Causes of short-circuit faults in the transformer

There are many and complicated reasons for Transformer internal faults and incidents caused by transformer outlet short circuit, which are related to structural planning, quality of raw materials, process level, operating conditions and other factors, but the selection of electromagnetic wire is the key. From the analysis of the transformer in recent years, we can see that there are several reasons related to the electromagnetic wire.

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

2. In fact, the magnetic field is distributed uniformly on the winding line and the leakage line of the transformer, which is based on the magnetic field distribution of the transformer Because of the elastic modulus factor of the cushion block, the axial cushion block is not equidistant, which will delay the resonance of the alternating force generated by the alternating magnetic leakage field, which is also the fundamental reason why the first deformation of the wire cake is located at the core yoke, transposition and corresponding parts with voltage regulating taps.

3. The influence of temperature on the bending strength and tensile strength of the electromagnetic wire is not considered in the calculation of short circuit resistance. According to the test results, the temperature of the electromagnetic wire has a great influence on its bending limit? 0.2. With the progress of the temperature of the electromagnetic wire, its bending strength, tensile strength and elongation decrease. At 250 ℃, the bending tensile strength decreases by more than 10%, and the elongation decreases by more than 40%. Under the extra load, the average winding temperature can reach 105 ℃ and the hottest spot temperature can reach 118 ℃. Generally, the transformer has reclosing process during operation, so if the short-circuit point cannot disappear for a while, it will receive the second short-circuit impact in a very short time (0.8s). However, the winding temperature increases sharply after the first short-circuit current impact. According to gbl094, the maximum allowable temperature is 250 ℃. At this time, the short-circuit resistance of the winding can be greatly reduced What kind of transformer recloser will cause short circuit.

4. The general transposition conductor has poor mechanical strength and is prone to deformation, loose strands and exposed copper when receiving short-circuit mechanical force. When selecting the general transposition conductor, because of the large current and the steep transposition climbing, the position will produce large torque. At the same time, due to the joint action of the amplitude direction and axial magnetic leakage field, the coil cake at the two ends of the winding will also produce large torque, resulting in distortion and deformation. For example, there are 71 transposition in A-phase common winding of Yanggao 500kV transformer, because thick general transposition conductor is selected, 66 transposition have different degrees of deformation. Other Wujing No.1l main transformer is also due to the selection of general transposition conductor. The two end winding cakes of high voltage winding at the iron core yoke have different overturning and exposing phenomenon.

5. The selection of soft conductor is also one of the main reasons for poor short circuit resistance of transformer. Because of the lack of early knowledge of this, or the difficulties in winding equipment and technology, the manufacturers refused to use semi-rigid conductors or there was no requirement in planning. From the point of view of the transformers with faults, they were all soft conductors.

6. Winding winding is loose, transposition or correction is not handled properly, too thin, resulting in electromagnetic wire hanging. From the point of view of the damage direction, the deformation often occurs in transposition, especially the transposition of transposition conductor.

7. If the winding turns or wires are not cured, the short circuit resistance is poor. There is no damage to the winding which has been treated by dipping paint in the early stage.

8. Improper control of the pre tightening force of the winding results in the mutual dislocation of the wires of the general transposition conductor.

9. In order to prevent the transformer from short circuiting, the effect of anti short circuit is not enough.

10. The pre tightening force acting on each winding or gear is not uniform, which results in the jumping of the coil during short-circuit impact, resulting in the excessive bending stress on the electromagnetic wire and deformation.

11. The external short-circuit incidents are frequent, and the accumulation effect of electrodynamic force after repeated short-circuit current shocks causes electromagnetic wire softening or internal relative displacement, which eventually leads to insulation breakdown.