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In many circumstances, three-phase AC power supply is not always the best choice for utility applications. In fact, there is a rating of three-phase to single-phase, three-phase to dual-phase and three-phase to multiphase transformers being in operation around the world, which are almost all made by general means. In spite of their huge cubage and weight, those transformers have other fatal characters for instance they can spread a fault current. EPT (Electronic Power Transformer, also called Power Electronic Transformer or Solid Transformer) has been proposed for years. In this paper, we use a module composed of the input three-phase PWM (Pulse Width Modulation) rectifier, medium frequency transformer and output single-phase inverters to structure a series of function SEPT (special EPT). We also discussed the modulation strategies of the SEPTs in this paper, and what's more, we expand the PARK transformation to four-phase and firstly proposed the transformation coefficients of four-phase AC system to dqg0 system. Several cases are being discussed in this paper, and the results of the simulation proved that SEPT has all the advantages EPT has, such as better steady-state, better waveforms, better power factor etc. compared with general special transformers.