Hybrid multilevel inverter drive with synchronous modulation and current waveform improvement

The use of multilevel converters is widespread in medium-voltage applications due to their inherent capability to share the voltage between the power devices. More than one multilevel topology can be combined to form a single Hybrid Converter with improved capabilities. In particular, the series connection of a neutral point clamped (NPC) as a main inverter, providing the active power, and a floating H-Bridge in each phase as conditioning inverters is analyzed in this paper. The function of these conditioning inverters is to compensate the harmonics generated by the main inverter, reducing the current distortion by delivering reactive power. The switching frequency of the main inverter is limited to reduce the switching losses by means of synchronous pulsewidth modulation. In this work, synchronous optimal pulsewidth modulation that minimize the weighted voltage THD in the NPC has been selected, while higher frequency subharmonic PWM is used for the floating H-Bridges. Design criteria for the selection of a proper value for the H-Bridge dc-link voltage and a simple control algorithm for its regulation are proposed. This scheme achieves currents with low distortion, resulting in a lower torque ripple and lower machine losses in a induction motor drive. Experimental results using a 2.5 kW converter prototype feeding a R-L load are shown to illustrate the operation of this topology and dynamic operation of an induction machine fed by this converter under V/f control is presented.