1. Overview of the 110 kV #1 main transformer overhaul project at an electrochemical plant in Jiangmen City
The #1 main transformer at the power plant is a three-phase, double-winding transformer, model SFT-40000/110, with a rated voltage of 110 kV and a rated capacity of 40000 kVA; the bushing model is BRDLW2-126. One day, at the customer's request, a preventative test was conducted on the main transformer, revealing that the dielectric loss and insulation resistance of the high-voltage side A and C phase bushings were substandard. After discussion, it was decided to repair the bushings. During routine or preventative tests of the main transformer, to promptly detect moisture in the insulation of capacitive bushings, according to the "Condition-Based Maintenance and Testing Procedures for Transmission and Transformation Equipment" (State Grid Corporation of China), the insulation resistance of the bushing's final screen to ground should be tested. When the insulation resistance is < 1000 MΩ, the dielectric loss factor of the final screen to ground should be measured, and its value should be > 0 and ≤ 1.5%.
Dielectric loss measurement of bushings can sensitively detect defects such as moisture in the insulation, overall equipment moisture absorption, and overheating aging. Dielectric loss is the energy loss that occurs within an insulating material under the influence of an external electric field due to the hysteresis effect of dielectric conductivity and polarization. The dielectric loss factor tanδ of oil-paper insulated bushings depends not only on the combined properties of the oil and paper but also on the moisture content and cleanliness of both insulating materials. Because moisture or other polar media have a relatively high relative permittivity, excessive moisture content will lead to a higher overall dielectric loss value. Therefore, to improve the dielectric loss level of bushings, it is necessary to start by reducing moisture content and improving cleanliness, and to strictly control all aspects.
After preliminary discussion, it was determined that the reason for the substandard dielectric loss and insulation of the #1 main transformer bushing in the power plant was due to poor bushing sealing, which caused the insulating oil inside the bushing to become damp.
2. Overhaul process of the #1 main transformer bushing at an electrochemical plant in Jiangmen City
2.1 First Repair
One day, the substation installation team of Jiangmen Power Engineering Transmission and Transformation Co., Ltd. disassembled the #1 main transformer bushing and transported it back to the company warehouse. The initial repair plan was to replace the insulating oil inside the bushing. The bushing was drained of oil, evacuated, and then refilled with oil. After these procedures were completed, a high-voltage test was conducted. The test data after the first repair was not significant; the insulation resistance between the end screen and ground of phases B and C of the bushing still did not meet the test specifications.
Was the problem a flaw in the testing method or dampness in the internal structure of the bushing leading to unsatisfactory dielectric loss measurements? When measuring the dielectric loss factor of a bushing, special attention must be paid to its placement. Because the capacitance of the main transformer bushing is very small, different placement positions can lead to significant differences in the measured results due to the influence of scattered impedances from the measuring electrodes and high-voltage electrodes to surrounding equipment, structures, walls, and the ground (if not fully grounded). Different positions will have different effects, often resulting in drastically different measurement results. Therefore, when measuring the dielectric loss of a bushing or conducting other tests, it is required that the bushing be placed vertically on a grounded bushing rack. The bushing should not be placed horizontally or suspended by insulating cables for measurement; otherwise, the measurement data will be inaccurate. The testing personnel then checked and reviewed the testing procedures, and found no problems in any of them.
Simply replacing the insulating oil inside the bushing was not ideal; further drying of the internal structure was necessary. After comprehensive consideration, it was decided to disassemble the bushing, placing the capacitor core along with the conductor into an oven for simultaneous heating and vacuuming—a two-pronged approach. Thus, a second repair was carried out.
