How to eliminate residual magnetism in power transformer cores

When residual magnetism in a power transformer core causes adverse effects, eliminating it becomes a necessary task. Residual magnetism is the result of hysteresis loss during the magnetization process of the transformer core. The magnetization process involves the orderly alignment of magnetic domains within the core material under an applied magnetomotive force (MMF), while hysteresis loss represents a form of energy loss.

Therefore, countermeasures to eliminate residual magnetism can be adopted based on its formation mechanism: applying a reverse DC MMF for demagnetization is theoretically feasible but difficult to control; increasing the ambient temperature of the core to accelerate the thermal motion of the core material's molecules, causing the orderly aligned magnetic poles to become disordered again, also presents control challenges; applying an AC MMF to eliminate residual magnetism is currently the preferred method. The AC MMF can be applied in the following forms:

(1) Direct no-load closing. If the impact of residual magnetism only triggers the heavy gas (Buchholz) protection, consideration should be given to switching the heavy gas protection to the signaling mode and performing a direct no-load closing. After a successful closing, the residual magnetism will be eliminated. If the overcurrent protection trips again, it is recommended not to attempt no-load closing again for demagnetization.

(2) Demagnetization by zero-voltage startup of the generator-transformer unit.

(3) External AC demagnetization at a reduced voltage level. This involves charging the transformer at no-load using an AC power source with a voltage lower than the transformer's rated voltage. This method is suitable for general substation transformers.