Local faults and repair process of transformer core

(1) When the insulation between the laminations of the upper yoke ages while the rest of the core remains normal: Simply remove the laminations of the upper yoke, clean off the old varnish, and re-varnish them. The other parts do not need to be touched.

(2) In older transformers, a ruptured insulating tube on a core bolt can short-circuit local laminations, causing partial inter-lamination short circuits in the core: During repair, the upper yoke must be removed, the winding of the faulty phase lifted out, and the core bolt with the ruptured insulating tube dismantled.

If there are only a few shorted laminations and the damage is not deep, a special scraper can be used to remove the adhering burrs, followed by applying varnish. Take this opportunity to remove all core bolts from the core limb and replace them with non-woven (glass fiber) bands for binding and clamping purposes. However, care must be taken to ensure the non-woven binding bands are not too thick, to avoid interfering with the winding installation. Generally, the thickness of the non-woven binding bands for distribution transformers should be kept between 1 to 2 mm. If necessary, multiple bands can be added to reduce the thickness of each individual band.

If the shorted laminations are severely burned and deeply fused together, the core limb laminations should be disassembled. Use a flat chisel to separate the shorted areas, remove the burrs, flatten the laminations, strip away the residual varnish, re-varnish them, and then dry them properly.

(3) Short circuits between core laminations caused by grounding faults: During treatment, use a thin flat chisel to separate the adhered laminations on the core surface, remove the burrs, and apply varnish.

(4) Loose yoke clamps or other metallic parts causing vibration, which leads to core overheating, discoloration, and burnt varnish. Upon inspection, if the fault is severe, remove and repair the damaged laminations while leaving the intact parts untouched. When reassembling the damaged laminations, interleave the repaired laminations with the intact ones. After the repair, core loss and temperature rise tests must be conducted.