Transformer strut accidents occur from time to time and have a growing trend. From the analysis of transformer pole accidents, insufficient short-circuit resistance has become the primary cause of power transformer accidents, causing great harm to the power grid and seriously affecting the safe operation of the power grid.
Transformer braces often experience the following accidents: multiple external short circuit impacts, gradually severe coil deformation, and ultimately insulation breakdown damage; External damage caused by frequent short circuit impacts in a short period of time; Damage caused by prolonged short circuit impact; One short circuit impact can cause damage. The main forms of transformer short-circuit damage are as follows:
1. Axial instability. This damage is mainly caused by the axial electromagnetic force generated by radial leakage, resulting in axial deformation of the transformer winding.
2. The wire cake is bent and deformed up and down. This damage is caused by the long-term deformation of the wire between two axial pads under the action of axial electromagnetic force due to excessive bending moment, and the deformation between the two pads is usually symmetrical.
3. Collapse of winding or wire cake. This type of damage is caused by the wires being squeezed or collided with each other under axial force, resulting in tilting deformation. If the wire is initially slightly inclined, the axial force promotes an increase in inclination, and in severe cases, it collapses; The larger the ratio of height to width of a wire, the more likely it is to cause collapse. In addition to the axial component, there is also a radial component in the end leakage magnetic field. The combined electromagnetic force generated by the leakage magnetic field in both directions causes the inner winding wire to flip inward and the outer winding to flip outward.
4. The winding is raised to support the pressure plate. This type of damage is often caused by excessive axial force or insufficient strength and stiffness of its end support components, or assembly defects.
5. Radial instability. This damage is mainly caused by the radial electromagnetic force generated by axial magnetic leakage, resulting in radial deformation of the transformer winding.
6. The elongation of the outer winding leads to insulation damage. The radial electromagnetic force attempts to increase the diameter of the outer winding, and excessive tensile stress on the wire can cause long-term deformation. This deformation is usually accompanied by wire insulation damage, resulting in inter turn short circuits. In severe cases, it can cause the coil to be embedded, disordered, and collapse, or even break.
7. The end of the winding is overturned and deformed. In addition to the axial component, there is also a radial component in the end leakage magnetic field. The combined electromagnetic force generated by the leakage magnetic field in both directions causes the winding wires to flip inward and the outer winding to flip outward.
8. The inner winding wires are bent or warped. The radial electromagnetic force reduces the diameter of the inner winding, and bending is the result of long-term deformation caused by excessive bending moment of the wire between two supports (inner braces). If the iron core is tightly bound and the winding radial brace is effectively supported, and the radial electric force is evenly distributed along the circumference, this deformation is symmetrical, and the entire winding is a polygonal star. However, due to the compression deformation of the iron core, the supporting conditions of the support bars are different, and the force along the circumference of the winding is uneven. In fact, local instability and warping deformation often occur.
9. The lead wire is fixed and unstable. This type of damage is mainly caused by the electromagnetic force between the leads, which causes lead vibrations and leads to short circuits between the leads.
Analysis of the reasons for the short circuit fault of transformer support bars:
There are many and complex reasons for internal faults and accidents caused by short circuits at the outlet of transformers, which are related to factors such as structural design, quality of raw materials, process level, and operating conditions. However, the selection of electromagnetic wires is crucial. In recent years, there has been a significant difference in the stress applied to the electromagnetic wire used for transformer static theory design compared to the actual operation.
(1) At present, the calculation programs of various manufacturers are based on idealized models such as uniform distribution of leakage magnetic field, equal diameter of wire turns, and equal phase force. However, in fact, the leakage magnetic field of transformers is not uniformly distributed and is relatively concentrated in the iron yoke, and the electromagnetic wires in this area are also subjected to significant mechanical forces; The transposed wire generates torque at the transposed position due to climbing changing the direction of force transmission; Due to the factor of the elastic modulus of the cushion blocks, the uneven distribution of the axial cushion blocks can cause delayed resonance of the alternating force generated by the alternating leakage magnetic field. This is also the fundamental reason why the wire cakes at the corresponding parts of the iron core yoke, transposition, and voltage regulating tap are deformed first.
