Assessment of Stresses on Power Transformer Winding Internal Insulation Under Resonant Overvoltages

Abstract

Windings of high voltage power transformers are complex oscillatory circuits, the natural frequencies of which are from a few up to several hundreds of kHz. It is well known that if the frequency of voltage oscillations at the input terminals of a transformer is close to one of the winding natural freguencies, such oscillations, can initiate the development of high-frequency resonant overvoltages inside the windings, which poses a potential danger to the internal insulation of the transformer. The most common reason for the appearance of voltage oscillations at the transformer terminals with frequencies from tens to hundreds of kHz is the multiple reflection of electromagnetic waves at the ends of the supply cable lines having a length from tens to hundreds of meters. With the increase of the cable lines rated voltages and with their growing application in recent years, there have been more and more cases of damages of the transformer internal insulation due to high-frequency resonant overvoltage inside the windings. In order to ensure the ability of transformers to withstand high-frequency stresses, it is important to evaluate the voltages affecting their internal insulation. Recently, the high-frequency white-box models of transformers have received great development. These models make it possible to quantitatively estimate resonant frequencies of transformers and to make qualitative analysis of resonant phenomena development in transformer windings. However, the limitations of these models, namely the omitting of frequency dependence of losses, do not allow them to provide a reliable assessment of the resonant-overvoltage magnitudes inside the windings. For a more accurate determination of the voltages at the sections of the windings, insulation, it is preferable to use direct measurements of voltages in the windings and transfer functions in a wide range frequency. The paper discusses the application of computational and experimental evaluation of voltages affecting the winding internal insulation by combination of results obtained by modeling and from available measurements