1. Dismantling and overhauling the transformer
(1) Handle work tickets and power outages, remove the external electrical connection leads and secondary wiring of the transformer, and conduct inspections and tests before maintenance.
(2) After part of the oil is drained, remove the casing, riser, oil conservator, cooler, gas relay, oil purifier, pressure relief valve (or safety air passage), joint pipe, thermometer and other ancillary devices, and carry out calibration and maintenance respectively. When the oil conservator is drained, check whether the indication of the oil level gauge is correct.
(3) Drain all the oil and dispose of it.
(4) Remove the off-excitation tap-changer operating lever; see DL/T 574-2010 “Transformer Tap-changer Operation and Maintenance Guidelines” for the removal methods of various on-load tap-changers; remove the bell hood (or body) after removing the middle waist flange or the connecting bolts of the large cover.
(5) Check the condition of the device body, tighten each part and test the insulation.
(6) Replace the sealing gasket, overhaul all valves, clean and overhaul the iron core, windings and fuel tank.
2. Transformer assembly
(1) Reinstall the bell (or body), and fill in oil as required after tightening the bolts.
(2) Install the casing after draining the oil in an appropriate amount, install the inner lead, and carry out the secondary oil injection.
(3) Install accessories such as coolers.
(4) Overall sealing test.
(5) Fill oil to the specified oil level line.
(6) After overhaul, conduct electrical and oil tests.
3. Precautions for transformer disassembly, maintenance and assembly
(1) The disassembled bolts and other parts should be cleaned, classified and properly kept. If they are damaged, they should be repaired or replaced.
(2) When disassembling, first disassemble the small instrument and casing, and then disassemble the large components. The assembly sequence is reversed.
(3) After the cooler, pressure relief valve (or safety air passage), oil purifier and oil conservator are removed, they should be sealed with a cover plate, and qualified transformer oil should be injected into the riser with current transformer (or take other moisture-proof sealing measures).
(4) Casing, oil level gauge, thermometer and other vulnerable parts should be properly stored after being removed to prevent damage and moisture. The capacitive casing should be placed vertically.
(5) Check the cooler, oil purifier and gas relay valves after assembly, and open or close them according to regulations.
(6) The upper vent holes such as the casing riser, the upper pipe hole cover, the cooler and the oil purifier should be exhausted several times.
Re-seal and wipe off any traces of oil until drained.
(7) When removing the non-excitation tap-changer operating lever, the position of the tap-changer should be recorded and marked; when the on-load tap-changer is removed, the tap should be placed in the middle position (or according to the manufacturer’s regulations).
(8) The components of the assembled transformer should be intact.
(9) Conscientiously do a good job of on-site record work.
