On 27 November 2020, in a 220kV substation #2 transformer elevated block CT direct resistance measurement found that part of the tap head direct resistance abnormal, after analysis and judgment 2K3 tap terminal poor contact, need to drain the body for oil processing, open the hand hole inspection found 2K3 terminal copper wire connector solder joints off, resulting in poor contact. The site by re-crimping copper nose for processing, back to the installation again after measuring the direct resistance, resistance value is normal, the defect is eliminated.

　　2 Process and analysis

　　A 220kV substation #2 transformer CT secondary terminal sealing ring aging serious, there is leakage of oil, the combination of transformer overhaul on the CT secondary terminal sealing ring for replacement, secondary line disconnect work by a power supply company secondary personnel responsible.

　　The CT nameplate shows that the neutral point of the MV side of the two groups of windings corresponding to each tap ratio and error level is the same, after repair for CT direct resistance measurement, as shown in Figure 1. During the measurement, it was found that the resistance values of the two pairs of taps 2K1-2K4 and 2K1-2K5 at the neutral point on the medium voltage side were abnormal, and there was a large deviation compared with 1K1-1K4 and 1K1-1K5, which did not meet the deviation requirement of less than 10%, and the measurement results are shown in Table 2.

　　The current and voltage measurement clamps were adjusted to ensure good contact with the measurement clamps and when they were measured again, it was found that 2K1-2K4 and 2K1-2K5 could not be measured, and the current could not be added. The next step was to investigate and measure the conduction of the other taps.

　　Based on the measurement results, it was suspected that there was a possible disconnection between 2K3-2K4. As the resistance values of these two groups of taps had been measured to be large before adjusting the clamps, it was initially judged that the 2K3 taps had poor contact with the terminals, causing disconnection of the 2K1-2K4 and 2K1-2K5 taps, resulting in the resistance values of these two groups of taps not being measured.

　　3 Defect handling

　　As the hand hole of the CT terminal block needs to be opened for inspection and processing, the body needs to be drained of oil. In order to reduce the amount of oil drained, the butterfly valve at the gas of the body was first closed before draining, and the oil draining was stopped when the oil level was lower than the hand hole.

　　After opening the hand hole, it was found that a weld inside the CT terminal 2K3 was disconnected, and the other end of the line was connected to the 2K4 terminal, the cause of the defect was the same as the initial judgment. From this can be seen, CT hard copper wire through the welding and copper connected, the connection there is a weak point, the force is easy to fall off. The transformer in 1997 production, has been operating for 23 years, due to the aging of the welding seam become brittle, in the CT terminal column unwiring process a little force easily lead to welding seam loose or even disconnected.

　　The site using copper nose crimping way to deal with, crimping before using sandpaper on the surface of the copper wire to polish, remove the oxide film, after processing back to install, back to install when pay attention to and other connected CT line group to maintain sufficient distance.

　　After the completion of the processing of its direct resistance test again, the test results are shown in Table 3 and Figure 6, with 1K1-1K4 and 1K1-1K5 contrast resistance value has a certain deviation, the reason for the large copper nose crimp copper contact resistance between the lines, although the resistance value deviation but the deviation is not large, to meet the deviation of less than 10% of the requirements, the defect eliminated.

　　4 Summary and preventive measures

　　(1) measured the resistance of 2K1-2K4 and 2K1-2K5 two pairs of taps, after adjusting the measurement clamp can not measure the resistance of these two pairs of taps, because the contact surface of the breakpoint extrusion together, so the resistance deviation is larger, and in the adjustment of the measurement clamp slightly drive the terminal 2K3, resulting in the contact surface of the breakpoint loosened, so the resistance value can not be measured.

　　(2) straight resistance measurement should be compared horizontally, deviation should be promptly investigated to find the cause, to prevent ignoring the case of poor contact of copper wire caused by straight resistance deviation, if not found in time, operation due to mechanical vibration, very easy to cause CT open circuit. ct operation does not allow open circuit, otherwise in the secondary winding will induct a very high voltage, damage the insulation of the winding, resulting in equipment damage, protection (3) In carrying out the elevation of the CT

　　(3) After carrying out the secondary disconnection of the elevated seat CT, CT terminal seal replacement, CT transformation and other work that may cause CT terminals to loosen and disconnect, CT direct resistance and insulation resistance measurement should be carried out to prevent CT disconnection.

　　(4) secondary personnel (by the city company secondary professional staff is responsible for) for elevated seat CT secondary unwiring, prohibit the use of electric spanners to loosen and tighten the terminal bolt, to prevent by a strong external force rotating terminal screw drive copper wire lead to CT secondary lead wire break.

