Minimizing Total Harmonic Distortion of a Two-Level Voltage Source Inverter Using Optimal Third Harmonic Injection

Full time-domain analysis of the impact of third-harmonic injection (THI) on the output phase voltage waveform of a two-level voltage source inverter is presented, and the total harmonic distortion (THD) performance of different pulsewidth modulation (PWM) methods is provided. Two well-known third harmonic injection PWM (THIPWM) schemes are adopted from the literature: THIPWM6 and THIPWM4, where the THI amplitude is one-sixth and one-fourth the amplitude of the sine-wave, respectively. In this article, three analysis-derived developments are provided. Global optimal THIPWM ensures minimum THD in all linear region, unlike other THIPWM schemes. A unique THI-level is selected for each modulation index from the proposed curve or the compacted lookup table. The best suboptimal THIPWM method is provided that employs a linear injection applied over the linear modulation. To achieve better THD performance and to satisfy different operation criteria, several suboptimal THIPWM methods are proposed. Comparisons between these schemes must define performance indices, and this is carried out using various objective functions, such as the sum of squared error and area under the curve. The impact on extending the linear region is also presented. This study examines when THIPWM is useful and which injection level is best. Experimental verifications support the presented analysis.