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Isolated operation of synchronous machines connected by scott transformer feeding two-phase unbalanced loads

Abstract:

This paper shows a detailed study of the electric traction system behavior and its effect on the operation of isolated generation plants. The electrical system and its generators are modeled using the space vector theory, which simplifies both steady state and transient analysis. Commonly, electrified traction systems use Scott transformers in order to convert the three-phase system into a two-phase network. In this configuration, each phase of the secondary transformer feeds a route section. The electrical unbalances originated by the traction system operation, as well as the harmonics injected by the traction drive converters, introduce negative sequence currents into the power system. These currents, flowing into the generators' stator windings produce increasing losses in the damping windings. Results identify the operating limits of an isolated electric plant using international standards, the maximum unbalances tolerated by generators and their relationship to load levels in each winding of the Scott transformer. Also, the negative sequence current flowing in the three-phase system introduced by the harmonics injected during the electronic converter operation is evaluated.