Simplified PWM Algorithm for Three-Level NPC Voltage Source Inverters

Conventional modulation techniques, such as sinusoidal pulse-width modulation (SPWM) and space vector pulse-width modulation (SVPWM), have demonstrated their effectiveness in the control of voltage source inverters (VSI). However, with the widespread use of multilevel inverters, the application of these traditional methods has become increasingly complex in terms of design and implementation. This paper presents a simplified pulse-width modulation (PWM) algorithm for a neutral-point clamped three-level VSI. By exploiting the symmetry of three-phase switching waveforms, the PWM algorithm is significantly simplified from six sectors to a single sector. Furthermore, a general mathematical model is derived based on the line-voltage volt-second balance principle. The degrees of freedom for common-mode injection and small vector distribution in the conventional method are interpreted as the third equation of the general model. As a result, the duty ratio for each phase state can be directly computed, facilitating implementation and enabling easy adaptation for other objectives, such as neutral point voltage control and common-mode voltage mitigation. The effectiveness of the proposed method is validated through theoretical analysis, simulations, and experiments.