Autotransformer-based 18-pulse rectifiers without using dc-side interphase transformers: Classification and comparison

Autotransformer-based 18-pulse rectifier systems are classified from three points of view: (a) DC-side connections of diode bridges, (b) the number of phases of the diode bridge input voltages, and (c) the symmetry of the input voltage systems. Five practical systems are chosen of which system structures, voltage synthesizing methods, equivalent transformer capacities, and simulated current waveforms are presented. The total harmonic distortion of line currents are compared when the supply voltages have pre-existing voltage-unbalance and -distortion resulted from fifth harmonics. Two schemes are recommended for applications required unregulated and regulated DC voltages.