Methods for handling local faults in transformer cores

(1) Insulation aging between upper yoke laminations: When the insulation between the upper yoke laminations has aged but the rest of the core is normal, simply remove the upper yoke laminations to scrape off the old varnish film and reapply varnish. The other parts do not need to be touched.

(2) Core bolt insulation failure in older transformers: In older transformers, the insulation tubes of the core bolts may crack, causing partial short-circuiting of the laminations and resulting in inter-laminar short circuits in parts of the core. During repair, the upper yoke must be removed, the winding of the faulty phase lifted out, and the core bolts with cracked insulation tubes removed.

If there are not many short-circuited laminations and the damage is not deep, a specialized scraper can be used to remove the burrs from the adhesion, followed by repainting. Take this opportunity to remove all core bolts from the core limb, and then use non纬带 (non纬带 is a specific material, usually translated as non-woven glass fiber tape or filament banding tape in this context) binding to replace the core bolts for tightening purposes. However, care must be taken to ensure the filament banding is not too thick, to prevent any impact on the windings. Generally, the thickness of the filament banding for distribution transformers should be between 1-2 mm. If necessary, several more bands can be added to reduce the thickness of each individual band. If the short-circuited laminations are severely burnt and deeply damaged, having fused together, the core limb laminations must be disassembled. Use a flat chisel to separate the short-circuited areas, remove the burrs, clean and level the surface, strip away the residual varnish film, reapply varnish, and then dry it.

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

(4) Loose yoke clamps or other metal parts: Loose yoke clamps or other metal components can cause vibration, leading to core overheating, discoloration, and scorched varnish film. Upon inspection, if the fault is severe, remove the faulty laminations for repair, leaving the intact parts untouched. When reassembling the damaged laminations, the repaired laminations should be interleaved and mixed with the intact ones. After the repair, core loss and temperature rise tests must be conducted.