Abstract
The generally accepted point of view is that the service life of a transformer is mainly determined by thermal aging, electrical insulation wear and mechanical wear. In most cases, this approach is confirmed during scheduled repairs or in case of damage. However, in a number of damages to powerful transformers, signs of defects in the electromagnetic system were found upon opening, such as short circuit in the grounding system of the active part mounting elements. All elements (yoke beams, half-bands ,band strips of rods, etc.) are grounded, and the system is designed so that there are no contours. Most often, the grounding system is divided into the NN and VN sides and brought out through insulators, but it is impossible to check whether there are non-design circuits in it. Careful inspection and damage analysis indicate that a closed circuit may form in the transformer core grounding system, in which currents of up to ten kilo amps are induced. The currents were calculated from the burnout of the copper bus bars connecting the vertical and horizontal mounting elements of the magnetic circuit. At the moment of contact rupture (burning of the last copper plate), the arc forms ionized plasma, which is attracted by a magnetic field or oil blast during abrupt decomposition into the winding and it damages the paper insulation. The mechanism of contour formation is clear and the main cause is vibration. The causes of vibration are close short circuits, often variable load, untimely pre–pressing. It is interesting to note the fact of damage and look for the cause when opening a transformer, but methods and means are needed to see the developing damage in advance and prevent transformers from failing in case of a total shortage. The paper describes the experience in detecting such types of defects during routine examinations of transformers. Control methods and methods for detecting defects are proposed.
Keywords
short circuit, transformer core, spectral analysis, electric discharge phenomena, sparking, arc
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