Analytical Approach to THD Formulation and Optimization in Sine-PWM-Based Single-Phase Multilevel Inverters With Variable DC Ratios

Asymmetric multilevel inverters (MLIs) with unequal voltage steps (UVSs) are often combined with the staircase modulation (SCM) technique to improve both voltage and current total harmonic distortion (THD)– an approach known as optimal minimization of THD (OMTHD). This is usually achieved by varying both the switching angles (SAs) and the dc source ratios (DCR), to obtain the desired modulation index (MI) while THD is minimized. In this article, a novel sine pulsewidth modulation (SPWM) based OMTHD is considered as an alternative to SCM-based OMTHD. Because only the DCRs need to be optimized instead of both SAs and DCRs, the OMTHD process is simplified, while voltage THD, current THD, as well as power efficiency, are improved, especially for the medium-low MI range, compared to conventional SPWM. An analytical formulation of phase-voltage THD for UVS-based SPWM MLIs is derived and utilized for the novel SPWM-based OMTHD. Validation is carried out by theoretical analysis, digital simulations, and real-time controller + Hardware-in-Loop (C-HIL)-based experiments.