Study of Special Unbalanced Load Modes of New Double-Autotransformer of the Flexible Alternating Current Link

Abstract

The purpose of the work is study of special asymmetric modes of new three-phase autotransformer device for flexible connection of alternative current power systems. The universal model was developed for the device, containing two autotransformers - the main phase-shifting and the additional regulating autotransformer. The model allows studying both symmetric and almost any asymmetric modes of the considered circuit. It became possible to connect asymmetrically the loads in different phases of the regulating autotransformer, as well as arbitrary connect the regulating autotransformer itself to the hexagon taps. According to the calculated values of currents, voltages and magnetic fluxes of the device the visual vectorial diagrams were constructed for the main and the regulating autotransformer, explaining specific of considered operating modes. It was shown that device’ currents in such modes contain symmetrical components of both zero and negative sequences. Also, at asymmetric connection of the regulating autotransformer, the relative magnetic flux of the legs of the main and regulating autotransformers are asymmetrical, but in main autotransformer their sum is approximately equal to zero and magnetic flux practically does not escape into the surrounding space. Radically different situation is observed in the regulating autotransformer. The significant zero-sequence fluxes arise with the release of significant (about 50%) magnetic flux into the surrounding space. This re-quires special measures to be taken to limit its harmful effects on the operation of the autotransformer.