In recent years, transformer accidents have occurred frequently and are on the rise. Analysis of these accidents reveals that insufficient short-circuit withstand capability has become the primary cause of power transformer accidents, posing a significant threat to the power grid and seriously affecting its safe operation. This article provides a categorized analysis of transformer damage accidents caused by external short circuits in the Shanghai Power Company over the past decade. It then proposes current problems in the selection of electromagnetic wires and measures to reduce such accidents, aiming to promote product improvement by manufacturers and to encourage operating units to further enhance their operational management. From January 1993 to December 2002, 17 transformers in the Shanghai power grid experienced short-circuit damage, accounting for 77.3% of all damage accidents, making it the primary cause of damage, with a total capacity of 2750 MVA. Among them, there were 2 instances of 500kV transformer damage, 13 instances of 220kV transformer damage, and 2 instances of 110kV transformer damage. One 220kV transformer and one 110kV transformer had their low-voltage windings severely deformed and required replacement. During transformer upgrades, deformation of the low-voltage windings was found in 4 220kV transformers, and during operation, signs of deformation were found in the 500kV windings of 2 transformers. Especially since 1995, transformer damage accidents have shown an upward trend, and the scope of their impact has been continuously expanding. The main forms of these accidents are: 1) Multiple external short-circuit impacts, leading to progressively severe coil deformation and ultimately insulation breakdown; 2) Frequent short-circuit impacts within a short period; 3) Damage caused by prolonged short-circuit impacts; 4) Damage caused by a single short-circuit impact. Based on recent years' transformer damage caused by outlet short circuits, the main forms of transformer short-circuit damage exhibit the following characteristics and causes. Axial instability damage primarily occurs due to the axial electromagnetic force generated by radial leakage magnetic flux, leading to axial deformation of the transformer windings. This type of accident accounts for 52.9% of all damage incidents. Bending deformation of the coils occurs because the conductors between two axial spacers undergo permanent deformation due to excessive bending moment under axial electromagnetic force; the deformation between the two coils is usually symmetrical. Winding or coil collapse occurs because the conductors are squeezed or impacted by each other under axial force, causing tilting deformation. If the conductors were originally slightly tilted, the axial force will increase the tilt, eventually leading to collapse; a larger height-to-width ratio of the conductors makes collapse more likely. In addition to the axial component, the end leakage magnetic field also has a radial component. The combined electromagnetic force generated by the leakage magnetic flux in both directions causes the inner winding conductors to flip inward and the outer winding to flip outward. Winding lifting that pushes open the pressure plate is often caused by excessive axial force or insufficient strength or rigidity of the end supports, or assembly defects. Radial instability, primarily caused by radial electromagnetic forces generated by axial leakage flux, leads to radial deformation of the transformer windings, accounting for 41.2% of all damage incidents. Elongation of the outer winding conductors causes insulation damage. The radial electromagnetic force attempts to increase the diameter of the outer winding; however, excessive tensile stress on the conductors can cause permanent deformation. This deformation is often accompanied by insulation damage, resulting in inter-turn short circuits. In severe cases, it can cause coil embedding, tangled coils, collapse, or even breakage. End-winding overturning deformation occurs because the end leakage magnetic field has both axial and radial components. The combined electromagnetic force from these two directions causes the winding conductors to overturn inwards, and the outer winding to overturn outwards. Bending or warping of the inner winding conductors causes the inner winding diameter to decrease due to radial electromagnetic forces. Bending is a result of excessive bending moments between the two supports (inner braces), leading to permanent deformation. If the core is sufficiently tightly bound and the radial braces provide effective support, and the radial electromagnetic force is evenly distributed along the circumference, this deformation is symmetrical, resulting in a polygonal star shape for the entire winding. However, due to the deformation of the iron core under pressure and the varying support conditions of the struts, the force along the winding circumference is uneven, often resulting in local instability and warping deformation. Lead wire instability, primarily caused by electromagnetic forces between leads, leads to vibration and short circuits; this type of accident is relatively rare. Based on recent years' transformer damage caused by outlet short circuits, the common locations of winding damage during a short circuit fault in a transformer are as follows. The reasons for deformation in the part corresponding to the iron yoke are : (1) The magnetic field generated by the short circuit current is closed through the oil and the box wall or iron core. Since the magnetic resistance of the iron yoke is relatively small, it is mostly closed through the oil circuit and the iron yoke. The magnetic field is relatively concentrated and the electromagnetic force acting on the coil is relatively large; (2) The gap of the inner winding is too large or the iron core is not tightly bound, which causes the iron core sheets to shrink and deform on both sides, resulting in the bending and deformation of the winding on the iron yoke side; (3) In terms of structure, the axial compression of the winding part corresponding to the yoke is the least reliable. The coil in this part is often difficult to achieve the required pre-tightening force, so the coil in this part is most prone to deformation. The voltage adjustment tap area and the part corresponding to other windings in this area are due to: (1) The ampere-turn imbalance causes the leakage magnetic field distribution to be uneven. The leakage magnetic field generated in the radial direction generates an additional axial external force in the coil. The direction of these forces always increases the asymmetry