Extending the Linear Modulation Range of a Five-Level Inverter Up To the Full Base Speed for Variable Speed Drive Applications

In this work, a hybrid five-level inverter topology with a single dc source is proposed along with a method to increase the linear modulation range of this topology up to full base speed with the elimination of lower order harmonics for variable speed drive applications. The five-level inverter topology consists of the cascade arrangement of two basic inverter topologies. The three-level flying capacitor stage with a nominal capacitor voltage of $V_{\text{dc}}/2$ is followed by the three-level H-bridge stage with a nominal capacitor voltage of $V_{\text{dc}}/8$. Using the proposed method, the peak phase fundamental voltage can be increased from 0.577$V_{\text{dc}}$ to 0.637$V_{\text{dc}}$ in the extended linear modulation region for the same dc input. Unlike conventional speed drives, where lower order harmonics are produced during operation in the extended modulation region (six-step mode), in the proposed topology, lower order harmonics are eliminated during operation in the extended linear modulation region. This ensures a better performance of the variable speed drives up to the full base speed. Experimental results to demonstrate both the steady-state and transient-state performance of the proposed topology are presented in this article.