The dismantling inspection and processing at the factory was found by disassembling the A-phase dry-type transformers. It was found that the low-voltage side branch windings that underwent short-circuiting were observed from the axial direction, and the multiple line segments turned over, which was more serious at both ends; Observing the windings in a variety of eight-point shape, and tightly clamped on the core column. The laminated paper end loops between the two lead wires at the lower end of the winding are thrown out so that the end wire segments sink and connect the lower iron yoke (with a burn on the iron yoke). There are bending deformations and damages such as the low-voltage side lead copper plate and the wooden support seen on site. The high and medium voltage windings and branch windings pulled out are all normal.
After the problem was found, major repairs to replace the windings were made. It is worth mentioning here that the problems found in the disassembled D-coherent transformer and the A-coherent transformer are similar, except that the low-voltage-side branch winding is also seriously deformed, but it has not been flipped in the axial direction (that is, it has not yet constituted a line segment. Short circuit). It was followed by a major repair of the replacement winding. Lessons to be learned The dry transformer group suffered such severe damage. The fundamental reason was the three-phase symmetrical short-circuit of the low-voltage side branch winding, and the dry transformer winding could not withstand the impact of such a large short-circuit current, but also exposed Some issues related to product manufacturing.
The design and manufacture of dry-type transformers are specified in the factory’s technical documentation: The system has a short-circuit capacity of 15400 MVA on the 500 kV side and 12500 MVA on the 220 kV side. The short circuit time is t<900K2s, where K is a multiple of the short circuit current. According to this, consider the duration of the short-circuit that a dry-type transformer can withstand: t = 900 K2 = 3.6 s (K = 1046.6 = 15.75), and the duration of this time is only 0.44 s. Dry-type transformer windings have been so severely damaged. The deformation is damaged.