The Method for Assessing the Permissible Overload Capacity of Power Transformers during High Geomagnetic Activity

The load capacity of five legged power transformer during geomagnetic disturbances was investigated. As an object of the research was chosen the 242/20 kV, 400 MVA, five legged power transformer. The aim of investigation was to determinate the acceptable level of power transformer load capacity by the criterion of the above-limit value of windings temperature excess over ambient temperature. To obtain quantitative estimates of the temperature criterion, additional losses due to eddy currents arising under the influence of geomagnetically induced quasi-direct currents was preliminarily determined and additional main active power losses caused by an operating current increase due to a multiple increase in the magnetizing current. The values of the geomagnetically induced quasi-direct currents in quantitative estimates corresponds to the recorded results of those currents in electric power systems. It was shown that the greatest increase in additional power losses is observed in grounded high-voltage windings, and in low-voltage windings the additional losses increased insignificantly. The permissible levels of the overload capacity of 242/20 kV, 400 MVA power transformer was established for different values of the geomagnetically induced quasi-direct currents and the ambient temperature. It was shown that increasing of load capacity above the set values could lead the overheating of power transformer windings, followed by thermal degradation of the main insulation and an increased risk of internal short-circuits.