The paper, titled "Control and Stability Analysis of Modular Multilevel Converter Under Low-Frequency Operation," was published in IEEE Trans. on Industrial Electronics, vol. 62, no.9, pp. 5329-5339, 2015.

The modular multilevel converter (MMC) has become one of the most attractive converter topologies for medium and high-voltage/power applications due to its modularity and scalability. The MMC has been widely investigated for high-voltage direct current transmission systems. The salient features of the MMC make it a potential candidate for medium-voltage adjustable speed drive systems. Nevertheless, one of the main technical challenges associated with the operation of MMC under constant-torque low-speed operation of the MMC-based adjustable-speed drive system is the large magnitude of the submodule (SM) capacitor voltage ripple due to the inverse dependence of the SM capacitor voltage ripple on the speed of the machine. This leads to increased rating values of the converter and/or instability. The paper presents novel control strategies to reduce the SM capacitor voltage ripple under constant-torque low-speed operation of an MMC-based adjustable-speed drive system. The paper also proves the stability of the proposed control strategies based on Lyapunov analysis of singularly perturbed nonlinear nonautonomous systems.

Debnath received his Ph.D. degree from the School of Electrical and Computer Engineering (ECE) at Purdue University in May 2015, under the supervision of Saeedifard, who previously worked at Purdue and is now an assistant professor in the Georgia Tech School of ECE. Debnath is currently a postdoctoral research associate at Oak Ridge National Laboratory, where he is involved in research on HVDC systems, wireless power transfer systems, and 3D printed solar inverters.