During transformer overhaul, the on-load tap changer undergoes disassembly, inspection, and reassembly. The major overhaul items are as follows:
What aspects should be considered when overhauling an on-load tap changer for a transformer?
1. Inspection, repair, and debugging of the tap changer core.
2. Cleaning, leak detection and maintenance of tap changer oil chamber.
3. Inspection, cleaning, lubrication and maintenance of the drive mechanism.
4. Inspection and maintenance of oil storage tanks and their accessories
5. Inspection, maintenance and calibration of oil flow control relays (or gas relays), overpressure relays and pressure relief devices.
6. Inspection of automatic control devices.
7. Treatment of insulating oil in oil conservator and oil chamber
8. Inspection, maintenance and debugging of electric mechanisms and other components.
9. Inspect the seals of all parts and deal with any oil leaks.
10. Inspection, maintenance and debugging of electrical control circuits.
11. Verification and debugging of the connection between the tap changer and the electric mechanism.
I. Key Operating Points of On-Load Tap Changer
1. Installation of combined on-load tap changer
(1) Unscrew the bolts that secure the switch head cover, and then remove the switch head cover, taking care to protect the sealing gasket.
(2) Check whether the slot on the drive shaft of the switch and the triangular mark on the support plate are aligned.
(3) Remove the locking plate of the position indicator, and then remove the tap position indicator.
(4) Remove the nuts connecting the switch body to the support plate. Attach a suspension line to the switch body and carefully lift it vertically. After lifting it out of the oil chamber, place it on a platform covered with plastic sheeting or on flat ground.
(5) Remove the oil suction pipe and take it out of the oil chamber.
(6) Check if the oil chamber of the switch is clean. If it is not clean, it should be cleaned.
(7) Use a special lifting plate to hang horizontally in the oil chamber, so that the support flange is supported and the tap changer is suspended. Then loosen the connecting bolts of the intermediate flange and the support flange to separate them.
(8) Gently lower the hoisting plate until the support flange places the tap changer onto the bracket mounted on the body of the device. At this point, the oil-saving tank can be lifted.
(9) After disconnecting the upper mailbox, perform the following checks on the selector:
1) Check if the connection wires (copper rods) between the selector and the toggle switch are loose.
2) Check whether the tap changer and selector stationary contact are in good condition, and whether the equalizing cover is intact.
3) Check whether the tap lead causes the bakelite strip of the selector to be subjected to horizontal tension (the horizontal tension should be close to zero).
4) Check whether the contact wire of the selector moving contact is located on the cylindrical part of the stationary contact and is at least 2mm away from the spherical part of the stationary contact.
(10) Connect a bridge circuit or use a multimeter to measure the resistance value of the transition resistor on the fixed main arc contact and the fixed transition contact. The measured value should deviate from the nominal value on the nameplate within ±10%.
(11) After the upper section oil tank is reinstalled, the oil chamber of the switch should be inspected and cleaned again, and rinsed with clean oil before installing the oil suction pipe.
(12) Use a special lifting plate to lift the tap changer, restore the connection between the intermediate flange and the support flange, and ensure a good seal.
(13) Hoist the switch and carefully lower it into the oil chamber. To ensure proper engagement of the coupling, gently rotate the insulating shaft of the switch to insert the coupling and lower the switch into place.
(14) Reinstall the nuts on the fixing bolts on the support plate to fix the switch to the support plate.
(15) Reinstall the position indicator plate and install the shaft head lock plate.
(16) Reinstall the switch head cover plate, tighten all the bolts on the switch head cover plate to make it firmly connected to the switch head flange and ensure good sealing.
(17) Install a bypass pipe on the pipe joint connecting the return oil pipe and the transformer tank on the switch head flange to connect the switch oil chamber and the transformer tank so that the oil chamber and the tank can be vacuum-filled at the same time. When the vacuum oil filling reaches the vent plug on the switch head cover, the bypass pipe should be removed, a sealing plate should be added, and then the vacuum oil filling should continue.
(18) Install the worm gear box and bevel gear box, install the horizontal shaft and vertical drive shaft with the coupling clamp, and install the rain cover.
(19) Use manual operation to switch between the forward and reverse directions to check the symmetry of the transmission mechanism. If it is not symmetrical, the vertical transmission shaft should be loosened and adjusted until the difference in the number of rotations during forward and reverse switching does not exceed one revolution.
(20) First perform the switching operation manually, then perform the switching operation using the electric mechanism. Check that the tap indicator and the electric mechanism position indicator are in the same position at each tap position.
