Space vector modulation strategy for three-phase multilevel current-fed inverter with unequal DC-link currents

Three-phase multilevel current waveforms with (2n + 1) current levels can be generated by connecting n three-phase three-level current-fed inverters (CFI) in parallel. However, it is found that the multilevel space vector diagram gets distorted and redundancy in the switching states is lost when the dc-link current of all the inverters is not the same. As a result, output current has low frequency harmonic components thereby deteriorating the THD. The presence of these low frequency components in the output currents can be avoided if specific switching state vectors are chosen and proper time sharing among these vectors is ensured while employing space vector modulation technique. Two methods to select suitable switching state vectors are proposed in this paper. In the first method, nearest switching state vectors are utilized to realize a reference current space vector. It is found that this method leads to reduced overall switching losses in the system. In the second method, the switching state vectors are chosen to eliminate low frequency pulsations in dc-link voltage of each inverter. It is observed that the sector identification and duty cycle calculation tasks are simplified in the second method. A 5-level CFI is simulated using MATLAB/Simulink.