Mathematical Modeling of Asymmetrical Operating Modes of Power Autotransformers

Abstract

When operating power autotransformers that have become widely used in networks of 110 kV and higher, it is often necessary to deal with open-phase modes caused by disconnections of one or two phases in the case of short circuits or in the case of single-phase repairs. This leads to asymmetry of the voltage of autotransformers, which affects the quality of power supply to consumers. For the autotransformer itself, an open-phase operation can be dangerous with respect to overloading of individual windings. The presence of an electrical connection between high and medium voltage causes the specifics in the analysis of their operating modes. In addition, while the theoretical analysis and the general approach to the calculation of asymmetrical modes of two-winding transformers are currently considered in sufficient detail, the task of analyzing the asymmetrical operating modes of three-winding autotransformers remains important. An approach to the calculation of autotransformer and combined asymmetrical operating modes of power autotransformers has been developed on the basis of the method of symmetrical components. The proposed model and obtained analytical expressions allow one to determine the currents and voltages of the phase windings of the autotransformer in a variety of asymmetrical modes of operation on the basis of a unified approach.