Diagnosis method of solid insulation fault of power transformer
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When a transformer internal fault involves solid insulation, it is generally considered to be quite serious, regardless of the nature of the fault. Because once the insulating properties of the solid material are destroyed, it is likely to further develop a breakdown of the main insulation or vertical insulation. Therefore, the influence caused by the deterioration of fiber materials is particularly valued in fault diagnosis. Furthermore, if it is determined whether a solid insulation is involved in the occurrence of an abnormality or fault in the transformer, the location of the fault is initially determined, which is helpful for the maintenance of the equipment.
In this paper, by studying the associated growth of other characteristic gas components and CO, CO2 when the fault involves solid insulation, a dynamic analysis of transformer insulation fault method is proposed. It also set out to establish a growth model for fault gas, which provides new criteria for predicting the development of faults.
1, the conventional method of judging the solid insulation failure CO, CO2 is an aging product of fiber materials, generally there is a large accumulation of non-fault conditions, it is often difficult to determine the analysis of the CO, CO2 content is due to the normal aging fiber material, Still a breakdown of the product.
Yuegang Shulang [1] studied the total oxides of carbon dissolved in oil using transformer unit paper and dissolved in oil, ie (COCO2) mL/g (paper), to diagnose solid insulation faults. However, the insulation structure, selected materials, and oil-paper ratio of transformers that have been put into operation vary greatly depending on the voltage level, capacity, model, and production process, and it is impossible to calculate the total mass of insulating paper in each transformer one by one. The practical operation is difficult and difficult to apply; and, considering that all the paper weights are reasonable when analyzing the overall aging, if the fault point only involves a small part of the solid insulation, it is difficult to use this method to consider the content of CO and CO2 separately. More effective.
IEC 599 [2] recommends using the CO/CO2 ratio as a criterion to determine the relationship between faults and solid insulation. When CO/CO2>0.33 or <0.09 is considered, it may indicate that there is a fiber insulation breakdown failure. In practice, this method also has considerable limitations [3]. This article has statistics on 59 cases of overheating faults and 69 cases of electrical discharge faults. The results show that the positive rate of the application 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 the discharge of the enclosure is only 23.1%.
2. Dynamic analysis method of solid insulation faults The new preventive test regulations stipulate that the transformers of 330kV and above are in operation for analysis of dissolved gas in oil every 3 months. However, many electrical power bureaus currently guarantee the safety of these important equipment. Some have reduced the time interval to 1 month. There are also some electric power bureaus that have launched an on-line monitoring of oil chromatography, which provides a good technical basis for continuous tracking of failures.
Internal faults involving solid insulation in power transformers include: discharges from enclosures, inter-turn short circuits, overheating of windings caused by overload or cooling failure, and partial discharge caused by poor insulation impregnation. Regardless of 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 the energy released at the fault point, releasing CO and CO2. However, their production is not isolated and will inevitably result from the insulating oil. The decomposition produces a variety of low molecular hydrocarbons and hydrogen, and can determine the cause of the failure by analyzing the associated growth between each characteristic gas and CO and CO2.
A quantitative standard is needed to determine whether the characteristic gases of the fault and the CO and CO2 content are concomitantly increasing. In this paper, statistical analysis of the results of continuous chromatographic monitoring of transformers is performed to obtain a statistical description of this standard. This can overcome the effect of the cumulative effect of dissolved gases and eliminate the random error of the measurement.
In this paper, Pearson product moment correlation is used to measure the degree of correlation between variables. The significance of the measured variable sequence pairs (xi, yi), i=1,..., Correlation coefficient γ selects two test levels: α=1% as a variable. Whether there is a significant correlation criterion, and α = 5% as a criterion for whether or not there is a correlation between variables. That is, when the correlation coefficient γ>γ0.01, the variables are considered to be significantly related; when γ<γ0.05, there is no clear correlation between the two. The values ​​of γ0.01, 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 and can better reflect the development process of the fault, it is possible to further determine whether the fault involves solid insulation by performing correlation analysis on the continuous monitoring value of the main characteristic gas and CO of the fault. When it is determined through other analysis methods that there is a discharge fault inside the device, the degree of correlation between CO and H2 can be used as a criterion for judging whether or not the electrical fault is related to solid insulation; and the overheating fault is based on the correlation between CO and CH4. Through the 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 faults, if the CO is not only strongly correlated with CH4, but also related to C2H4, it indicates that the fault point has a higher temperature; In the case of a discharge fault, if the CO and H2 and C2H2 have a strong correlation, the nature of the fault may be a spark discharge or an arc discharge.
3. Development Trend of Failure After confirming the type of failure, if you can further understand the development trend of the failure, it will be helpful for the reasonable arrangement of the maintenance plan. The rate of gas production as an important parameter for judging the degree of damage caused by gas production in oil-filled equipment is very valuable for analyzing the nature of the fault and the degree of development (including the power, temperature, and area of ​​the fault source, etc.) [4].
Through regression analysis, these three typical patterns can be summarized as:
(a) Positive quadratic: The variation of total hydrocarbons over time is roughly Ci=a.t2+b.t+c (a>0), ie, the rate of gas production γ=a.tb increases, proportional to time . This often corresponds to a sudden failure. The failure power and the area involved are constantly increasing. This type of failure growth is often very dangerous.
(b) Negative quadratic: The variation of total hydrocarbons and gas production rates is the same as (a), except that a<0. That is, after the total hydrocarbon Ci increases to a certain extent, it fluctuates around this value without significant change. Mostly correspond to gradually weakened or temporary faults, such as overheating of windings in the event of a system short circuit and partial discharges in the event of a system overvoltage.
(c) Primary type: The linear growth model is a form of gas production that corresponds to the point of failure that exists stably. The variation rule of the total hydrocarbon is Ci=k.t+j, and the gas production rate is a fixed constant k. Usually, the fault is considered to be serious only when the fault gas production rate k or the total hydrocarbon Ci is greater than the attention value.
In this paper, statistics on the corresponding relationship between the increase in the total hydrocarbon content of transformers and the severity of faults in 59 cases of overheating faults and 69 cases of discharge faults are presented. Table 2 shows the results.
4. The example analysis of the growth model of fault gas production is positive quadratic, and the gas production rate shows a significant growth trend in a short period of time. It is a rapid development failure, reflecting the failure power and the area involved in the failure. Bigger.
On March 14, 1985, a core inspection revealed that there were 7 layers of burns, perforations, creepage, and other obvious dendritic discharge traces in the seven layers between the high-voltage coil and the low-voltage coil, which was a fault in the screen discharge and was consistent with the analysis results.
5. Conclusions a. The generation of dissolved gas in power transformer oil always has its own internal causes. According to the associated growth of gas and CO, which are the main features of the fault, it can be judged whether the fault point involves solid insulation. This method is basically not affected by the cumulative effect, there is no limit on the value of attention, you can analyze the changing law of dissolved gas at any time, in a timely manner to detect possible latent failures.
b. For the power transformer in operation, the gas production process of the fault does not always grow linearly, and there are other growth modes. The statistical results show that if the total hydrocarbon content increases in a positive quadratic form, most of them are serious destructive failures; when the faulty gas production increases linearly, the failure point is relatively stable; if the total hydrocarbons show negative quadratic growth, most of them are Temporary failure is generally not harmful.
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