Cause of transformer short-circuit fault

There are many and complex reasons for transformer internal faults and incidents caused by transformer outlet short circuit, which are related to structural planning, raw material quality, process level, operating conditions and other factors, but the selection of electromagnetic wire is the key. From the dissection of the transformer in recent years, the analysis of its incident shows that there are roughly the following reasons related to the electromagnetic line.

1. The electromagnetic line selected based on the static theoretical planning of the transformer has a large difference in stress acting on the electromagnetic line during practical operation.

2, the current accounting procedures of the manufacturers are based on the uniform distribution of the leakage magnetic field, the same turn diameter, equal phase of the force and other idealized models, and in fact, the transformer leakage magnetic field is not uniform distribution, in the yoke part is relatively concentrated, the electromagnetic line in the area by the mechanical force is also large; Transposition wire at transposition because climbing will change the direction of force transmission, and produce torque; Because of the factor of the elastic modulus of the pad, the axial pad is not evenly dispersed, which will cause the alternating force generated by the alternating leakage magnetic field to delay resonance, which is also the fundamental reason why the wire cake in the core yoke, the transposition place, and the corresponding part of the pressure regulation tap is the primary deformation.

3. The influence of temperature on the bending and tensile strength of the electromagnetic wire is not considered when the short-circuit resistance can be calculated. The anti-short circuit ability planned at normal temperature can not reflect the practical operation condition. According to the test results, the temperature of the electromagnetic line is the limit of its submission. With the temperature improvement of the electromagnetic line, its bending strength, tensile strength and elongation are reduced, and the bending tensile strength at 250℃ is reduced by more than 10% compared with that at 50℃, and the elongation is reduced by more than 40%. The transformer in practice operation, under the additional load, the average winding temperature can reach 105℃, and the most hot temperature can reach 118℃. General transformer operation has a reclosing process, so if the short circuit point can not disappear for a while, it will accept the second short circuit impact in a very short period of time (0.8s), but because of the first short circuit current impact, the winding temperature increases sharply, according to the rules of GBl094, the maximum allowed 250℃. At this time, the anti-short circuit of the winding can be greatly reduced, which is why the short-circuit incident is mostly generated after the transformer reclosing.

4, the selection of general transposition wire, poor mechanical strength, in the acceptance of short circuit mechanical force prone to deformation, loose, copper exposure phenomenon. When the general transposition wire is selected, because the current is large and the transposition climb is steep, the part will produce a larger torque, and at the same time, the line cake at the two ends of the winding will also produce a larger torque, resulting in distortion and deformation because of the joint action of the amplitude and axial leakage magnetic field. For example, the A-phase common winding of Yanggao 500kV transformer has A total of 71 transpositions, because the thicker general transposition wire is selected, of which 66 transpositions have varying degrees of deformation. The other Wujing 1l main transformer is also because of the selection of general transposition wires, and the two ends of the high voltage winding in the core yoke part have different flipping and exposing phenomena.

5, the selection of flexible wires is also one of the main reasons for the formation of transformer short-circuit resistance. Because of the lack of knowledge in the early stage, or the difficulties in the winding equipment and process, the manufacturer is unwilling to use semi-hard wires or there is no requirement in this respect when planning, and the transformers that cause problems are soft wires.

6. The winding is loose, the transposition or correction climb is handled improperly, is too thin, and the electromagnetic line is suspended. From the point of view of the damage of the end, 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. There is no damage to the windings treated by dipping.

8. Improper control of the preload force of the winding forms the mutual dislocation of the wires of the general transposition wires.

9, the suit gap is too large, resulting in insufficient support on the electromagnetic line, which increases the potential for transformer short-circuit resistance.

10, the action in each winding or each file preload is not uniform, short circuit impact to form the pulse of the wire cake, resulting in excessive bending stress on the electromagnetic line and deformation.

11, the external short-circuit incident is frequent, the accumulation effect of electric power after repeated short-circuit current impact causes the electromagnetic line to soften or internal relative displacement, which eventually leads to insulation breakdown.