The core should be flat with intact insulation varnish film and tightly stacked laminations. The silicon steel sheets on the edges should not be curled up or wavy. The surface of all core parts should be free of oil sludge and impurities. There should be no short circuits or搭接 (lapping) between laminations, and the joint gaps must meet the required specifications.
Good insulation must be maintained between the core and the upper/lower clamps, square irons, pressure plates, and base plates.
There must be a clear and uniform gap between the steel pressure plate and the core. The insulating pressure plate should remain intact without any damage or cracks, and have appropriate tightness.
The steel pressure plate must not form a closed loop and should be grounded at a single point.
After disconnecting the connecting tabs between the upper clamp and the core, as well as between the steel pressure plate and the upper clamp, measure the insulation resistance between the core and the upper/lower clamps, and between the steel pressure plate and the core. There should be no significant change compared to previous test results.
Bolts should be securely tightened. The positive and negative pressure screws and lock nuts on the clamps must be free of looseness, have good contact with the insulating washers, and show no signs of discharge burns. There should be sufficient clearance between the negative pressure screws and the upper clamp.
Core bolts (through-bolts) should be tightened, and their insulation resistance should show no significant change compared to previous tests.
The oil passages must be unobstructed. Oil duct spacers should not be dislodged or blocked and must be arranged neatly.
The core is allowed to have only one grounding point. The grounding tab should be made of purple copper (pure copper) sheets with specified thickness and width, inserted between 3 to 4 levels of core laminations. For large transformers, the insertion depth should be no less than 80mm. The exposed part must be insulated to prevent short-circuiting the core.
It should be securely fastened with sufficient mechanical strength, well insulated, not forming a closed loop, and not in contact with the core.
