Three-Phase/Six-Phase Conversion Autotransformers

The first commercial demonstration of six-phase transmission, the Goudey-Oakdale three-phase/six-phase project, used two delta wye-grounded three-phase transformers to realize the required phase/voltage conversion. This paper proposes the usage of autotransformers for phase/voltage conversion for the actual three-phase/six-phase systems to be built in the future. The design considerations of the three-phase/six-phase conversion autotransformers are described. Topology-based, nonlinear, core-type conversion autotransformer EMTP models are developed. The behavior of the conversion autotransformers in a practical three-phase/six-phase system is simulated. These include transformer energization, short circuits, voltage regulation, open conductor, and ferroresonance. The simulation results show that the autotransformer is a viable choice for the conversion transformer. Since the cost ratio of the proposed conversion autotransformers with respect to the three-winding conversion transformers is approximately 64%, the new design is superior economically to other transformer designs, and will help push the HPO technology.