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Experimental Study on Influence of Gas Decomposition in SF6 Electrical Equipment

Author:Winfoss Seeker Technology (Shanghai) Co., Ltd.Release time: 2018-05-17 Browsing

The contents of SO2 and H2S in the SF6 gas decomposition at the opening of the SF6 breaker were studied in detail. The effects of breaking current and arcing time on the contents of SO2 and H2S in SF6 decomposition products were studied by a large number of experiments. Contrastive experimental studies were conducted on adsorbents and non-adsorbents in arc extinguishers. Arcs were studied and analyzed. The effect of energy on the decomposition of SF6 gas and the relationship between SO2 and H2S content in SF6 decomposer over time with adsorbents and non-adsorbents. According to the content of SF6 decomposed material, it can provide reference for SF6 equipment status judgment or defect, fault judgment and on-line detection.

SF6 gas has excellent insulation properties and arc extinguishing performance. SF6 electrical equipment uses SF6 gas as insulation medium and arc extinguishing medium. SF6 electrical equipment has been widely used in the field of high pressure and ultra-high pressure. It has the advantages of small size, light weight, large capacity, quick set-up and less maintenance [1-2]. Some existing SF6 electrical equipments have defects or failures, which affect the normal operation of the equipments and even cause accidents. Therefore, discovering the internal defects or faults of SF6 electrical equipments in time is of great significance to ensure the safe operation of the equipments and the power grid. SF6 gas is decomposed under the action of arc, discharge and overheating. Diagnosing whether or not there is a fault inside the device by detecting the content of SF6 decomposition product will become an effective method for on-line monitoring and fault diagnosis of SF6 electrical equipment [3-5].

The situation of SF6 gas decomposed material under the action of breaking the arc has been a lot of research results in the experiment and theory, but there are not many researches in practical application. SF6 gas decomposition products under the arc are mainly sulfides and fluorides. Sulfides mainly include SO2, H2S, SF4, SO2F2, SOF2, etc.; fluorides mainly include HF, CF4, and metal fluorides [6-7]. The SF4 and SOF2 contents in the above-mentioned decomposition products are relatively large but unstable, and they quickly produce stable SO2 and HF under the influence of oxygen (O2) and water gas (H2O) impurities. HF is highly corrosive and can react with internal metals. The reaction, usually detected after the interruption or failure, has a small or undetectable content of hydrogen fluoride (HF); the CF4 content in the fresh gas is very high (about 200ul/l), but the increase in the arc is relatively small , it can not be used as a test gas. The content of other gaseous decomposition products inside the equipment is very small. Therefore, it is feasible to study the decomposition of SF6 gas using SO2 and H2S as detection gases [8-9]. The detected SO2 concentration is the total amount of direct decomposition and hydrolysis products, and H2S is a characteristic component of the decomposition of the solid insulating material. This article has carried on the detailed research to the SO2 and H2S content in the decomposition of SF6 gas when the SF6 breaker is opened from the actual application. The experimental research and analysis of the breaking current, the arcing time, the arc energy, the adsorbent and other factors on the SF6 The influence of the decomposition product provides reference for SF6 equipment status judgment or defect, fault judgment and on-line detection according to the SF6 decomposition content.

The curve of SO2 and H2S in decomposition products shows that the content of SO2 produced by the direct decomposition of SF6 during arc burning is very small, and most of SO2 is produced by further reaction of other intermediate decomposition products. Experimental studies have confirmed that the main gas products of SF6 decomposition under arc high temperature are SF4 and SOF2, but SF4 and SOF2 quickly hydrolyze to produce stable SO2 and HF [10-12]. It is generally considered that H2S gas is a characteristic component of the decomposition of the solid insulating material, and its content is related to the type of insulating material. The insulating material of the gas outlet of the SF6 circuit breaker used in this test is PTFE.