2.2 Second Repair
The bushing disassembly begins at the top where the bushing cap is installed. Remove the top nut, and be careful to apply even force when removing the explosion-proof membrane. Remove the bushing oil chamber nut and double nuts, remove the oil chamber, and measure the position of the spring nut (for easy reinstallation). Expose the internal structure at the top of the bushing and remove the upper spring. After removing the spring, prepare to lift and dismantle the bushing, and remove the bushing porcelain insulator. When lifting the porcelain insulator, ensure that the insulator is vertical to avoid damaging the capacitor core inside. The lead-out end of the capacitor core must be removed before removing the porcelain insulator. Be especially careful when removing it, as the end is very thin and fragile. After exposing the capacitor core, remove the bottom equalizing ring, remove the end nut, remove the lower porcelain insulator of the bushing, and finally remove the mounting flange, as shown in Figure 1.
[Technical] Analysis and Maintenance Methods for Insulation Faults in Main Transformer Bushings
[Technical] Analysis and Maintenance Methods for Insulation Faults in Main Transformer Bushings
The bushings for phases A, B, and C were disassembled. After each phase bushing was disassembled, its capacitor core was hoisted into a container, and transformer oil was injected into the bottom (to improve the heating effect). The container was then covered and hoisted into an oven. The temperature was controlled at 100-130℃ for heating and vacuuming. The parts of each bushing were categorized and stored in a special storage tray, as shown in Figure 2.
[Technical] Analysis and Maintenance Methods for Insulation Faults in Main Transformer Bushings
[Technical] Analysis and Maintenance Methods for Insulation Faults in Main Transformer Bushings
After each phase bushing is dried, it is installed, vacuumed, and oiled. Finally, dielectric loss and insulation resistance tests are conducted. The test data are very accurate. The dielectric loss value is comparable to that of some new bushings when they leave the factory. The insulation resistance value is greatly improved compared to before the repair. It also passes the AC withstand voltage test.
The test data for the two maintenance procedures performed on the bushing are shown in Tables 1 and 2. The second maintenance fully met the specifications.
3. Fault Analysis of Bushings
3.1 Analysis of the Main Causes of Common Transformer Bushing Faults
Analysis of common transformer bushing faults reveals four main causes of bushing defects: First, incomplete vacuuming during manufacturing or overhaul leaves residual air between the bushing panels, leading to partial discharge and insulation breakdown. Second, substandard gasket quality or excessive operation causes gasket aging and sealing failure. Poor sealing at the top of capacitive bushings can lead to water ingress and insulation breakdown, while poor sealing at the bottom can cause oil leakage and a drop in the insulating oil level. Third, years of neglect degrade the bushing's insulation performance, damaging the porcelain and potentially causing breakdown. Dirt on the bushing surface absorbs moisture, increasing conductivity and reducing insulation resistance. This not only increases the risk of surface flashover and tripping but also increases leakage current, causing the bushing to overheat, damaging the porcelain, and even breaking down. Flashover also damages the bushing surface. Fourth, improper installation methods or angles can cause defects in the transformer.
3.2 Analysis of the bushing fault of the No.1 main transformer in the power plant
The main cause of the main transformer bushing failure was poor bushing sealing and insulation failure. There are two main reasons for the bushing sealing and insulation failure: one is that the installation personnel may have lacked experience, operated improperly, or the bolts may not have been tightened properly, resulting in ineffective insulation sealing; the other is that it may be due to years of disrepair, operation beyond the cycle period, or quality problems or aging of the rubber gasket.
Based on the analysis of the causes, corresponding countermeasures were formulated from the perspective of maintenance and repair. Keep the porcelain bushing surface clean, free from cracks, damage, and discharge marks; replace replaceable gaskets such as those for oil drain holes to maintain good sealing and prevent leakage, improving the bushing's sealing performance to prevent water and oil leakage. Strengthen the micro-water test of insulating oil, shorten the test cycle, and check the looseness and sealing of the top terminal block during maintenance, keeping detailed maintenance records. For capacitor bushings and oil-filled bushings with normal oil levels, replenish or replace the oil promptly. Apply anaerobic adhesive to the copper pipe threads of the bushing to increase the strength of the terminal block and copper pipe. If tgδ exceeds the standard or has serious defects, disassemble and dry the bushing, ensuring the bushing and oil tests are passed.