of generating these forces. The axial external force and the axial internal force generated by the normal radial leakage flux cause the coil to bend vertically and compress the pad of the coil. In addition, these forces are also partially or completely transmitted to the iron yoke, trying to make it leave the core column, resulting in the coil deforming or flipping towards the middle of the winding; (2) In order to achieve ampere-turn balance or the required insulation distance in the tap section, the coil in this part often needs to add more pads. The thicker pads cause the force transmission to be delayed, and thus the impact on the coil is also greater; (3) After the winding is assembled, the center reactance height cannot be ensured, which further aggravates the ampere-turn imbalance; (4) After running for a period of time, the thicker pads naturally shrink more, which aggravates the ampere-turn imbalance on the one hand, and aggravates the jumping when subjected to short circuit force on the other hand; (5) In order to achieve ampere-turn balance in the design time, the electromagnetic wire in the tap section is selected with a narrower or smaller cross section, which has low resistance to short force. The deformation in this part is common in the transposition of transposed conductors and the standard transposition of single helix. The transposition of a transposed conductor involves a steeper ramp than that of a regular conductor, resulting in opposing tangential forces at the transposition points where the coil radii differ. These equal but opposite tangential forces cause the inner winding to deform towards a smaller diameter, while the outer winding, by maintaining a uniform coil radius, straightens the transposed section. The inner transposition deforms towards the center, while the outer transposition deforms outward. Furthermore, the thicker the transposed conductor, the steeper the ramp and the more severe the deformation. Additionally, the presence of an axial short-circuit current component at the transposition point further exacerbates the coil deformation. A standard transposition of a single helix occupies one turn, creating an imbalance of ampere-turns at that location. Combined with the deformation characteristics of a transposed conductor, this makes the coil more prone to deformation at that point. Lead wires are commonly found in windings with a beveled helix structure. In this structure, the imbalance of ampere-turns between the two helical ends results in a large axial force, and the presence of axial current causes a lateral force at the corner of the lead wire, leading to twisting deformation. Furthermore, residual stress exists in helical windings during the winding process, causing the windings to strive to return to their original shape. Therefore, helical windings are more prone to twisting and deformation under the impact of short-circuit current. Short circuits are commonly seen between low-voltage leads. Due to the low voltage and high current flow in low-voltage leads, and the 120-degree phase difference, the leads attract each other. If the leads are not properly fixed, a phase-to-phase short circuit can occur. Analysis of Transformer Short Circuit Fault Causes: The causes of internal transformer faults and accidents due to short circuits at the transformer outlet are numerous and complex. They are related to factors such as structural design, raw material quality, manufacturing process, and operating conditions, but the selection of the electromagnetic wire is crucial. Analysis of transformer accidents dissected in recent years reveals the following main reasons related to the electromagnetic wire: The electromagnetic wire selected based on the transformer's static theoretical design differs significantly from the stress acting on the electromagnetic wire during actual operation. Currently, the calculation programs of various manufacturers are based on idealized models such as uniform distribution of leakage magnetic field, identical coil diameter, and equal-phase forces. However, in reality, the leakage magnetic field of a transformer is not uniformly distributed; it is relatively concentrated in the yoke section, where the electromagnetic wire experiences greater mechanical force. At the transposition point, the direction of force transmission changes due to the incline, generating torque. Due to the modulus of elasticity of the pads, the unequal distribution of axial pads causes delayed resonance of the alternating force generated by the alternating leakage magnetic field. This is the fundamental reason why the coils at the yoke section, transposition point, and corresponding locations with voltage regulating taps deform first. The short-circuit withstand capability calculation does not consider the effect of temperature on the bending and tensile strength of the electromagnetic wire. The short-circuit withstand capability designed for room temperature cannot reflect actual operating conditions. Based on test results, what is the yield limit of the electromagnetic wire due to temperature? The impact of 0.2 is significant. As the temperature of the electromagnetic wire increases, its bending and tensile strength, as well as its elongation, all decrease. At 250℃, the bending and tensile strength decreases by more than 10% compared to 50℃, and the elongation decreases by more than 40%. In actual operation, the average winding temperature of a transformer under rated load can reach 105℃, and the hottest spot temperature can reach 118℃. Transformers generally undergo a reclosing process during operation. Therefore, if the short circuit point cannot be eliminated immediately, it will be subjected to a second short circuit impact in a very short time (0.8s). However, due to the rapid increase in winding temperature after the first short circuit current impact (according to GB1094, the maximum allowable temperature is 250℃), the winding's short-circuit withstand capability has significantly decreased. This is why short circuit accidents are more common after transformer reclosing. Using ordinary transposed conductors results in poor mechanical strength, making them prone to deformation, strand breakage, and copper exposure when subjected to short-circuit mechanical forces. When using ordinary transposed conductors, the large current and steep transposition ramps generate significant torque at these locations. Simultaneously, the coils at both ends of the winding also experience substantial torque due to the combined effects of radial and axial leakage magnetic fields, leading to twisting and deformation. For example, in the Yanggao 