Over the years, transformer accidents have occurred from time to time, and the trend appears to be increasing. Analysis of transformer accident cases shows that insufficient short-circuit resistance has become the primary cause of transformer accidents, causing significant harm to the power grid and seriously affecting the safe operation of the grid. The following will provide a detailed analysis of the causes of transformer short-circuit faults.
There are many and complex reasons why short circuits at transformer outlets lead to internal faults and accidents. These causes are related to factors such as structural design, the quality of raw materials, process levels, and operating conditions, but the selection of electromagnetic wires is critical. Based on dissections of transformers in recent years, the electromagnetic wires chosen according to the transformer’s static theoretical design differ significantly from the stresses they encounter during actual operation.
1. Currently, the calculation programs of various manufacturers are based on idealized models that assume uniform distribution of leakage magnetic fields, wires of identical diameter, and equal-phase forces. In reality, however, the transformer’s leakage magnetic field is not uniformly distributed and is relatively concentrated in the iron yoke. The electromagnetic wires in this area experience greater mechanical forces. Additionally, transposed wires at the crossover points experience torque due to force transmission changes caused by the incline. The elastic modulus of the wedges and the uneven spacing of axial wedges can cause delayed resonance of alternating forces generated by the alternating leakage magnetic field. This is the fundamental reason why the winding layers at the iron yoke, crossover points, and those with tap changers deform first.
2. The calculation of short-circuit resistance does not consider the effect of temperature on the bending and tensile strength of electromagnetic wires. The short-circuit resistance designed at normal temperature does not reflect the actual operating conditions. According to experimental results, the temperature of the electromagnetic wire has a significant impact on its 0.2% yield limit. As the temperature of the electromagnetic wire increases, its bending strength, tensile strength, and elongation all decrease. At 250°C, the bending and tensile strengths are much lower than at 50°C, and elongation decreases by more than 40%.
In actual operation, the transformer, under rated load, can have an average winding temperature of 105℃ and a hotspot temperature of 118℃. Generally, transformers experience reclosing during operation. Therefore, if a short-circuit point cannot be cleared immediately, the transformer will be subjected to a second short-circuit shock within a very short period (0.8s). However, due to the rapid increase in winding temperature after the secondary short-circuit current impact, according to GB1094, a temperature of 250℃ is allowed. At this point, the winding's short-circuit resistance has significantly decreased, which explains why short-circuit accidents occur more frequently after transformer reclosing.
3. Using ordinary transposition conductors, which have poor mechanical strength, can easily lead to deformation, strand separation, and copper exposure when subjected to short-circuit mechanical forces. When using ordinary transposition conductors, due to high currents and steep transposition slopes, this area will generate a large torque. Additionally, at the ends of the winding coils, due to the combined effect of radial and axial leakage magnetic fields, large torque will also be generated, leading to twisting and deformation.
For example, in the A-phase common winding of the Yanggao 500kV transformer, there are a total of 71 transpositions. Because relatively thick ordinary transposition conductors were used, 66 of these transpositions show varying degrees of deformation. Furthermore, for Wujing No. 1 main transformer, it was also due to the use of ordinary transposition conductors; both ends of the high-voltage winding coil in the yoke area of the core exhibit various levels of turning and exposed conductor incidents.
4. The use of soft conductors is also one of the main reasons for the poor short-circuit resistance of transformers. Due to insufficient awareness in the early stages, or difficulties in winding equipment and technology, manufacturers were generally reluctant to use semi-hard conductors or the design had no such requirements. All transformers that experienced faults were using soft conductors.
5. Loose winding, improper transposition handling, and overly thin construction cause the conductors to be suspended. From the locations of the accident damage, deformations are mostly found at transposition points, especially at the transposition areas of the transposition conductors.
