1. Introduction Oil-filled equipment such as power transformers, instrument transformers, and circuit breakers are key components in substations. Oil leakage from these devices can affect the safe operation of electrical equipment, thus attracting significant attention from the power industry. Statistics show that oil leakage accounts for approximately 65% ​​of all electrical equipment defects. Currently, the North China Oilfield power grid has 42 substations, including one 220 kV substation, five 110 kV substations, and 36 35 kV substations. There are a total of 1850 oil-filled devices. According to the operations department, approximately 640 devices are experiencing oil leakage, with about 420 of them suffering severe leakage. Annual power outages due to oil leakage occur at a maximum of 5 times for 110 kV and above main transformers and at a maximum of 30 times for 35 kV main transformers. Up to 7 pieces of electrical equipment are burned out annually due to oil leakage. This has significantly impacted oilfield production and daily life. Therefore, the North China Oilfield power grid urgently needs to address the problem of oil leakage in its electrical equipment. [b]2 Steps, methods, and technical requirements for treating oil leaks in electrical equipment using sealant.[/b] To solve the problem of oil leaks in electrical equipment, special sealant materials for oil-sealing have been developed both domestically and internationally. These are two-component sealant adhesives formulated using scientific methods with polymer materials as the matrix. Examples include TS518 emergency repair agent, TS528 oil surface repair agent, and PSI pressurized sealant sticks, all produced and distributed by Beijing Tianshan New Material Technology Co., Ltd. The steps for treating oil leaks in electrical equipment using sealant are as follows: 2.1 Locating the leak point. There are five methods: ① Visual inspection: Generally, the wettest spot is the leak point, but this is not very accurate; ② Powder application: Apply chalk near the leak point and observe where it gets wet first; ③ Water flushing: Suitable for locating leak points in the cooler pipes of a forced-oil circulating transformer radiator. If the oil leak is severe and many pipes are not visible, the principle of oil being lighter than water can be used for inspection. When flushed with water, oil floats on the surface. Carefully observe the water surface; if an oil layer is present, it indicates a leak. ④ Touch test: During inspection, touch the surface from top to bottom with your hand or a tool to locate the leak point. ⑤ Nitrogen pressurization method: Fill the oil tank with nitrogen gas and check for leaks. The pressure is generally 50 kPa (0.5 atmospheres). Before pressurization, the pressure release device and oil level indicator should be sealed tightly. The technical requirement at this time is that the leak point must be accurately identified. 2.2 Cleaning the leak point. The purpose is to remove the oxide layer, oil stains, paint, and other dirt to expose the metal at the leak point. There are two cleaning methods: ① Mechanical cleaning method: Clean the surface with an electric grinder, angle grinder, file, gauze, etc., and then clean with gasoline or acetone. ② Chemical cleaning method: Clean the surface with hydrochloric acid or sulfuric acid corrosion. Generally, these two methods are used in combination. The technical requirements at this time are as follows: the area cleaned around the leak point should be more than 10 times the area of ​​the leak hole, and the surface should show a metallic luster (only transformer oil is allowed). In addition, to increase the adhesion surface with the sealant, the metal surface should be rough, and if necessary, pitting can be applied to the metal surface. 2.3 Leak sealing. Two methods are introduced here: ① Using TS528 oil surface repair agent to seal the leak. First, mix TS528 light red A glue and light blue B glue in a 1:1 weight ratio, then apply the glue to the leak hole. Check the sealing effect after a few minutes. If leakage still occurs, remove the glue and re-seal; if leakage has stopped, reinforce with TS518. The reinforcement method is to mix TS518 A and B glues in a 1:1 weight ratio, apply evenly to TS528, and remove excess glue. At an ambient temperature of 25℃, the curing time of TS528 is about 5 minutes, and the curing time of TS518 is about 10 minutes. After curing, it can be used for a long time. The technical requirements at this time are: the sealing operation must be completed within 5 minutes after the adhesive is mixed, otherwise the