The change trend of SO2 and H2S was basically the same with the increase of time after breaking, and the content increased rapidly. After 6 hours, the SO2 content reached the maximum value of 96.57uL/L, and the H2S content reached the maximum value of 13.14uL/L at 7 hours. After reaching the maximum value, the content of the decomposition product rapidly decreased again under the effect of the adsorbent. After 200 hours, the H2S content was reduced to 0, but the SO2 content tended to be stable at 3.14 uL/L. After 30 days, the SO2 was still stable at 3.14uL/L, and the H2S content was zero. The current of 5kA (peak) was re-interrupted without changing the adsorbent, and the adsorbent still had an adsorption function. The adsorbent was saturated, but the concentration of SO2 stabilized at 3.43uL/L. The test and analysis confirmed that the adsorption of SO2 on the adsorbent used in this experiment was physical adsorption. When the amount of adsorbent adsorbed reached a certain degree, the adsorption rate of the adsorbent on SO2 and the concentration of SO2 in SF6 gas reached a dynamic equilibrium. The magnitude of the equilibrium concentration is related to the degree of saturation of the adsorbent and the gas pressure. In the experiment, it was found that when the breaking current is less than 15 kA (effective value), the decomposition product will become 0 under the adsorption of the adsorbent, while when the breaking current is greater than 20 kA (effective value), SO2 still remains after up to one month of adsorption.

3.2 Influence of Breaking Current on SO2 and H2S Content

In the absence of adsorbent, the magnitude of the breaking current (effective value) influences the fitted curve of the decomposition product content. Due to the discrete time of the operation of the circuit breaker operating mechanism, the arcing time at each breaking current is different, but the difference is not significant. The arcing time is controlled between 9.7ms and 10.2ms, which is basically a half-cycle burning arc. . The value at each breaking current is the value at which the decomposition product is stable when no adsorbent is used. From Fig. 5, the contents of SO2 and H2S increase with the increase of the breaking current, and the increasing speed is faster and faster. The graphs of SO2 and H2S content fitting curve are all second-order curves, indicating decomposition products and currents. The square of the square is proportional to this, which is consistent with the theory.

When the adsorbent has a breaking current (effective value), it has a fitting curve for the content of the decomposition product. The arcing time is controlled between 9.6ms and 10.1ms. Since the content of the decomposed substance changes rapidly with time when there is an adsorbent (see FIG. 4), the value at each breaking current shown in FIG. 6 is the maximum value of the decomposed substance content with the adsorbent. From Fig.6, the contents of SO2 and H2S increase with the increase of breaking current. The increasing speed is slow when the breaking current is less than 5kA, and basically changes linearly when it is 5 to 15kA, and when the breaking current is greater than 15kA. The growth is very rapid. This is caused by the adsorption rate of the adsorbent, and the adsorption rate of the adsorbent is related to the concentration of the decomposed material. When the breaking current is less than 5kA, the concentration of decomposition products is small, and the adsorbent quickly adsorbs completely; when the breaking current is 5 to 15kA, the decomposition product concentration is kept within a certain range. At this time, the adsorption rate of the adsorbent increases with the concentration of the decomposition product. The increase; breaking current in 20kA or more decomposition of the concentration of large, the adsorption rate of the adsorbent will no longer increase with the increase in concentration.

When the adsorbent was compared to the non-adsorbent, the maximum measured value of the decomposed content was significantly smaller at the same large breaking current, and was less than a quarter of the measured value when no adsorbent was used. This is due to the fact that the adsorbent absorbs impurities such as moisture in the SF6 gas and reduces the direct decomposition of the SF6 gas; on the other hand, the adsorbent is adsorbed by the adsorbent during the measurement process. The adsorbent has these two functions.

4 Conclusion

The experimental study of SF6 decomposition products under different parameters shows that the SO2 content directly generated under the arc is less, and most of them are produced by the continued hydrolysis of intermediate products, and the content of H2S is smaller than that of SO2. For the SF6 circuit breaker with a rated short-circuit current of 25 kA, the content of SO2 and H2S both increase after the arc breaks without an adsorbent, and reaches a stable value within several tens of hours; when the adsorbent is present, the content of the decomposed substance rapidly increases and then rapidly decreases. When the current is less than 15kA (effective value), the decomposition product finally absorbs to zero, and when the current is greater than 20kA, the decomposition product remains.

Under the condition that the arcing time is the same and the arc voltage change trend is consistent, the content of the decomposed material is proportional to the square of the breaking current. Under the condition of the same breaking current, the content of decomposition products increases with the increase of arcing time, but it grows slowly near 10ms, and rapidly increases near the peak value of the breaking current such as 5ms and 15ms. In the absence of adsorbents, the content of decomposition products is proportional to the arc energy, and the decomposition energy per kJ is approximately between 15 uL/L and 20 uL/L. The adsorbent has a great influence on the content of the decomposed material. When the adsorbent is used, the measured value is approximately 1/5 to 1/4 of the measured value without the adsorbent. The adsorption rate of the adsorbent is related to the content concentration of the decomposed material, and decomposes with the adsorbent. The increase in the concentration of the material increases.

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