(2) The calculation of short-circuit resistance did not consider the effect of temperature on the bending and tensile strength of the electromagnetic wire. The short-circuit resistance designed at room temperature cannot reflect the actual operating situation. According to the test results, the temperature of the electromagnetic wire has an impact on its yield limit? 0.2 has a significant impact. As the temperature of the electromagnetic wire increases, its bending strength, tensile strength, and elongation all decrease. At 250 ℃, the bending tensile strength decreases by more than 40% compared to at 50 ℃, while the elongation decreases by more than 40%. The actual operating transformer, under rated load, has an average winding temperature of 105 ℃ and the hottest point temperature of 118 ℃. Generally, transformers have a reclosing process during operation. Therefore, if the short-circuit point cannot disappear for a short time, it will immediately withstand the second short-circuit impulse in a very short period of time (0.8s). However, due to the sharp increase in winding temperature after the previous short-circuit current impulse, according to GBl094, 250 ℃ is allowed. At this time, the short-circuit resistance of the winding has significantly decreased, This is why short-circuit accidents occur more frequently after transformer reclosure.
(3) The use of ordinary transposed wires has poor mechanical strength and is prone to deformation, loose strands, and exposed copper when subjected to short-circuit mechanical forces. When using ordinary transposed wires, due to the high current and steep transposition climb, a large torque will be generated in this area. At the same time, the wire cake at the two ends of the winding will also generate a large torque due to the combined effect of amplitude and axial leakage magnetic field, resulting in distortion and deformation. For example, the A-phase common winding of Yanggao 500kV transformer has a total of 71 transpositions. Due to the use of thicker ordinary transposition wires, 66 transpositions have varying degrees of deformation. In addition, due to the use of ordinary transposed wires, the high-voltage winding wire cakes at the yoke of the iron core of the Wujing 1l main transformer have different overturning and exposed phenomena.
(4) The use of soft wires is also one of the main reasons for the poor short-circuit resistance of transformers. Due to insufficient understanding of this in the early stages, or difficulties in winding equipment and technology, manufacturers were unwilling to use semi hard wires or had no requirements for this aspect in the design. From the perspective of the faulty transformers, they were all soft wires.
(5) The winding is loosely wound, improperly transposed, and too thin, causing the electromagnetic wire to hang in the air. From the location of the accident damage, deformation is more common at the transposition point, especially at the transposition point of the transposed wire.
(6) The winding turns or wires are not cured and have poor short-circuit resistance. There was no damage to the windings that were treated with impregnation paint in the early stages.
(7) Improper control of the pre tightening force of the winding causes the wires of ordinary transposed wires to be misaligned with each other.
(8) The gap between the sets is too large, resulting in insufficient support on the electromagnetic line, which increases the hidden danger of the transformer's short-circuit resistance.
(9) The uneven pre tightening force acting on each winding or gear causes the wire cake to jump during short circuit impact, resulting in excessive bending stress acting on the electromagnetic wire and deformation.
(10) External short circuit accidents are frequent, and the accumulation effect of electric force after multiple short circuit current surges causes electromagnetic wire softening or internal relative displacement, ultimately leading to insulation breakdown.
Common parts of transformer support bar short circuit damage
Corresponding to the part under the iron yoke. The reasons for the deformation of this part include: (1) the magnetic field generated by the short-circuit current is closed through the oil and the box wall or iron core. Due to the relatively small magnetic resistance of the iron yoke, it is mostly closed between the oil circuit and the iron yoke, and the magnetic field is relatively concentrated. The electromagnetic force acting on the wire cake is also relatively large; (2) The gap between the inner winding sets is too large or the iron core binding is not tight enough, resulting in the shrinkage and deformation of the two sides of the iron core plate, causing the winding on the yoke side to warp and deform; (3) In terms of structure, the axial compression of the corresponding winding part of the yoke is the least reliable