There are many and complex reasons for the internal faults and troubles of the transformer caused by the short circuit at the outlet of the transformer. It is related to the structural planning, the quality of raw materials, process level, operating conditions and other factors, but the selection of electromagnetic wire is the key. According to the analysis of transformer accidents in recent years, there are roughly the following reasons related to electromagnetic wires.
1. The electromagnetic line selected based on the static theoretical planning of transformer is quite different from the stress acting on the electromagnetic line during practical operation.
2. At present, the calculation procedures of various manufacturers are based on the idealized models of uniform distribution of leakage magnetic field, the same wire turn diameter and equal phase force. In fact, the leakage magnetic field of transformer is not evenly distributed, which is relatively concentrated in the yoke part, and the electromagnetic wires in this area are also subjected to large mechanical force; At the transposition point, the climbing of transposition conductor will change the transmission direction of force and produce torque; Because of the elastic modulus factor of the cushion block and the unequal dispersion of the axial cushion block, the alternating force generated by the alternating leakage magnetic field will delay resonance, which is also the fundamental reason for the primary deformation of the wire cake at the iron core yoke, transposition and the corresponding parts with voltage regulating tapping.
3. The influence of temperature on the bending and tensile strength of electromagnetic wire is not considered in the calculation of short-circuit resistance. The short-circuit resistance planned under normal temperature can not reflect the actual operation. According to the test results, the temperature of the electromagnetic wire has no effect on its compliance limit? 0.2 has a great impact. With the improvement of the temperature of the electromagnetic wire, its bending strength, tensile strength and elongation decrease. The bending tensile strength at 250 ℃ decreases by more than 10% and the elongation decreases by more than 40%. For the transformer in practical operation, under additional load, the average winding temperature can reach 105 ℃ and the hottest spot temperature can reach 118 ℃. Generally, the transformer has reclosing process during operation. Therefore, if the short-circuit point cannot disappear for a while, it will immediately accept the second short-circuit impact in a very short time (0.8s). However, because the winding temperature increases sharply after the impact of the first short-circuit current, the maximum allowable temperature is 250 ℃ according to the rules of gbl094. At this time, the anti short-circuit ability of the winding has decreased greatly, This is why most of the short-circuit accidents occur after transformer reclosing.
4. The general transposition conductor is selected, which has poor mechanical strength and is prone to deformation, loose strand and copper exposure when receiving short-circuit mechanical force. When selecting the general transposition conductor, due to the large current and steep transposition climbing, this part will produce large torque. At the same time, the wire cake at the two ends of the winding will also produce large torque due to the combined action of amplitude and axial leakage magnetic field, resulting in distortion and deformation. For example, there are 71 transpositions of phase a common winding of Yanggao 500kV transformer, because thicker general transposition conductors are selected, of which 66 transpositions have varying degrees of deformation. In addition, Wujing 1L main transformer is also due to the selection of general transposition conductor, and the wire cakes at the two ends of high-voltage winding at the iron core yoke have different overturning and wire exposure.
5. The selection of flexible conductor is also one of the main reasons for the poor short-circuit resistance of transformer. Because of the lack of knowledge in the early stage, or the difficulties in winding equipment and technology, the manufacturers are unwilling to use semi-hard conductors, or there are no requirements in this regard in planning. From the perspective of faulty transformers, they are all soft conductors.
6. The winding is loosely wound, the transposition or correction climbing position is not handled properly, it is too thin, and the electromagnetic wire is suspended. From the damage direction of the incident, the deformation is mostly seen at the transposition, especially at the transposition of the transposition conductor.
7. The winding turns or wires are not cured, and the short-circuit resistance is poor. None of the windings treated by dipping paint in the early stage is damaged.
8. Improper control of the pre tightening force of the winding results in the dislocation of the wires of the general transposition wires.
9. The clearance of the suit is too large, resulting in insufficient support on the electromagnetic line, which increases the hidden danger to the anti short circuit ability of the transformer.
10. The preload acting on each winding or gear is uneven, and the runout of wire cake is formed during short-circuit impact, resulting in excessive bending stress acting on the electromagnetic line and deformation.
11. External short-circuit events occur frequently. The accumulation effect of electrodynamic force after repeated short-circuit current impact causes electromagnetic wire softening or internal relative displacement, which eventually leads to insulation breakdown.