After the transformer windings carry currents, a leakage magnetic field (axial and radial) will be established in the μ0 medium in their space and in the space they envelop; the windings in this field will in turn be subjected to force , This force is called "Lorentz force" or electric force. Electromechanical stress in the transformer winding material, and some spread to other components of the transformer. In the rated current, electrodynamic force is not large; but short-circuit, electric power will surge, can make the transformer failure or even damage.
Transformer windings are made of copper segments separated by winding pads. The dynamic characteristics of such systems vary during the short circuit because the resilience of the insulation pad is related to its degree of compaction, ie to the force. Electric power itself is not constant, but according to the complex law changes. The winding has several natural oscillation frequencies. If a frequency of the electromotive force coincides with the natural oscillation frequency, resonance will occur, increasing the pressure beyond the static pressure. The instability in the winding is the superposition of two processes, electromagnetic and mechanical.
Long-term practical experience and short-circuit strength test shows that the transformer in a sudden short circuit, the winding damage is the result of radial force and axial force short circuit. In dual-winding transformers, the axial force along the windings places the windings under pressure or tension. The direction of the pulling force is toward the iron yoke, which is insulated from the ends of the windings by the iron yoke to the iron core clamping device.
When the pulling force is greater than the mechanical strength of the structural member, the winding, the pressure plate and the clamping member and other components may be deformed. In severe cases, the upper iron yoke can be jacked up to destroy the entire core structure and pull the entire winding. The radial force along the radial direction of the winding causes the inner winding to be stressed and the outer winding to be pulled. When the tensile force is greater than the tensile stress of the wire, the winding is deformed and the insulation is broken. The entire main and longitudinal insulation structures are damaged, and even the wire is severed.
In addition, due to the short-circuit current flowing through the winding than the rated current of several times, the load loss will be several times larger than the rated operation, and the rapid rise of the winding temperature, so if the failure can not be eliminated in the shortest possible time, the transformer there may be burned.
Although the study of electrodynamic forces acting on the transformer windings for a long time has been quite a long time, the whole process is very complicated. So far, the windings are usually regarded as fixed when calculating the mechanical strength of the transformers. The electrodynamic force acting on the winding is considered as a constant corresponding to the maximum current flowing in the winding. In other words, dynamic problems are replaced by static ones.
