[Technology] Main transformer casing insulation fault analysis and maintenance method

Source: Transformer Technology Magazine

Main transformer casing insulation fault analysis and maintenance method

Jiangmen Power Engineering Transmission and Transformation Co., Ltd.

Xiao Qiong

Abstract: Failure or abnormality of the casing does not necessarily constitute an accident. However, if it cannot be eliminated in time for routine maintenance, it will often cause very serious consequences, strengthen the routine maintenance and management of the casing, and adopt the necessary testing methods. It is the responsibility of the operation and maintenance personnel to detect problems early and eliminate hidden dangers. Introduce the daily maintenance project of 110 kV #1 main transformer casing of an electro-chemical plant in Jiangmen City. Through the discovery of transformer casing fault, fault cause analysis and refurbishment, it is found that the A and C phase casing dielectric loss is unqualified and set. The reason for the decrease in the insulation performance of the pipe is that the casing is not tightly sealed, and the insulating oil inside the casing is damp. The experience of this fault handling has far-reaching reference for the maintenance and high-test work of the main transformer casing in the future.

Key words: casing; damp; dielectric loss; overhaul

1. Overview of 110 kV #1 main transformer maintenance project of an electro-chemical plant in Jiangmen City

The electrification plant #1 main transformer becomes a three-phase double-winding transformer, the main transformer model is SFT 40000/110, the rated voltage is 110 kV, the rated capacity is 40,000 kVA, and the casing type is BRDLW2 126. On a certain day, the main transformer was subjected to a preventive test at the request of the customer. It was found that the high-voltage side A and C phase casing dielectric loss and the final screen insulation resistance were unqualified. After discussion, it was decided to repair the casing. In the routine routine or preventive test of the main transformer, in order to find out the moisture condition of the capacitor casing insulation in time, according to the "Provisional Test Procedure for Power Transmission and Transformation Equipment Status" (State Grid Corporation), the end of casing should be Screen-to-ground insulation resistance is tested. When the insulation resistance is < 1000 MÎ©, the dielectric loss factor of the final screen to ground should be measured. The value should be > 0 and â‰¤ 1.5%.

Casing dielectric loss measurements are sensitive to the presence of moisture in the insulation and the overall moisture and overheating of the equipment. Dielectric loss is the energy loss that occurs inside an insulating material under the action of an external electric field due to the hysteresis effect of dielectric conductance and dielectric polarization. The dielectric loss factor tan Î´ of the oil-paper insulation sleeve depends on both the oil-to-paper properties and the water content and cleanliness of the two insulation materials. Because the relative dielectric constant of moisture or other polar medium is large, too much content will result in a large dielectric loss value of the entire product. Therefore, in order to improve the dielectric loss level of the casing, it is necessary to strictly control all aspects from reducing water content and improving cleanliness.

After preliminary discussion, it was concluded that the cause of dielectric loss and insulation failure of the main transformer casing of the Electrochemical Plant was due to the tight seal of the casing and the dampness of the insulating oil inside the casing.

2, Jiangmen City, an electrification plant #1 main transformer casing repair process

2.1 First repair

On one day, the Jiangmen City Power Engineering Transmission and Transformation Co., Ltd. substation installation class removed the #1 main transformer casing and transported it back to the company warehouse. The initial maintenance plan is to replace the insulating oil inside the casing. The casing is drained, vacuumed, and oiled. After the above treatment, a high pressure test was carried out. The test data after the first repair is not obvious. The final screen-ground insulation resistance of the casing B and C phases has not met the requirements of the test specification.

Is it a flaw in the test method or the internal structure of the casing is also affected by the moisture loss? When measuring the bushing dielectric loss factor test, pay special attention to the position where it is placed, because the capacitance of the main transformer bushing is small, when the placement position is different, because the measuring electrode and the high voltage electrode pair are not completely grounded around the equipment, frame, wall And the influence of the ground's scattered impedance on it will lead to a big difference between the measured results of the casing. The location will be different, the impact will be different, and often there will be very different measurement results. Therefore, it is required to place the casing vertically on the casing frame that has been grounded when measuring the casing damage or other tests of the casing. The casing cannot be placed horizontally or suspended with an insulating cable. Otherwise, it will result in inaccurate measurement data. Therefore, the test personnel examined and reviewed the test, and there was no problem in all aspects.

