, it is likely to further develop into a breakdown accident of the main insulation or the longitudinal insulation. Therefore, the influence caused by the deterioration of fiber materials is particularly valued in fault diagnosis. Moreover, if it is possible to determine whether the transformer is involved in solid insulation or abnormality, it will initially determine the location of the fault, which is very helpful for equipment maintenance work.
In this paper, a method for dynamic analysis of transformer insulation faults is proposed by studying the associated growth of other characteristic gas components with CO and CO2 when the fault involves solid insulation. And set up a growth model of fault gas, providing a new criterion for predicting the development of faults.
1. Conventional method for judging solid insulation failure
CO and CO2 are aging products of fiber materials. Generally, they are accumulated in a large amount under non-fault conditions. It is often difficult to judge whether the CO and CO2 content obtained by the analysis is due to the normal aging of the fiber material or the decomposition product of the fault.
Yue Gang Shu Lang [1] studied the total amount of carbon oxides, ie (CO + CO 2 ) mL / g (paper), which was re-decomposed and dissolved in oil using a transformer unit paper to diagnose solid insulation faults. However, the insulation structure, material selection and oil-paper ratio of the put-in transformer vary greatly with voltage grade, capacity, model and production process. It is impossible to calculate the total quality of insulation paper in each transformer one by one. The actual operation is difficult and difficult to apply; and it is reasonable to consider the total paper weight in the analysis of the overall aging. If the fault point only involves a small part of the solid insulation, it is difficult to use CO/CO2 content alone. More effective.
IEC 599 [2] recommends using the CO/CO2 ratio as a criterion to determine the relationship between fault and solid insulation. It is considered that CO/CO2>0.33 or <0.09 indicates that there may be fiber insulation breakdown failure, and in practice this method also has considerable limitations [3]. In this paper, 59 cases of overheat fault and 69 discharge faults were counted. The results show that the positive rate of CO/CO2 ratio is only 49.2%. This method has a high recognition rate of 74.5% for suspension discharge faults, but the positive rate for perimeter discharge is only 23.1%.
2. Dynamic analysis method of solid insulation fault
The new preventive test procedure stipulates that transformers of 330kV and above in operation should be analyzed for dissolved gases in oil every three months. However, many electric power bureaus have shortened the time interval to ensure the safety of these important equipments. For 1 month. Some electric power bureaus have already launched an online monitoring of oil chromatography, which provides a good technical basis for continuous tracking of faults.
The faults involving solid insulation inside the power transformer include: fence discharge, turn-to-turn short circuit, overheating of the winding caused by overload or poor cooling, partial discharge caused by poor insulation impregnation, and the like. Whether it is an electrical fault or an overheat fault, when the fault point involves solid insulation, the oil paper insulation will be cracked under the action of releasing energy at the fault point, releasing CO and CO2. However, their production is not isolated, it is inevitable because of the insulating oil. The decomposition produces a variety of low molecular hydrocarbons and hydrogen, and can be used to determine the cause of the failure by analyzing the associated growth of each characteristic gas with CO and CO2.
A quantitative criterion is required to determine whether the characteristic gas of the fault and the CO and CO2 content are accompanied by an increase. This paper obtains a statistical description of this standard by correlating the results of continuous chromatographic monitoring of transformers. This can overcome the effects of the dissolved gas accumulation effect and eliminate the random error interference of the measurement.
In this paper, the Pearson product moment correlation is used to measure the degree of correlation between variables. The sequence of the measured variable pairs (xi, yi), i = 1, ..., the correlation coefficient γ is selected two levels of test: α = 1% as a variable Whether the relevant criteria are significant, and α=5% is the criterion for whether there is correlation between the variables. That is, when the correlation coefficient γ>γ0.01, the variables are considered to be significantly correlated; when γ<γ0.05, there is no clear correlation between the two. The values ​​of γ0.01 and γ0.05 are related to the number of samples N, which can be obtained by checking the correlation coefficient test table.
Since CO is an intermediate product of cellulose degradation, it can better reflect the development process of the fault. Therefore, by correlating the main characteristic gas of the fault with the continuous monitoring value of CO, it can be further judged whether the fault involves solid insulation. When other analytical methods are used to determine that there is a discharge fault inside the equipment, the correlation degree between CO and H2 can be used as a criterion for judging whether the electrical fault is related to solid insulation; and the superheat fault is based on the correlation between CO and CH4. Analysis of 59 cases of overheating faults and 69 cases of discharge faults.
This method can reflect the severity of the fault to a certain extent. In the case of overheating fault, if CO has a strong correlation with CH4, it is also related to C2H4, indicating that the temperature of the fault point is higher; In the case of a discharge fault, if CO has a strong correlation with H2 and C2H2, the nature of the fault may be spark discharge or arc discharge.
3 development trend of failure
After confirming the type of failure, further understanding of the development trend of the failure will help to rationalize the maintenance plan. The gas production rate is an important parameter to judge the degree of gas-induced fault damage in oil-filled equipment, and it is valuable for analyzing the nature and development degree of faults (including the power, temperature and area of ​​the fault source) [4].
Through regression analysis, these three typical models can be summarized as:
(a) Positive quadratic form: The variation of total hydrocarbons with time is roughly Ci=a.t2+b.t+c(a>0), that is, the gas production rate γ=a.t+b is continuously increasing, which is proportional to time. This often corresponds to sudden failures, and the fault power and the area involved are constantly increasing. This type of failure growth pattern is often very dangerous.
(b) Negative quadratic form: The change law of total hydrocarbon and gas production rate is the same as (a), except that a<0. That is, after the total hydrocarbon Ci increases to a certain extent, it fluctuates near this value and no significant change occurs. More corresponding to the form of gradual weakening or temporary failure, such as winding overheating in the case of system short circuit and partial discharge occurring under system overvoltage.
(c) One-time type: the linear growth model, which is a form of gas production corresponding to a stable fault point. The variation law of total hydrocarbons is Ci=k.t+j, and the gas production rate is a fixed constant k. Usually, the fault is considered serious only when the fault gas production rate k or the total hydrocarbon Ci is greater than the attention value.
In this paper, the correspondence between the growth pattern of total hydrocarbon content and the severity of faults in 59 cases of overheating fault and 69 discharge fault transformers is statistically analyzed. The results are shown in Table 2.
4, case analysis
The growth model of fault gas production is a positive quadratic type, and the gas production rate shows a significant growth trend in a short period of time. It is a rapidly developing fault, reflecting that the area involved in fault power and faults is constantly increasing.
On March 14, 1985, the core inspection showed that there were 7 layers of high-voltage coils and low-voltage coils with obvious dendritic discharge marks such as burns, perforations, creepage, etc., which were faults in the surrounding screen discharge, which were consistent with the analysis results.
5 Conclusion
a. There is always an intrinsic cause of the dissolved gas in the power transformer oil. According to the main characteristic of the fault and the accompanying growth of CO, it can be judged whether the fault point involves solid insulation. This method is basically not affected by the cumulative effect, and there is no limitation of the attention value. The variation law of the dissolved gas can be analyzed at any time, and potential latent faults may be found in time.
b. For the power transformers in operation, the gas production process of the faults does not all increase linearly, and there are other growth modes. The statistical results show that if the total hydrocarbon content increases in a positive quadratic form, it is mostly a serious destructive fault; when the fault gas production increases linearly, the fault point is relatively stable; if the total hydrocarbons are negative quadratic, most of them are Temporary failures are generally less harmful.
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