Introduction
In order to keep the dimensions of the device at an acceptable level, the modern transformer is designed with a more compact insulation, and the thermal and electrical stress levels required for insulation between its internal components are significantly increased during operation. 110kV and above the level of large-scale power transformers mainly use oil-paper insulation structure, the main insulating material is insulating oil and insulation paper, cardboard.
When the transformer internal failure involves solid insulation, regardless of the nature of the fault, is usually considered to be quite serious. Because once the insulation properties of solid materials are damaged, it is likely to further develop into the main insulation or vertical insulation breakdown. So the impact of fiber material degradation caused by the fault in the special attention. Moreover, if you can determine whether the transformer abnormal or fault involved in solid insulation, it is initially identified the fault location, the equipment maintenance work is very helpful.
In this paper, a method of dynamic analysis of transformer insulation failure is proposed by studying the related growth of other characteristic gas components and CO and CO2 when the fault involves solid insulation. And the establishment of fault gas growth model, to predict the development of the failure to provide a new criterion.
1, To determine the solid insulation fault routine method
CO, CO2 is the aging product of fiber material, generally in the case of non-fault also has a lot of accumulation, it is often difficult to determine the analysis of the CO, CO2 content is due to the normal aging of fiber materials, or failure of decomposition products.
(CO + CO2) mL / g (paper) is used to diagnose solid insulation faults. The results show that the total amount of carbon oxides in the oil is reduced. However, the insulation structure of the transformer, the choice of materials and oil and paper ratio with the voltage level, capacity, type and production process vary widely, it is impossible to calculate the total weight of each transformer insulation paper, the method because of The actual operation is difficult and difficult to apply; and it is reasonable to consider all the paper weight in the analysis of the overall aging. If the fault point involves only a small part of the solid insulation, it is difficult to use this method to consider CO and CO2 More effective.
IEC599 [2] recommended the ratio of CO / CO2 as a criterion to determine the relationship between failure and solid insulation. It is assumed that CO / CO2> 0.33 or <0.09 indicates that there may be a fiber insulation breakdown failure. In practice, this method also has considerable limitations [3]. In this paper, 59 cases of superheat failure and 69 cases of discharge failure were counted. The results show that the positive rate of CO 2 / CO2 ratio is only 49.2%. This method has a high accuracy of 74.5% for the suspension discharge fault, but the positive rate of the screen discharge is only 23.1%.
2, Solid insulation fault dynamic analysis method
The new precautionary test procedure stipulates that the 330kV and above level of the transformer every 3 months to conduct a dissolved gas analysis of oil, but many of the current power bureau to ensure the safety of these important equipment, and some have shortened the time interval For 1 month. There are some electric bureau has carried out the oil chromatography on-line monitoring attempt, which for the realization of continuous fault tracking, provides a good technical foundation.
Power transformer internal solid insulation involved in the fault include: fence discharge, inter-turn short circuit, overload or poor cooling caused by winding overheating, insulation damage caused by partial discharge and so on. In the event of electrical failure or overheating failure, when the fault point involves solid insulation, the oil and paper insulation will crack and release CO and CO2 under the action of the energy released at the point of failure, but their production is not isolated, Of the decomposition of a variety of low molecular hydrocarbons and hydrogen, and by analyzing the characteristic gas and CO and CO2 associated with the growth between the situation, to determine the cause of the malfunction.
To determine whether the characteristic gas and CO and CO2 content of the fault are accompanied by growth, a quantitative standard is required. In this paper, the correlation analysis of the results of continuous chromatographic monitoring of transformers is carried out to obtain a statistical description of this standard. This can overcome the effects of the cumulative effect of the dissolved gas and eliminate the random error of the measurement.
In this paper, Pearson product correlation is used to measure the degree of correlation between variables. The significance of two pairs of test variables (xi, yi), i = 1, ..., correlation coefficient γ is chosen as the variable by α = 1% Whether there is a significant correlation between the criteria, and whether α = 5% as a criterion for whether the variables are relevant. That is: when the correlation coefficient γ> γ0.01, that the variables are significantly related; γ <γ0.05, the two are not clearly associated. Γ0.01, γ0.05 value and the number of sampling N, can be obtained by checking the correlation coefficient test table.
Because CO is the intermediate product of cellulose degradation, it can reflect the development process of the fault. Therefore, it is possible to judge whether the fault involves solid insulation by analyzing the correlation between the main characteristic gas and the continuous monitoring value of CO. When the other methods are used to determine the existence of discharge faults within the equipment, the correlation between CO and H2 can be used as the criterion to judge whether the electrical fault is related to the solid insulation. The superheat fault is judged by the correlation between CO and CH4. Through the analysis of 59 cases of superheat failure and 69 cases of discharge failure.
This method can reflect the severity of the fault to a certain extent. In the case of overheat failure, if CO has not only a strong correlation with CH4, it is also related to C2H4, indicating that the temperature of the fault point is higher; Discharge failure, if CO and H2 and C2H2 have a strong correlation, indicating that the nature of the failure may be spark discharge or arc discharge.
3, The development trend of failure
After confirming the type of failure, if you can further understand the development trend of the failure, will help the maintenance plan reasonable arrangements. The gas production rate as an important parameter to determine the degree of gas-bearing failure in oil-filled equipment is valuable for analyzing the nature and degree of development (including the power, temperature and area of the fault source).
Through regression analysis, these three typical models can be summarized as follows:
(A) Positive quadratic: the change of total hydrocarbon with time is approximately Ci = a.t2 + b.t + c (a> 0), that is, the gas production rate γ = a.t + b is increasing, which is proportional to time. This often corresponds to sudden failures, fault power and the area involved are constantly increasing, this failure growth model is often very dangerous.
(B) Negative quadratic: The change of total hydrocarbon and gas production rate is the same as that of (a), except that a <0, ie, the total hydrocarbon Ci increases to a certain extent and fluctuates near the value without any significant change. And more with the gradual or temporary failure of the corresponding form, such as in the case of short circuit in the case of winding overheating and over-voltage system occurs in the case of partial discharge.
(C) A type: the linear growth model, is a stable existence of the corresponding point of the gas production. The change rule of the total hydrocarbon is Ci = k.t + j, and the gas production rate is a fixed constant k. It is usually considered that the fault is serious only when the fault gas production rate k or the total hydrocarbon Ci is greater than the attention value.
4, The case analysis
The growth model of fault gas production is positive secondary type, and the gas production rate shows a significant growth trend in a short time. It is a rapid development fault, which reflects the increase of fault power and fault.
March 14, 1985 for hanging core inspection found that high-voltage coil and low-voltage coil between the screen there are seven layers of different degrees of burns, perforation, creeping and other obvious dendritic discharge traces, is a screen discharge failure, consistent with the analysis results.
5 Conclusion
A, power transformer oil dissolved gas production has its own internal reasons, according to the main characteristics of the failure of the gas and CO associated with the growth of the situation, you can determine whether the fault point involves solid insulation. This method is basically not affected by the cumulative effect, there is no limit to the value of attention, you can analyze the law of dissolved gas at any time, in time to find possible latent failure.
B, the operation of the power transformer, the failure of the gas production process is not all linear growth, there are other growth patterns. The results show that if the total hydrocarbon content is positive and secondary, the fault is relatively stable when the fault gas is linearly increased. If the total hydrocarbon is negative quadratic growth, Temporary failure, the general harm is not.
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