2. Installation of composite on-load tap changer
Its characteristics include the direct switching selector and the switching switch being integrated into one unit. The oil in the tap changer tank is isolated from the transformer body oil. The switching operation is completed simultaneously with the tap selection, resulting in a larger size. However, since voltage regulation is completed in one step, the structure is simple, the cost is low, and the operation process is simple. It is suitable for transformers with low current and low voltage, and is generally used on on-load tap-changing transformers with a few voltage taps (mostly 7). The steps for installing the composite on-load tap changer are as follows:
(1) Adjust the tap position to the rated tap position.
(2) Remove the switch head cover. Before removing it, drain the oil or air.
(3) Loosen the hinge nut of the oil suction pipe elbow.
(4) Move the counter's fitting into the switch head.
(5) Remove the locking screw of the gear mechanism from the switch head and loosen the spring.
(6) Remember the position markings of the gear mechanism, remove the fixing screws of the gear mechanism, and then pull the gear mechanism upwards. Record the position of the support crossbar.
(7) Disassemble the suction pipe. When pulling it out, first insert a screwdriver into the upper groove and pry up the suction pipe, then grab the second groove with your hand and pull out the suction pipe.
(8) Fix the special lifting tool to the coupling end face of the selector switch body with 3 screws. The stop plate of the lifting tool must be inserted into the middle of the crossbar.
(9) Twist the lifting device clockwise so that the moving contact of the selector is between the two phases, that is, in its lifting position.
(10) Hang the load on the lifting device and slowly and carefully pull the selector switch body upwards. During the pulling process, be careful to maintain sufficient clearance between the moving contact and the equalizing ring and the oil chamber to avoid collision.
(11) Place the selector switch body at the inspection location and inspect and clean it.
(12) Screw the four M8 bolts on the special tool for lifting the oil chamber into the bolt holes of the support flange. Lift the oil chamber, remove the bolts with locking washers connecting the support flange and the switch head flange, and then place the oil chamber on the support plate on the clamping line. At this time, the upper section of the oil tank can be lifted.
(13) After cleaning the oil chamber, reassemble according to the reverse procedure described above. However, the following points should be noted:
1) After inserting the oil suction pipe, press the top of the oil suction pipe firmly to make the selector switch body coaxial with the oil chamber.
2) Use a lifting tool to turn the switch to the calibration position.
3) The gear mechanism should be adjusted to the calibration position so that it can be inserted and coupled with the selector switch body.
4) Before installing the switch cover, place the sealing gasket to ensure the cover is sealed.
5) Finally, check through the inspection window on the switch head cover to see if it is in the calibration position.
(14) Install a bypass pipe so that the oil chamber and oil tank can be filled with oil at the same time, and install the drive shaft and rain cover at the same time.
(15) Check the symmetry of the transmission mechanism.
(16) Trial operation
II. Installation, Inspection, and Adjustment of Tap Changers
(1) Inspect all components of the tap changer, including the switching switch or selector switch, tap selector, changeover selector, etc., for damage or deformation.
(2) Inspect all insulation components of the tap changer. There should be no cracks, creepage, or moisture.
(3) Check that all fasteners on the tap changer are properly tightened.
(4) Check that the contacts of the tap changer and their wiring are intact, have good contact, and are correctly and securely connected. If necessary, measure the contact resistance, contact pressure, and stroke of the contacts. Check that there are no broken strands in the copper braided wire.
(5) Check whether the transition resistor is broken or loose, and measure the value of the transition resistor. The resistance value should meet the requirements.
(6) Check the insulation distance of each part of the tap lead.
(7) The length of the tap lead should be appropriate so that the tap changer is not subjected to tension.
(8) Check that the valve between the disassembly switch and its oil tank is open.
(9) Tap changer sealing inspection. With the transformer body and its oil conservator filled with oil, drain the insulating oil from the tap changer oil chamber and check for any leakage in the oil chamber. Finally, conduct an overall sealing inspection, including accessories and all pipes, and ensure there is no oil leakage.
(10) Clean the tap changer oil chamber and core, inject standard insulating oil, and the oil level in the oil tank should be adapted to the ambient temperature.
(11) When evacuating the transformer, the tap changer oil chamber should be connected to the transformer body. When performing vacuum oil filling of the tap changer, both the transformer body and the tap changer oil chamber must be evacuated simultaneously. For tap changers with explosion-proof membranes, the explosion-proof membranes should be removed and replaced with sealing plates. If the tap changer oil conservator cannot withstand this vacuum value, the pipes leading to the oil conservator should be disconnected, and all valves and vent plugs affecting the vacuum should be closed. When performing atmospheric pressure oil filling of the tap changer, an vent should be provided to prevent the pressure relief device from being damaged.
