A Quasi-Z-Source Four-Switch Three-Phase Inverter With Null Vector Capability

In this article, a novel structure for a four-switch three-phase inverter with the ability of creating a null vector is proposed. This structure utilizes a symmetrical quasi-Z-source network and generates zero output voltage during the shoot-through state, which is similar to the null vector in the conventional six-switch inverter. This vector is used for the remaining time for each switching period. This means that the proposed structure has several advantages over the conventional four-switch inverter, in which two active vectors with opposite directions are necessary for synthesizing a null vector. These two vectors increase the switching losses due to the current circulation through these active states. Moreover, the number of switching states increases because of applying two opposite vectors. However, this problem does not exist in the proposed topology. To decrease the output current distortion and to minimize the number of switching events per each cycle, a new switching pattern is proposed, too. Also, fluctuations of the two split capacitors in the dc-link are considered in the closed-loop modulation method, which balances the output voltages and decreases current THD by 0.7%. To validate the proposed topology and the given pulsewidth modulation approach, its mathematical analyses are given here. Also, simulations and experimental verification have been performed on a 1-kW prototype inverter.