In recent years, overhauling and dissecting transformers and analyzing their accidents have revealed that there are several reasons related to electromagnetic wires.
1 The electromagnetic wire selected based on the static theory design of the transformer is different from the stress acting on the electromagnetic wire during actual operation.
2 At present, the calculation programs of various manufacturers are based on idealized models such as uniform distribution of leakage magnetic field, the same diameter of wire turns, and equal phase force. However, in fact, the leakage magnetic field of the transformer is not evenly distributed. The yoke part is relatively concentrated, and the electromagnetic force in this area is also affected by the mechanical force; the transposition conductor at the transposition position will change the force transmission direction due to climbing, and generate the torque; due to the coefficient of the elastic modulus of the spacer, the axial pad Unequally distributed blocks will cause the alternating force generated by the alternating leakage magnetic field to delay resonance, which is why the line cake at the yoke part of the iron core, the transposition part, and the corresponding part of the pressure tap change is first deformed. the reason.
(3) The effect of temperature on the bending and tensile strength of electromagnetic wire was not considered when calculating the short-circuit resistance. The short-circuit resistance designed at room temperature does not reflect the actual operation. According to the test results, the temperature of the magnet wire has a great influence on its yield limit 0.2. With the increase of the temperature of the magnet wire, its bending strength, tensile strength and elongation rate Both decreased, the bending strength at 250°C decreased by more than 10% at 50°C, and the elongation decreased by more than 40%. The actual operation of the transformer, under rated load, the average winding temperature up to 105 °C, the hottest point temperature up to 118 °C. Normally, the transformer is re-closed during operation, so if the short-circuit point does not disappear for a moment, it will withstand a second short-circuit shock in a very short time (0.8s), but due to the impact of the first short-circuit current, The temperature of the winding increases sharply. According to the provisions of GBl094, the maximum allowable temperature is 250°C. At this time, the short-circuit resistance of the winding has been greatly reduced. This is why most of the short-circuit accidents occur after the reclosing of the transformer.
4 using ordinary transposed conductors, poor mechanical strength, when subjected to short-circuit mechanical force prone to deformation, loose strands, exposed copper phenomenon. When using common transposed conductors, due to the large current, the steep slope of the transposition will produce a large torque at this part, and at the same time, the line cakes at the two ends of the windings will also produce a combination of axial and axial leakage magnetic fields. Larger torque causes distortion. For example, Yang Yang's 500 kV transformer has a total of 71 transpositions in the A-phase common winding. Due to the use of thicker transposed conductors, 66 transpositions have varying degrees of deformation. In addition, the No. 1l main change of Wusong is also due to the phenomenon that the high-voltage winding two-end line cake in the yoke part of the iron core has different flipping lines due to the use of ordinary transposing wires.
5 The use of soft wires is also one of the main reasons for the poor resistance of transformers to short circuits. Due to the lack of early understanding of this, or the difficulty of winding equipment and processes, manufacturers are reluctant to use semi-rigid wire or no design requirements in this regard, from the point of failure of the transformer is a soft wire.
6 winding winding loose, transposition or correcting improper handling of climbing, too thin, resulting in electromagnetic wire hanging. From the point of view of accident damage, deformation is more common at the transposition, especially at the transposition of the transposed conductor.
7 The winding wire or the wire is not cured between the wires, and the short-circuit resistance is poor. None of the early dip-lacquered windings were damaged.
The improper control of the 8 windings leads to misalignment of the conductors of common transposed conductors.
9 Kit clearance is too large, resulting in insufficient support on the electromagnetic wire, which increases the potential for the transformer to withstand short-circuit.
10 The role of the preload in the various windings or various files is not uniform, and the line cake is caused to jump when a short-circuit impact occurs, causing the bending stress acting on the electromagnetic wire to be too large and being deformed. Information Sources:
11 External short-circuit accidents are frequent. After the multiple short-circuit current surges, the accumulation of electromotive force causes the magnet wire to soften or internal relative displacement, eventually leading to insulation breakdown.
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