What are the requirements for inspecting and maintaining a transformer's no-load tap changer?

The tap changer operating handle shall be equipped with a protective cover. The flange at the handle shall be well sealed with no oil leakage. Locking screws shall securely fasten both the handle and the drive mechanism, and the handle rotation shall be smooth without binding. The position indicator on the handle shall be clear, accurate, and consistent with the tap voltage regulation range of the winding. Limit stops shall be provided at both extreme positions.

The insulating cylinder of the tap changer shall be intact and undamaged, with good insulation properties, and its support bracket shall be firmly secured. The allowable exposure time of the tap changer to air shall be the same as that of the core assembly. If the tap changer is disassembled during maintenance and cannot be reinstalled promptly, it shall be immersed in qualified transformer oil.

All tap changer insulation shall be in good condition, securely bound, neatly arranged, and all joints shall be well soldered with no signs of desoldering or overheating.

The surfaces of all fixed contact posts and moving contact rings shall be smooth, free of oil deposits, oxidation films, or burn marks. The silver-plated layer on the contact surfaces shall show no signs of peeling.

Rotate the tap changer through all tap positions to inspect the contact condition between each moving contact ring and moving contact post, as well as the spring condition. Check the contact pressure—when tested with a feeler gauge, it should not be possible to insert the gauge between the contact surfaces. The contact resistance between any two fixed contact posts shall not exceed 500 μΩ. After inspection, the tap changer shall be returned to its original operating position.

If the tap changer is removed during maintenance, clear markings and records must be made. After reinstallation, the voltage ratio shall be measured and verified.

As part of preventive maintenance, the tap changer shall be rotated annually: move it 10–15 times back and forth from its operating position to remove oil sludge, oxidation films, or other deposits from the contact surfaces through friction. Then return it to the operating position and measure the DC resistance, which must be acceptable (i.e., not higher than the previous measurement result).