This paper proposes a novel discontinuous pulse-width modulation (DPWM) method to reduce the current harmonics and improve the system efficiency for a single-phase three-level neutral-point clamped inverter. In single-phase inverters, the unipolar pulse-width modulation (UP-PWM) method is commonly used. However, this method has the disadvantage of power losses due to numerous switching operations. Conventional DPWM methods reduce the power losses and improve efficiency but increase the current total harmonic distortion (THD). To overcome these weaknesses, this paper proposes a hybrid DPWM switching method combining two PWM methods: the UP-PWM method and the conventional DPWM method called one-pole clamped PWM method. Since the proposed DPWM method offers all the advantages of both PWM methods, the optimal performance-with regard to the power losses and current THD-is obtained. The combination of two PWM methods is investigated by analyzing the power losses and current THD. Based on the analysis, the process determining the optimal operating condition is introduced. The effectiveness of the proposed DPWM method is demonstrated through simulations and experiments.
Manuscript received June 30, 2017; revised October 10, 2017 and December 28, 2017; accepted January 5, 2018. Date of publication January 17, 2018; date of current version August 7, 2018. This work was supported in part by a grant from R&D Program of the Korea Railroad Research Institute, Republic of Korea and in part by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2016R1A2B4010636). Recommended for publication by Associate Editor J. Liu. (Corresponding author: Kyo-Beum Lee.) J.-S. Lee is with the Railroad Safety Research Division, Korea Railroad Research Institute, Uiwang 16105, South Korea (e-mail: ljs@krri.re.kr).
