Fault Analysis of 35kV Outdoor Vacuum Circuit Breaker Pillar Porcelain Sleeve

In order to further clarify the failure mechanism of the pillar porcelain insulator, the field inspection, the microscopic morphology analysis and material analysis of the ceramic shell of the pillar under a 35kV outdoor vacuum circuit breaker were carried out. The results show that the direct cause of the failure is the discharge between the high voltage conductor inside the porcelain sleeve and the weak part of the porcelain sleeve, and the high temperature arc leads to the bursting of the porcelain sleeve. The indirect cause of the failure is the loose porcelain of the porcelain sleeve, the existence of internal layering, uneven composition and micro-pores and other macro and micro defects, and the original defects of the inner surface of the porcelain sleeve, such as local unglazed and poor quality of the porcelain sleeve manufacturing process. In the long-term outdoor operation, the external moisture and gas penetrate into the porcelain sleeve, which leads to the deterioration of the insulation performance of the porcelain sleeve, and eventually leads to failure.

1. Introduction ZW7-40.5 outdoor vacuum circuit breaker uses vacuum as the extinguishing medium, and the moving end of the extinguishing chamber is connected with the output shaft of the operating mechanism through the crank arm and the insulating tie rod.The overall structure of the circuit breaker is porcelain bushing pillar type, the upper porcelain bushing is the arc extinguisher chamber porcelain bushing, and the lower porcelain bushing is the pillar porcelain bushing.The three-phase porcelain bushing is installed on a frame, and the three-phase current transformer is installed in the porcelain bushing of the lower pillar, which is connected with the main circuit of the circuit breaker(Figure 1).The upper and lower porcelain sleeves are filled with vacuum insulating silicone grease with excellent insulating properties [1][2][3][4].High voltage circuit breaker porcelain casing is often made of high strength alumina ceramic, which has good chemical stability, good insulation performance, high mechanical strength advantages, ceramic casing product performance is directly related to the service life of the entire equipment [5][6][7][8].The common failure causes of outdoor circuit breakers are flange cracking, porcelain sleeve deformation and cracking, cement expansion, aging, rust, etc.In a 110 kV transmission line, seven circuit breaker failures have occurred, of which the porcelain sleeve cracking fault accounts for 41%.

Fault Situation
In A 110kV substation, the A-phase porcelain sleeve of 35kV circuit breaker burst, and the partial porcelain sleeve between the third parachute skirt and the lower flange fell off and flew out, and the internal insulation silicone grease leaked, resulting in the equipment being forced to shut down.On-site inspection of the faulty circuit breaker shows that the direct cause of the fault is that the high voltage conductor inside the porcelain sleeve of the pillar under the circuit breaker dissolves with the porcelain sleeve, and the high temperature arc generated by the discharge causes the porcelain sleeve to burst and the internal insulation silicone grease to flow out.

Macroscopic inspection
Two typical samples were obtained by sampling the shed porcelain bushing, and the inspection results were as follows: Figure 2 shows the macroscopic morphology of sample 1# sampled on site.There are large-area arc burning traces on the inner wall of the porcelain sleeve of the sample.Along the section length of about 50.89mm, the fracture surface of the porcelain sleeve is mostly gray, and there are soot attachments deposited on the surface of some areas.There are obvious differences between the section morphology and other parts.The three parts of the 1# sample were examined respectively, as shown in Figure 2b, 2c and 2d.As can be seen from Figure 1b, the enamel of the inner wall of the sample is burned and melted, forming a large number of pits of different sizes.There is a smooth surface at the edge of the end face, which is different from the glaze melt mark, indicating that there is no glaze or uneven material.In Figure 1c, the red area at the root of the umbrella skirt has a smooth surface, hard texture, a large number of small holes on the surface, the back and bottom are gray and white, the red material is not evenly distributed, the surface is uneven, there is a local bulge, and the edge has an obvious black border with the porcelain body, indicating that the material there is abnormal.Figure 2d is a locally enlarged image of the normal area of the skirt section.It can be seen from the figure that there are many small holes on the surface of the sample, and the largest hole is about 0.1mm in diameter.3 shows the macroscopic appearance of sample 2#.There are traces of local arc burning and an unglazed area on the inner wall of the sample, as shown in parts 1 and 2 of Figure 3a.It can be seen that there are a large number of pores on the glaze of the arc burn site, which is caused by the melting of the glaze after burning at high temperature.There is a surface sag of about 17.92mm in length and 2mm in depth on the inner wall of the site 2. The color of this area is the same as that of the porcelain body, which is gray and white, indicating that the surface is not glazed, which is the defect of the original process.Figure 3b shows the side macro morphology of sample 2#.It can be seen from the figure that there is a region of the side section of the sample with a round and smooth surface, which is different from the rough normal fracture surface, indicating that the porcelain body in this part is discontinuous and is a defect of the original process.It can be concluded from the macroscopic inspection results of the sample that the faulty porcelain bushing has some original process defects such as uneven material, discontinuous porcelain body, unglazed surface and a large number of small holes.

