Quality Standards for Core Maintenance of Power Transformers

Transformer Core Inspection and Assembly Requirements

• The iron core should be flat with intact insulation coating, tightly stacked laminations, and no curling or waviness at the edges of silicon steel sheets. All core surfaces must be free of oil, dirt, and impurities. There should be no short circuits or bridging between laminations, and joint gaps must meet specifications.

• Good insulation must be maintained between the core and upper/lower clamping plates, square iron pieces, pressure plates, and base plates.

• There must be a clear, uniform gap between steel pressure plates and the core. Insulated pressure plates must remain intact without damage or cracks, and maintain appropriate tightness.

• Steel pressure plates must not form closed loops and should be grounded at a single point.

• After disconnecting the connection strips between the upper clamping plate and core, and between the steel pressure plate and upper clamping plate, measure the insulation resistance between the core and clamping plates, and between the steel pressure plate and core. Results should show no significant changes compared to previous tests.

• All bolts must be securely tightened. Forward and reverse pressure pins and lock nuts on clamping plates must not be loose, should maintain good contact with insulating washers, and show no discharge burn marks. Reverse pressure pins must maintain adequate distance from the upper clamping plate.

• Core-through bolts must be tightly secured with insulation resistance values showing no significant changes compared to previous test results.

• Oil channels must remain unobstructed with oil duct spacer blocks properly secured, free from blockages, and neatly arranged.

• The core must be grounded at only one point using a purple copper strip 0.5mm thick and no less than 30mm wide, inserted between 3-4 layers of core laminations. For large transformers, insertion depth must be no less than 80mm. Exposed portions must be insulated to prevent core short circuits.

• Components must be securely fastened with sufficient mechanical strength, maintain good insulation without forming electrical loops, and must not contact the core.

• Insulation must be in good condition with reliable grounding.