adhesive will fail to cure. The mixing and sealing process should be timely, rapid, and accurate. ② Use American-made PSI pressurized sealing adhesive sticks for sealing. First, cut the adhesive stick according to the size of the leak hole, knead the cut adhesive stick evenly, and make the two different layers of adhesive paste fully mixed. Then, squeeze the prepared adhesive paste onto the leak hole, and moisten the surface of the adhesive paste with water to remove excess adhesive paste. Check the sealing effect after a few minutes. If leakage still occurs, the adhesive should be removed and the sealing should be repeated. It can be put into use after curing at an ambient temperature of 25℃ for about 3 minutes and 1 hour. The technical requirements at this time are: the sealing operation must be completed within 3 minutes after the adhesive is mixed, otherwise the adhesive paste will fail to cure. The mixing and sealing process should be rapid, timely, and accurate. [b]3 Tips for using sealing adhesive[/b] 3.1 External cover method. Usually, transformer radiators, bushing risers, and bushings have vent bolts, and there are also oil drain bolts at the bottom of the radiator. If leakage occurs due to insufficient compression of the gaskets on the vent bolts and drain bolts, tightening the bolts will solve the problem. However, if leakage is caused by other reasons, such as excessive compression of the gaskets or cracks due to aging and loss of elasticity, there are two methods: One is the conventional method, which involves shutting off the power, draining the oil, and replacing the gaskets. This method requires a long power outage and involves a large workload. The other method is the external cover method shown in Figures 1-3. This method involves machining an iron cover according to the dimensions of the vent bolt (or drain bolt) in Figure 1, and then using sealant to attach the cover to the vent bolt (or drain bolt), as shown in Figure 3. During a major overhaul of the equipment, the iron cover is then removed for thorough repair. In some equipment, such as the 6KV side handhole flange and riser flange of a transformer, the bolts are welded internally (the structure is shown in Figure 4). If the welding process is not meticulous, resulting in pinholes or cracks, oil will enter the bolt hole through the pinholes and flow out along the bolt, causing oil leakage. At this point, an iron cover as shown in Figure 2 can be machined according to the external dimensions of the screw in Figure 4. The iron cover is then glued to the screw with sealant, as shown in Figure 5. During major repairs, the iron cover can be removed and the screw re-welded. 3.2 Clamp Method When oil leaks from the internal pipes of a transformer cooler, the distance between the pipes is very small, making it difficult to handle with ordinary tools. In order to seal the leak without interrupting power or draining oil, sealant can be used with a special clamp as shown in Figure 6. The clamp is clamped onto the pipe, and then the clamp bolts are tightened appropriately to seal the leak hole with sealant, thus stopping the leak. 3.3 Adsorption Method When sealing leaks at ambient temperatures below 0℃, the sealant has a long curing time. If the internal pressure of the electrical equipment oil tank is high, oil may seep out from the sealant before it is fully cured. After the sealant cures, oil gaps will form in the sealant, allowing oil to continue to seep out. To solve this problem, as shown in Figure 7, after applying sealing adhesive to the surface of the leak, cover it with a 0.75mm thick sheet of iron, and then place a permanent magnet on top to attract the iron sheet to the oil tank. The pressure generated by the permanent magnet will stop the leakage. After the adhesive cures, remove the magnet. 4. Application of Oil-on-Site Leakage Sealing Technology in the North China Oilfield Power Grid In 1998 and 1999, the North China Oilfield conducted leakage sealing tests on 30 leaking points in several substations in the oilfield power grid, including Ren Dong 220KV, Refinery 110KV, Dianzhuang 110KV, Liulu 110KV, Renba, and Yanling, which had serious oil leakage. Quality tracking inspections were also conducted, and the results are shown in Table 1. Statistics show that the success rate of leakage sealing was 93.4%, and the failure rate was 6.6%. The following two examples illustrate the leakage treatment process. In October 1998, during the handling of an oil leakage defect in the No. 2 main transformer of the 110kV substation at the refinery, the two flanges of the riser connecting pipes of the