The transformer will inevitably suffer from the impact of various short-circuit fault currents during operation, especially the outlet short circuit and near field short circuit, which will do the greatest harm to the transformer. The transformer winding will bear huge, uneven axial and radial electric stress. When there are weak links in the internal mechanical structure of the winding, winding deformation will inevitably occur. It includes axial and radial dimension changes, body displacement, interturn short circuit, winding distortion, bulge, etc. The following conditions may occur when the transformer winding continues to operate after deformation:
(1) The transformer was immediately damaged. There was a 110 kv transformer in our bureau that was subjected to short circuit in the near area. After successful reclosing, the gas acted more than 20 seconds later. After that, the winding deformation was checked and the transformer was returned to the factory for re winding;
(2) The insulation material of the transformer is damaged or the insulation distance is changed due to the winding deformation, resulting in the reduction of the insulation strength. Under the long-term normal voltage or over-voltage, the insulation may eventually be broken down. In such cases, the insulation defects can be detected by conventional methods such as electrical test and oil test;
(3) After the winding is deformed, the insulation condition is not damaged, but the mechanical strength of the coil decreases. When the winding suffers a short circuit fault again, it will be damaged because it cannot withstand the huge electric force. In this case, the insulation is not damaged, and there is no problem reflected in the conventional electrical test and oil test. Only by means of winding deformation test can correct conclusions be drawn. At the same time, this kind of situation is also common, because many transformers will not be damaged after only one short circuit, while transformers in operation may have suffered multiple short circuit impacts, their mechanical strength has decreased, and even slightly deformed. However, since the deformation fault in their internal parts cannot be detected through routine tests, their status cannot be judged before overhaul and lifting inspection, which is a serious potential accident.
Therefore, actively carrying out transformer winding deformation testing, timely finding and handling problematic transformers, and carrying out targeted lifting inspection can save a lot of manpower and material resources, and also play an extremely important role in preventing transformer accidents.
