There are many reasons for transformer internal faults and events caused by short circuit at transformer outlet, which are related to structural planning, quality of raw materials, technological level, operating conditions and other factors, but the selection of electromagnetic wire is the key. From the analysis of the transformer in recent years, it can be seen that there are roughly the following reasons for the electromagnetic wire.

1. The electromagnetic wire selected based on the static theoretical planning of the transformer is quite different from the stress acting on the electromagnetic wire during practical operation.

2. At present, the accounting procedures of various manufacturers are compiled on the basis of idealized models such as uniform distribution of leakage magnetic field, same diameter of wire turns and equal phase force. In fact, the leakage magnetic field of transformer is not uniformly distributed, and the electromagnetic wire in this area is relatively concentrated in the iron yoke part, and the mechanical force received by the electromagnetic wire in this area is also relatively large. The transposed conductor will change the direction of force transmission at the transposed position because of climbing, and generate torque; Because of the factor of the elastic modulus of the cushion block, the axial cushion block is spread at unequal distances, which will delay the resonance of the alternating force generated by the alternating magnetic flux leakage field, which is also the root cause of the primary deformation of the wire cake at the corresponding part of the iron core yoke, the transposition and the voltage regulation tap. The effect of temperature on the bending and tensile strength of the magnet wire is not considered when calculating the short-circuit resistance. According to the normal temperature planning of the anti-short circuit can not reflect the practice of operating conditions, according to the test results, the temperature of the magnet wire on its yield limit? 0.2 has a great influence, with the temperature of the magnet wire progress, its bending, tensile strength and elongation decreased at 250°C bending tensile strength than at 50°C decreased by more than 10%, elongation decreased by more than 40%. The practical operation of the transformer, under additional load, the average temperature of the winding can reach 105 'C, the hottest spot temperature can reach 118°C. Generally, transformers have a reclosing process during operation, so if the short circuit point-time cannot disappear, it will receive the second short circuit impact in a very short time (0.8s). However, after being impacted by the first short circuit current, the winding temperature increases sharply. According to the rules of GBI094, the maximum tolerance is 250°C, and the short circuit resistance of the winding has been greatly reduced, this is why the majority of short-circuit incidents occur after the reclosing of the transformer.

4. The general transposition conductor is selected, which has poor mechanical strength and is prone to deformation, scattered strands and copper exposure when receiving short-circuit mechanical force. When choosing a general transposition wire, because the current is large and the transposition climbing is steep, this part will produce a larger torque, and at the same time, the wire cake at the two ends of the winding, because of the combined effect of the amplitude and axial leakage magnetic fields, it will also produce Larger torque, causing distortion. For example, the-phase common winding of Yanggao 500kV transformer has 71 transposition, because the thicker general transposition conductor is selected, 66 of which have different degrees of deformation. Other Wujing No.1 main transformer is also due to the selection of general transposed wires, and the phenomenon of different overturning and exposing wires in the two-end wire cakes of the high-voltage winding at the yoke part of the iron core.

5, the selection of soft wire, but also the formation of the transformer short-circuit to one of the main reasons for the poor. Because of the lack of knowledge about this in the early stage, or the difficulties in winding equipment and technology, the manufacturers are not willing to use semi-hard wires or there is no such requirement at all in planning. Judging from the transformers that produce defects, they are all soft wires.

6. The winding system is relatively loose, and the transposition or correction climbing place is improperly handled, which is too thin to form a suspended electromagnetic wire. From the point of view of the damage, the deformation is more common at the transposition, especially at the transposition of the transposition wire.

7. The winding turns or wires are not cured, and the short circuit resistance is poor. The winding treated with dipping paint in the early stage is free of damage.

8. Improper control of the preload of the winding causes the wires of the general transposition wires to be misplaced with each other.

9. The gap of the suit is too large, resulting in insufficient support on the electromagnetic wire, which increases the hidden danger for the transformer to resist short circuit.

10. The pre-tightening force acting on each winding or each gear is uneven, and the line cake is formed during short-circuit impact, resulting in excessive bending stress acting on the electromagnetic wire and deformation.

11, external short-circuit trouble frequently, repeated short-circuit current impact after the accumulation effect of electric power caused by the electromagnetic wire softening or internal relative displacement, eventually leading to insulation breakdown.