Microscopic morphology analysis by scanning electron microscopy
The samples of normal section, red area, smooth surface and inner discharge surface of porcelain bushing were analyzed by SEM.The scanning microscopic image of the sample is shown in Figure 4 It can be seen from Figure 4a that the normal section sample of the porcelain jacket has a rough surface, directional fracture texture, and a large number of pores are uniformly distributed, indicating that the porcelain of the porcelain jacket is loose and the density is not high.As can be seen from Figure 4b, there are also a large number of pores on the surface of the sample in the red area.Compared with the sample in the normal section, the pore size is larger, the pore density is smaller, and the porcelain density is relatively high, indicating that the porcelain sintered by the porcelain bushing is not uniform.As can be seen from Figure 4c, there are also a large number of pores on the smooth surface, and a large number of uneven pits are distributed on the surface, but the surface is relatively smooth and flat, indicating that the abnormal section existed before the fracture.It can be seen from Figure 4d that the glaze after discharge cauterization has a smooth surface with a large number of bubbles and pits, which are caused by the release of melting gas on the glaze caused by high discharge temperature

Energy spectrum analysis
As above, energy spectrum analysis was performed on surface elements and their distributions at four locations of the sample.Figure 5 is an example of detailed surface distribution diagram of elements.The elements on the surface of the samples of normal section, smooth surface and discharge part of the porcelain sleeve are mainly O(oxygen), Si(silicon) and Al(aluminum), and the distribution of elements on the surface of the sample is uniform as a whole, while the distribution of elements on the surface of the sample in the red area is uneven.The elements of O, Al and K( potassium) increase significantly in the lower right area of the sample, and the distribution of Si elements is relatively uniform.It shows that the distribution of O, Al and K elements is not uniform in the sintering process of this region.At the same time, the main element contents of the four samples were compared, and the results were shown in Table 1.The O element content on the surface of the normal section sample was significantly higher than that of the other three samples, while the Si element content was lower than that of the other three samples, indicating that the material of different parts of the porcelain liner sample was uneven.The red region samples contain higher Si content and the least O content.There is a large amount of Cu (copper) element on the surface of the discharge part of the inner wall of the porcelain sleeve, which is caused by the melting and evaporation of the bronze inside the porcelain sleeve due to high temperature during the discharge process, and the sputtering deposit on the inner surface of the porcelain sleeve.
Figure 5. Scanning Distribution of elements on the surface of the samples Through the analysis of energy spectrum, it can be concluded that the distribution of each element in the sintering process of the porcelain sleeve is not uniform, and the material of different parts of the porcelain sleeve is different.

Conclusion
After macroscopic inspection, SEM micromorphology and energy spectrum analysis, it is determined that the porcelain sleeve is relatively loose, has internal stratification, uneven composition and micro-pores, and there are some original defects on the inner surface of the porcelain sleeve, such as local unglazed surface and poor manufacturing process quality.Due to the macroscopic and microscopic defects in the porcelain sleeve, during long-term outdoor operation, external moisture and gas infiltrate the porcelain sleeve, resulting in the deterioration of the insulation performance of the porcelain sleeve.Under the action of the electric field, discharge occurs between the internal conductor and the weak part of the porcelain sleeve.The discharge results in the local high temperature of the porcelain sleeve and the deterioration of the insulating silicone grease performance.Finally, under the action of internal pressure, the porcelain sleeve burst.

Suggestions
(1) Porcelain sleeve manufacturers strengthen the quality control of porcelain sleeves in the firing process to ensure product quality.
(2) Protective measures should be taken during the transportation of porcelain sleeve products to prevent severe vibration or collision of porcelain sleeve.
(3) Product users strengthen the quality sampling inspection of the porcelain sleeve of the warehousing equipment to ensure that the quality of the warehousing equipment meets the standard requirements.
(4) We should pay close attention to the operation status of the batch of equipment, especially for those that already have silicone grease leakage and porcelain sleeve cracks, timely carry out equipment power outage maintenance and flaw detection.

Figure 2 . 1 #
Figure 2. 1# Sample macroscopic examination diagram (a.Overall topography of the sample; b.Sample inner wall discharge site topography; c.Topography of red area of umbrella skirt section; d.Topography of normal section of umbrella skirt Figure3shows the macroscopic appearance of sample 2#.There are traces of local arc burning and an unglazed area on the inner wall of the sample, as shown in parts 1 and 2 of Figure3a.It can be seen that there are a large number of pores on the glaze of the arc burn site, which is caused by the melting of the glaze after burning at high temperature.There is a surface sag of about 17.92mm in length and 2mm in depth on the inner wall of the site 2. The color of this area is the same as that of the porcelain body, which is gray and white, indicating that the surface is not glazed, which is the defect of the original process.Figure3bshows the side macro morphology of sample 2#.It can be seen from the figure that there is a region of the side section of the sample with a round and smooth surface, which is different from the rough normal fracture surface, indicating that the porcelain body in this part is discontinuous and is a defect of the original process.It can be concluded from the macroscopic inspection results of the sample that the faulty porcelain bushing has some original process defects such as uneven material, discontinuous porcelain body, unglazed surface and a large number of small holes.

Figure 3 .
Figure 3. Macroscopic examination of inner wall and side of 2# sample

Figure 4 .
Figure 4. Scanning microscopic images of samples (a.Micrograph of the normal fracture site of the porcelain bushing; b.Micrograph of the red region; c.Smooth surface micrograph; d.Micrograph of discharge parts) By SEM micromorphology analysis, it can be concluded that the porcelain sleeve has the original defects of loose porcelain, low density and abnormal section.

Table 1
Sample surface element content percentage (%)