How to Overhaul a Transformer Core

The transformer core serves as the mechanical backbone of the entire transformer and is a crucial component that provides the magnetic circuit for the transformer. Therefore, inspecting the transformer core is a key step in ensuring the safe operation of the transformer. The inspection of the core involves the following 8 steps:

1. Inspect the external appearance of the core for smoothness, checking for any short circuits or discoloration, discharge burn marks, and ensuring that the insulation paint film is intact. Clean the top of the upper yoke and the bottom of the lower yoke from any oil or debris using a clean white cloth or foam plastic. If there are raised or irregular areas in the laminations, use a wooden or copper mallet to flatten them.

The core should be smooth, with intact insulation paint, tightly packed laminations, and the silicon steel sheets on the edges should not be raised or form a wavy pattern. The surfaces of the core should be free of oil and impurities, with no short circuits or overlap between laminations, and the gap between joints should meet the required standards.

2. Check the tightness and insulation condition of the upper and lower clamps, square iron, and winding pressure plates. Inspect the insulation plates for any electrical burns or discharge marks.

To facilitate monitoring the insulation condition of the core during operation, a small sleeve can be added to the transformer cover during major maintenance, connecting the core grounding wire (sheet) to the ground.

l Ensure good insulation between the core and upper/lower clamps, square iron, plates, and base plate.

l There should be a noticeable uniform gap between the steel pressure plate and the core. The insulation pressure plate should be intact, without damage or cracks, and securely fastened.

l The steel pressure plate should not form a closed circuit and should have one grounding point.

l After opening the connecting pieces between the upper clamp and the core and between the steel pressure plate and the upper clamp, measure the insulation resistance between the core and upper/lower clamps and between the steel pressure plate and the core. Compare the results with previous tests; there should be no significant changes.

3. Check the contact situation of pressure nails and insulation washers. Use a specialized wrench to tighten the bolts of the upper and lower clamps, square iron, pressure nails, etc. Ensure that the bolts are securely fastened, and there is no loosening of the positive and negative pressure nails and locking nuts on the clamp. The contact with insulation washers should be good, without any discharge burn marks, and the reverse pressure nails should have sufficient distance from the upper clamp.

4. Use a specialized wrench to tighten the through-bolts of the upper and lower core one by one. During the tightening process, intermittently check and measure the insulation of the through-bolts. Ensure that the tightness of the through-bolts meets the requirements to prevent loosening. After tightening, perform insulation resistance testing and compare the results with previous tests. If there is no significant change in insulation resistance, it indicates that the insulation performance of the through-bolts is good and undamaged. If any abnormalities are found, further inspection and appropriate repair measures are necessary.

5. Inspect the oil paths between the core and the core-to-clamp. The oil paths should be unobstructed, with oil duct pads intact and arranged neatly.

6. Check the connection and insulation of the core grounding pieces. The core should only have one grounding point, using a copper piece with a thickness of 0.5mm and a width not less than 30mm, inserted between 3-4 levels of the core. For large transformers, the insertion depth should be no less than 80mm, and the exposed part should be insulated to prevent core short circuits.

7. Inspect the pull plates and steel straps of the core with a non-hole structure. They should be securely fastened with sufficient mechanical strength, have good insulation, not form a closed circuit, and not come into contact with the core.

8. Check the insulation and grounding of the core electric field shielding. Ensure that the core electric field shielding is intact, without any signs of damage, rupture, aging, or detachment. Confirm that the grounding connection between the core and clamps is secure, with no looseness or corrosion.

By following these 8 steps, you can effectively inspect and maintain the transformer core to ensure its proper functioning and safety.

LuShan, est. 1975, is a Chinese professional manufacturer specializing in power transformers and reactors for 49 years. Leading products are single-phase transformer, three-phase transformers, DC inductors, AC reactors, filtering reactor, expoxy resin high-voltage transformer and intermediate, high-frequency products. Our transformers and reactors are widely used in 10 application areas: rapid transit, construction machinery, renewable energy, intelligent manufacturing, medical equipment, coal mine explosion prevention , excitation system, vacuum sintering, central air conditioning.

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