B and C phase bushings on the 35kV side of the main transformer were leaking oil at a rate of 28 drops/min, and the flange of the riser connecting pipe of the neutral point bushing was leaking oil at a rate of 11 drops/min. The transformer tank and radiator fins were covered with sludge, making the appearance extremely dirty. The main transformer was forced to stop operation twice for oil leakage repair. During the first shutdown, the oil and paint on the three flanges were removed, and the flange connection sides were sealed with TS528 and fiberglass tape. Special bolts and threaded screw covers were also machined and used in conjunction with TS528 sealing flange bolts. After the treatment, there was no further oil leakage at that time. However, due to the large diameter of the flange face and the large exposed area, after the main transformer was put back into operation, the sealing adhesive cracked locally due to vibration, and oil leakage reappeared. During the second shutdown, the connecting butterfly valve was first closed, then the three flanges were disassembled, and the gaskets were removed. Inspection revealed that the oil leak was caused by the gaskets being too thin and undercompressed, failing to provide a seal. We applied TS528 evenly to the gaskets and reassembled the three flanges, finally eliminating the main transformer oil leak completely. In the handling of the oil seepage defect in the No. 1 main transformer of the 110kV substation at the refinery, two vent bolt gaskets on the upper part of the main transformer radiator were severely leaking oil, and a pinhole appeared in the weld of the A-phase bushing riser on the 110kV side. The main transformer tank and radiator were contaminated with sludge, resulting in a dirty appearance, forcing the main transformer to be shut down. Based on the site conditions, replacing the vent bolt gaskets would require draining the radiator, a large workload and time-consuming process. We used TS528 to seal the vent bolts and gaskets together. For the pinhole, we used TS518 sealant, and reinforced both with TS518 reinforcing adhesive. The leakage defect of the No. 1 main transformer was eliminated in about half an hour. 5. Benefit Analysis 5.1 Economic Benefit Calculation Assuming 100 fewer incidents of electrical equipment oil leakage defect repair are handled annually, each incident requiring one vehicle and four people, with a cost of 180 yuan per vehicle and 23.85 yuan per man-day, the annual reduction in vehicle and labor costs would be: 100 × (180 + 4 × 23.85) = 27,500 yuan. Assuming a 110KV 31500KVA power transformer experiences 3 fewer power outages annually, and a 35KV 4000KVA power transformer experiences 10 fewer power outages annually, with each incident lasting 8 hours, a cost of 0.4 yuan per kilowatt-hour, an average power factor of 0.9, and a transformer utilization rate of 0.8, the annual reduction in main transformer power outage losses would be: (3×31500+10×4000)×8×0.4×0.9×0.8=309,900 yuan. Assuming an annual reduction of 2 damaged 110KV and above voltage and current transformers (70,000 yuan each), 4 damaged 35KV voltage transformers (8,000 yuan each), and a reduction of approximately 40,000 yuan in material and transformer oil losses, then the annual reduction in losses is: 2×7+4×0.8+4=212,000 yuan. Therefore, the total annual economic benefit is: 27,500+309,900+212,000=550,000 yuan. 5.2 Social Benefit Analysis: The application of oil-sealed leakage plugging technology significantly reduces the failure rate of electrical equipment, reduces equipment downtime and the number of outages, reduces substation switching operations, and lowers the possibility of misoperation. This not only contributes to ensuring continuous and reliable power supply for oilfield life and production but also lays the foundation for creating leak-free standardized substations in the future. [b]6 Conclusion[/b] Oil-on-line sealing technology is one of the effective methods for eliminating oil leaks in electrical equipment. Its unique advantages include speed, convenience, reduced power outage time and frequency, and even eliminating the need for power outages altogether. However, it also has limitations. Applying this technology cannot eliminate oil leaks in all equipment. For example, oil leaks in transformer butterfly valve gaskets or transformer tank cover gaskets currently require replacement of the gaskets for a complete solution. Therefore, oil-on-line sealing technology still needs continuous exploration and refinement in practice to generate greater economic and social benefits in the power supply industry.