Causes of Transformer Winding Deformation

When the short circuit fault current strikes the transformer in operation, the huge short circuit current flows through some winding of the transformer. Under the action of the coil current and the leakage magnetic field, the coil will be subjected to the combined action of strong radial force and axial force.

Radial Force

It is mainly caused by the axial leakage flux between inner and outer coils, which is shown in Figure 1. For double-coil transformer, the outer coil is stretched by radial force, thus compressing inner coil. If some kind of disturbance is suddenly added to the inner ring, the pressure on the line ring will produce minimal deviation, which will cause partial bending of coil wires. Inevitably, the bending moment will be further increased, eventually showing a "avalanche". The coil developed from round shape to irregular plum flower or drum shape, completely losing radial mechanical strength.

Axial Force

The axial force is mainly caused by end winding leakage flux and magnetic flux of unbalanced ampere turns, which is shown in Figure 2. If axial ampere turns are totally balanced and symmetrical, the directions of axial force point to inner side and the axial force has no external effect.

(2) If the height of winding is different, and the upper and lower positions are asymmetrical, the height difference will increase. Under the influence of axial force, it will deform and further increase the asymmetry, which is shown in Figure 3. This is a cumulative effect. When a short circuit current flows the next time, it will produce more axial electric power so that the destruction of mechanical structure accelerates.

(3) As shown in Figure 4, the winding axial ampere turns are distributed in an unbalanced manner. The regulating winding occupies the axial space and the transverse leakage flux will produce. The axial stress will be generated at the end of the high voltage winding while the axial force of the low voltage winding will keep the internal balance and the external stress will not occur.

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