The effect of simply replacing the insulating oil inside the casing is not satisfactory, and the internal structure of the casing is further dried. After comprehensive consideration of various aspects, it is finally decided to disassemble the casing and put the casing capacitor core together with the conductor. Into the oven, while heating it, while vacuuming it, a two-pronged strategy is implemented. Thus, a secondary repair work was carried out.

2.2 Second repair

The disassembly of the casing begins with the removal of the cap from the top of the casing. The top nut is removed. When removing the explosion-proof membrane, it is necessary to pay attention to uniform force; remove the casing oil chamber nut, double nut, remove the oil chamber, and measure the position of the spring nut. (In order to restore the installation), the internal structure of the top end of the casing is exposed, and the upper spring of the casing is removed; after the spring is removed, the casing is ready to be lifted and the casing is removed, and the porcelain bottle is removed; the porcelain insulator must be vertical to avoid damage to the capacitor inside. The core, the end of the capacitor core should be removed before the porcelain sleeve is removed. Be careful when removing it, because the end is very slender and fragile; the exposed capacitor core, remove the bottom pressure equalizing ring, remove the end nut, remove the sleeve The porcelain insulator in the lower part of the tube is finally removed from the mounting flange, as shown in Figure 1.

[Technology] Main transformer casing insulation fault analysis and maintenance method

The A-phase, B-phase, and C-phase casings were disassembled. After each phase of the casing was removed, the capacitor core was hoisted into the tank, and the transformer oil was injected into the bottom (the effect was better when heating). The upper cover is hoisted into the oven, the temperature is controlled at 100 ~ 130 Â° C, heating and vacuuming, and the parts of each sleeve are stored and stored by special storage trays as shown in Figure 2.

[Technology] Main transformer casing insulation fault analysis and maintenance method

After each phase of the casing is dried, it is installed, vacuumed, oiled, and finally subjected to dielectric loss and insulation resistance test. The test data obtained is very accurate, and the dielectric loss value can be comparable to the dielectric loss of some new casings. The insulation resistance value is greatly improved compared to before maintenance and passed the AC withstand voltage test.

The test data for the 2 maintenance of the casing is shown in Table 1 and Table 2. The second repair fully meets the specifications.

3, casing failure analysis

3.1 Analysis of the main causes of common faults in transformer bushings

Through the analysis of the common faults of the transformer bushing, it is not difficult to find that there are four main reasons for the general formation of casing defects: First, because the casing is not completely vacuumed during the factory or overhaul, the residual air between the casing screens is generated, resulting in local Discharge, causing insulation breakdown; second, due to the quality of the rubber pad is not closed or the over-period operation causes the rubber pad to age, resulting in seal failure. If the capacitor casing is poorly sealed at the top, it may cause insulation breakdown after water inlet, and the lower rubber pad If the sealing is poor, the casing will seep oil, which will lead to the oil level of the casing insulating oil. Thirdly, due to the long-term disrepair, the insulation performance of the casing will decrease, causing porcelain damage and even breakdown, and the surface of the casing will be absorbed. After moisture, the conductivity will increase, which will lead to a decrease in insulation resistance, which is not only prone to surface flashover, causing tripping, but also due to increased leakage current, resulting in heating of the insulating sleeve, causing porcelain damage and even breakdown. At the same time, flashover will damage the surface of the casing; fourthly, it will cause defects to the transformer due to improper installation methods or angles.

3.2 Analysis of the failure of the main transformer casing of the electrification plant #1

Because the main reason for the failure of the main transformer casing is due to the tight seal of the casing and the failure of the insulation, the main reason for the failure of the casing seal insulation is two aspects: one may be due to the lack of experience of the previous installers and the operation is not Specifications, or insufficient bolt tightening force, invalidation of the insulation seal; on the other hand, it may be due to years of disrepair, over-period operation or quality problems due to the rubber pad or aging.

Through analysis of the reasons, the corresponding countermeasures have been formulated from the aspect of maintenance. Keep the appearance of the porcelain sleeve clean, no cracks, damage and discharge marks; replace the oil pad and other adjustable rubber pads, keep the seal good, no leakage, improve the sealing performance of the casing, prevent the casing from seeping and leaking oil, and make the casing No leakage. Strengthen the micro-water test of insulating oil, shorten the test period, carry out loosening and sealing inspection of the top terminal block of the casing during maintenance, and make a maintenance record. The oil level of the capacitor casing and the oil-filled casing is normal. It is necessary to replenish the oil or replace the new oil in time, and apply anaerobic glue on the copper pipe thread of the casing to increase the rigidity of the terminal block and the copper pipe. If tgÎ´ exceeds the standard or has serious defects, it must be disintegrated and dried, and the casing and oil test are qualified.