4. Key points that maintenance personnel should focus on during future main transformer overhaul operations.
Pay attention to the installation environment of the sleeve to prevent it from getting damp during construction. The installation process should be carried out in a clean, dry environment with a suitable temperature, preferably controlled at 10 to 15 degrees Celsius above the ambient temperature.
Because the temperature of the capacitor core inside the bushing can reduce the impact of moisture when it meets the installation conditions, it is best to heat the capacitor core and components inside the bushing to 100-130°C before installation to remove moisture from their surface and complete the assembly as soon as possible before the temperature drops.
Pay close attention to the installation details of the bushing to prevent top seal failure. The top seal of the bushing is generally divided into the seal of the bushing itself and the seal of the bushing lead wire. Nowadays, most substation main transformer oil conservators are equipped with elastic corrugated plates at the top, which, together with the compression spring, regulate temperature changes. When maintenance personnel assemble the elastic corrugated plates, the sealing gasket on the oil conservator must be properly fitted with the sealing ring between the nut on the conduit to prevent the corrugated plate from cracking. The bushing lead wire is generally a cable-driven structure, and the seal between the top terminal block and the conductive head must be tight; otherwise, rainwater will seep into the transformer's interior along the bushing conductive head, the top terminal block, and the cable through the conduit.
If moisture from the environment gets into the base of the transformer leads, it will cause the transformer to become damp and break down, leading to a power outage. Therefore, bolts must be tightened to ensure a good seal.
Pay attention to the sealing of the grounding bushing to prevent it from getting damp. During maintenance, the bushing should be laid horizontally with the end-screen bushing facing upwards. Remove the bushing and check the insulation of the lead-out flexible wire from the end-screen, then perform any necessary repairs. When the bushing is in operation or undergoing withstand voltage testing, ensure that the external grounding cover of the bushing is properly grounded.
Pay attention to the tightness of the equalizing ball to prevent discharge events. The equalizing ball is installed at the tail of the central conduit and can be screwed up and down along the conduit axis. The equalizing ball must be tightened; otherwise, a discharge event will occur between the bushing and the equalizing ball. The equalizing ball must meet the electrical strength requirements. The position of the equalizing ball can adjust the creepage distance between the bushing tail and the winding, the insulation distance of the tank wall, and improve the radial potential distribution. If improperly adjusted, discharge will occur on the ball surface, causing dielectric breakdown, which is very harmful to the electrical performance of the bushing. The condition and quality of the oil valve and oil plug must be good, and there should be no rust. The quality of the rubber gasket should also be good.
Pay close attention to the operational details of the handover test to prevent human error. Personnel responsible for the handover test should take care to prevent the guide rod from rotating or the grounding lead from being broken when disconnecting or connecting the leads of the small bushing of the end screen. After the test, the original state should be restored, and a multimeter should be used to measure whether the end screen has been reliably grounded. When the oil sampling work is finished, remember to tighten the oil sample valve. When disconnecting or connecting the leads, the test personnel should pay attention to the protection of the bushing when moving it up or down to prevent damage. Pay attention to the oil level in the bushing; consider adding a little extra oil before taking the oil sample and replenish oil in a timely manner.
5. Conclusion
This was a very successful bushing overhaul. Through this overhaul, the bushing maintenance level has been greatly improved, and it has a very important reference value for future main transformer maintenance or overhaul work.
Transformer maintenance may cause defects due to insufficient experience of maintenance personnel or improper maintenance methods. These defects are often hidden and difficult to detect. Only by strengthening the professional and technical level of maintenance personnel, improving their ability to detect equipment defects, carrying out maintenance and repair according to the manufacturer's requirements, conducting tests according to the preventive testing procedures, and developing scientific and effective handling methods can we ensure the good operation of the equipment.