500kV transformer, the A-phase common winding has 71 transpositions; due to the use of thicker ordinary transposed conductors, 66 of these transpositions exhibited varying degrees of deformation. Similarly, in the Wujing No. 11 main transformer, the use of ordinary transposed conductors resulted in varying degrees of wire overturning and exposure at both ends of the high-voltage winding in the core yoke area. The use of soft conductors is also a major cause of poor short-circuit withstand capability in transformers. Due to insufficient early understanding of this issue, or difficulties in winding equipment and processes, manufacturers were unwilling to use semi-rigid conductors or did not include this requirement in the design. Transformers that have experienced failures all used soft conductors. Loose winding, improper handling of transposition or correction ramps, and excessive thinness can also cause the electromagnetic wires to be suspended. From the location of the accident damage, deformation is mostly seen at the transposition point, especially at the transposition point of the transposition conductor. The winding turns or conductors were not cured, resulting in poor short-circuit resistance. None of the windings that were previously impregnated with varnish were damaged. Improper control of the winding preload caused the conductors of the ordinary transposition conductors to misalign. The excessive gap between the sets resulted in insufficient support on the electromagnetic wire, which increased the risk to the transformer's short-circuit resistance. Uneven preload on each winding or each span caused the coil to jump during short-circuit impact, resulting in excessive bending stress on the electromagnetic wire and deformation. Frequent external short-circuit accidents and the cumulative effect of electromotive force after multiple short-circuit current impacts caused the electromagnetic wire to soften or internal relative displacement, ultimately leading to insulation breakdown. Recommendations for ordering (1) When selecting equipment, the existing product structure should be fully considered, redundant functions should be eliminated, and a reliable structure should be selected. After fully considering the short-circuit capacity of the power grid and the dynamic stability performance of the product, the product parameters should be determined. The tap changer should be configured reasonably according to the actual needs of the power grid. The requirements for performance parameters should be adapted to the current manufacturing level and material conditions. (2) Prioritize the use of product designs that have passed the short-circuit type test, and conduct random short-circuit withstand tests on the products to ensure product consistency. (3) Use fully self-cooled transformers. Because fully self-cooled transformers have lower cooling efficiency, use more copper, and are heavier than transformers with other cooling methods, they have stronger short-circuit withstand capability. In response to the aforementioned causes and problems of short-circuit faults, the product design should take improvement measures in all aspects of electrical and structural design. The dispersion of processes and materials should be fully considered, and sufficient margin should be left in key parts. When there is a contradiction between advanced technology and product reliability, reliability should be given priority. The design should be based on high-temperature conditions (250℃～350℃) for short-circuit withstand capability design, and short-circuit withstand capability verification calculations should be performed for special parts (such as transposition and spiral joints). If the inner coil must have a tap, an independent voltage regulating winding structure should be given priority. Simultaneously, the use of ordinary transposed conductors should be prohibited, and self-adhesive transposed conductors and composite conductors of semi-hardness or higher should be selected as much as possible; the inner support hard cylinder of 35kV and below windings should use epoxy glass fiber insulating cylinders with low dielectric loss and no partial discharge; axial clamping should preferably use spring clamping nails. Regarding manufacturing processes, to address the aforementioned process defects and deficiencies, process level should be improved, process execution discipline strengthened, and effective control of the product manufacturing process ensured. Regarding materials, self-adhesive transposed conductors and composite conductors of semi-hardness or higher should be selected as much as possible. High-density integral pads equidistant from oil channels should be used. For 35kV and below inner windings, epoxy glass fiber cylinders should be given priority as the inner support insulating cylinder for the windings. For installation, to ensure the quality of transformer installation, an installation method of seller responsibility can be adopted, and the seller must be responsible for the quality of the entire installation work. During on-site core lifting inspection, the pre-tightening force of the transformer body should be checked to ensure that the transformer body is in a tight state. Given the current poor resistance of transformers to external short circuits, the negative factors associated with automatic reclosing or forced commissioning after a system short-circuit trip should be considered in operational management. Otherwise, it may exacerbate transformer damage or even render repair impossible. Operational departments can, based on the probability of instantaneous automatic elimination of short-circuit faults, cancel the use of automatic reclosing for nearby overhead lines (e.g., within 2km) or cable lines, or appropriately extend the closing interval to reduce the harm caused by failed reclosing. Furthermore, transformers that have tripped due to short circuits should be tested and inspected as much as possible. Research and operation units, manufacturers, and material suppliers should collaborate closely in conjunction with accident analysis to continuously develop new processes and materials, improve product design, and enhance the short-circuit resistance of transformers to meet operational needs. In summary, many factors contribute to faults or accidents, but the structural design and manufacturing process of transformers remain the primary factors. Some problems have also been exposed in operational management. Besides some structural factors that are not fully understood, problems in design and operational processes deserve the attention of manufacturers and operating units.