　　Power knowledge of transformer elevated seat CT direct resistance abnormal situation processing

　　November 27, 2020, in a 220kV substation # 2 transformer elevated seat CT straight resistance measurement found part of the tap head straight resistance abnormal, after analysis and judgment 2K3 tap terminal poor contact, the need for the body to drain oil processing, open the hand hole inspection found 2K3 terminal copper wire joint solder joints off, resulting in poor contact. The site by re-crimping copper nose for processing, back to the installation again after measuring the direct resistance, resistance value is normal, the defect is eliminated.

　　2 Process and analysis

　　A 220kV substation #2 transformer CT secondary terminal sealing ring aging serious, there is leakage of oil, the combination of transformer overhaul on the CT secondary terminal sealing ring for replacement, secondary line disconnect work by a power supply company secondary personnel responsible.

　　The CT nameplate shows that the neutral point of the MV side of the two groups of windings corresponding to each tap ratio and error level is the same, after repair for CT direct resistance measurement, as shown in Figure 1. During the measurement, it was found that the resistance values of the two pairs of taps 2K1-2K4 and 2K1-2K5 at the neutral point on the medium voltage side were abnormal, and there was a large deviation compared with 1K1-1K4 and 1K1-1K5, which did not meet the deviation requirement of less than 10%, and the measurement results are shown in Table 2.

　　The current and voltage measurement clamps were adjusted to ensure good contact with the measurement clamps and when they were measured again, it was found that 2K1-2K4 and 2K1-2K5 could not be measured, and the current could not be added. The next step was to investigate and measure the conduction of the other taps.

　　Based on the measurement results, it was suspected that there was a possible disconnection between 2K3-2K4. As the resistance values of these two groups of taps had been measured to be large before adjusting the clamps, it was initially judged that the 2K3 taps had poor contact with the terminals, causing disconnection of the 2K1-2K4 and 2K1-2K5 taps, resulting in the resistance values of these two groups of taps not being measured.

　　3 Defect handling

　　As the hand hole of the CT terminal block needs to be opened for inspection and processing, the body needs to be drained of oil. In order to reduce the amount of oil drained, the butterfly valve at the gas of the body was first closed before draining, and the oil draining was stopped when the oil level was lower than the hand hole.

　　After opening the hand hole, it was found that a weld inside the CT terminal 2K3 was disconnected, and the other end of the line was connected to the 2K4 terminal, the cause of the defect was the same as the initial judgment. From this can be seen, CT hard copper wire through the welding and copper connected, the connection there is a weak point, the force is easy to fall off. The transformer in 1997 production, has been operating for 23 years, due to the aging of the welding seam become brittle, in the CT terminal column unwiring process a little force easily lead to welding seam loose or even disconnected.

　　The site using copper nose crimping way to deal with, crimping before using sandpaper on the surface of the copper wire to polish, remove the oxide film, after processing back to install, back to install when pay attention to and other connected CT line group to maintain sufficient distance.

　　After the completion of the processing of its direct resistance test again, the test results are shown in Table 3 and Figure 6, with 1K1-1K4 and 1K1-1K5 contrast resistance value has a certain deviation, the reason for the large copper nose crimp copper contact resistance between the lines, although the resistance value deviation but the deviation is not large, to meet the deviation of less than 10% of the requirements, the defect eliminated.

　　4 Summary and preventive measures

　　(1) measured the resistance of 2K1-2K4 and 2K1-2K5 two pairs of taps, after adjusting the measurement clamp can not measure the resistance of these two pairs of taps, because the contact surface of the breakpoint extrusion together, so the resistance deviation is larger, and in the adjustment of the measurement clamp slightly drive the terminal 2K3, resulting in the contact surface of the breakpoint loosened, so the resistance value can not be measured.

　　(2) straight resistance measurement should be compared horizontally, deviation should be promptly investigated to find the cause, to prevent ignoring the case of poor contact of copper wire caused by straight resistance deviation, if not found in time, operation due to mechanical vibration, very easy to cause CT open circuit. ct operation does not allow open circuit, otherwise in the secondary winding will induct a very high voltage, damage the insulation of the winding, resulting in equipment damage, protection (3) In carrying out the elevation of the CT

　　(3) After carrying out the secondary disconnection of the elevated seat CT, CT terminal seal replacement, CT transformation and other work that may cause CT terminals to loosen and disconnect, CT direct resistance and insulation resistance measurement should be carried out to prevent CT disconnection.

　　(4) secondary personnel (by the city company secondary professional staff is responsible for) to raise the seat CT secondary disconnect, prohibit the use of electric spanners to loosen and tighten the terminal bolt, to prevent by a strong external force to rotate the terminal screw drive the copper wire leads to CT secondary lead wire disconnection.