Comparative analysis of special transformers for four-phase power transmission systems

Four-phase transmission can be a viable alternative to traditional three-phase lines, as shown in recent papers, but it requires the solution of some problems. One is the phase transformation between the three- and the four-phase section: the choice of the best typology of the needed special transformers, not a novelty in the literature, is not trivial. Previously the authors have treated the equivalent mathematical representation of the special transformers in terms of nodal admittance matrix for understanding the relationship between the symmetrical components of the two systems. Recently novel solutions have been proposed that differ remarkably from the earliest records dating back to the beginning of the past century. In the present paper an analysis is performed in order to capture in a systematic way the underlying principles of the phase transformation, trying to exploit a general framework for comparing every solution with the other ones.