Calculation and Experimental Assessment of the Voltages on the Longitudinal Insulation of the Windings of Station Service Voltage Transformers under Resonant Overvoltages

Abstract

Abstract—Under high-frequency oscillations of grid voltage, resonant overvoltages may develop inside transformer windings if the frequencies of the grid voltage oscillations are close to the resonant frequencies of the transformer windings. For transformer equipment—in particular, station service voltage transformers, which, in virtue of the specifics of their operation, may be subjected to the action of high-frequency oscillatory voltages—to withstand internal resonance, it is necessary to assess potential impacts on the internal insulation of their windings under resonance as early as the design stage. The transformer models applied in practice for numerical simulation of high-frequency transients in the transformer windings do not allow accurate reproduction of resonant voltage rises, as they do not provide for rigorous consideration of the frequency dependence of the losses. Direct measurements of the winding voltages under resonance are an alternative to the calculations; however, in practice, such measurements are difficult to make and it is not always possible to measure voltages that affect the insulation in the parts of the windings of interest. In the article, the possibility of determining the impacts on the longitudinal insulation of the windings under resonance by supplementing the measured results with analytical solutions of equations of transients in the windings is studied. The approaches considered in this paper are applicable not only to the station service voltage transformers, but also to other types of the transformer equipment.