4. The key points that maintenance personnel should pay attention to during the main overhaul operation in the future.

Pay attention to the installation environment of the casing to prevent it from getting wet during the construction process. It is necessary to have clean and dry conditions and suitable temperature during the installation process. It is best to control the assembly at 10 to 15 Â°C higher than the ambient temperature.

Because the temperature of the capacitor core in the casing can reduce the influence of moisture under the installation conditions, it is best to heat the capacitor core and parts inside the casing to 100 ~ 130 Â°C before installation to eliminate the moisture on the surface. The assembly is completed when the temperature has not decreased.

Focus on the mounting details of the casing to prevent the top seal from failing. The seal on the top of the casing is generally divided into a seal of the casing itself and a seal of the casing lead. Most of the substation's main transformer conservator is equipped with elastic corrugated plates at the top, and the elastic corrugated plates and the compression springs adjust the temperature change together. When the maintenance personnel assemble the elastic corrugated board, the sealing gasket on the oil conservator should be matched with the sealing ring between the nuts on the duct to prevent the corrugated board from cracking. The bushing lead is generally a cable-through structure, and the sealing between the top wiring board and the conductive head is strict. Otherwise, rainwater will penetrate into the interior of the transformer along the bushing conductive head, the top wiring board and the cable along the duct.

If the moisture in the environment enters the root of the transformer, it will cause the transformer to break down and cause power failure. Therefore, it must be tightened with bolts to ensure a good seal.

Pay attention to the seal of the grounded small casing to prevent the casing from getting wet. When the maintenance personnel are inspecting, the casing should be horizontally laid down, so that the small screen of the last screen is facing upwards, the small casing is removed, and the insulation of the soft wire drawn from the last screen is checked, and the corresponding repair is performed. When running the casing or when doing the withstand voltage test, it should be noted that the outer grounding cover of the casing should be well grounded.

Pay attention to the tightness of the equalizing ball to prevent discharge events. The pressure equalizing ball is mounted at the tail of the central duct and can be pivoted up and down along the axial direction of the duct. The pressure equalizing ball must be tightened, otherwise, a casing and equalizing ball discharge event will occur. The equalizing ball must meet the electrical strength requirements. The position of the equalizing ball can adjust the creepage distance between the tail of the casing and the winding and the insulation distance of the tank wall, and improve the radial potential distribution. If improperly adjusted, the spherical surface will discharge, causing dielectric breakdown. This situation is very harmful to the electrical performance of the bushing; the state and quality of the oil valve and oil plug are good, there should be no rust; the quality of the rubber pad should also be good.

Pay attention to the operational details of the handover test to prevent human defects. The person in charge of the handover test shall take care to prevent the guide rod from rotating or twisting the grounding lead when removing the lead wire of the small screen at the end of the screen. After the end of the test, it shall be restored to its original state, and the multimeter shall be used to measure whether the final screen has been reliably grounded; At the end of the oil sample work, remember to tighten the oil sample valve; when disassembling and connecting the leads, the experimenter should pay attention to the casing protection to prevent damage to the casing; pay attention to the oil level of the casing and consider before taking the oil sample. Add a little more oil and replenish it in time.

5, the conclusion

It can be said that this is a very successful casing refurbishment. Through this refurbishment work, the maintenance level of the casing has been greatly improved, and it has a very important reference for the future development of the main transformer maintenance or overhaul work.

Transformer overhaul due to inadequate experience of maintenance personnel and improper maintenance methods may cause defects in the transformer. These defects are often hidden and difficult to detect. Only the professional technical level of the maintenance personnel is enhanced, and the ability to find equipment defects is improved. Maintenance and overhaul, test according to the precautionary test procedures, and develop scientific and effective treatment methods to ensure the good operation of the equipment.

references

[1] China Southern Power Grid Co., Ltd. Quality Acceptance and Evaluation Standard for 10 kVï½ž500 kV Transmission and Distribution and Distribution Engineering [S]. 2012.

[2] GB 50150-2016. Electrical equipment installation engineering electrical equipment handover test standard [S]. 2016.

[3] Dong Qiguo. Power Transformer Fault and Diagnosis [M], Beijing: China Electric Power Press, 2000.

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