Line current harmonics reduction in hybrid multilevel converters using phase-shifting transformers

This paper proposes a new arrangement to implement the dc voltage sources of hybrid multilevel inverters in active power transfer applications, using only uncontrolled rectifiers and phase-shifted secondary windings. As each cell of a hybrid multilevel inverter usually processes distinct power levels, this paper then proposes a new design methodology to determine the phase shifts among the secondaries of the input isolation transformer, even when the active power levels processed by the uncontrolled rectifiers are unequal. A set of phase angles is obtained to minimize, and even to eliminate, desired low-order harmonics of the input current, in particular the fifth harmonic, since this harmonic component presents an amplitude considerably higher than the amplitude of the other harmonics. In consequence, the proposed design methodology increases the input power factor and reduces the input current THD. Simulation and experimental results are presented to demonstrate the feasibility of the proposed structure.