An Improved PWM Scheme for Three-Level Inverter Extending Operation Into Overmodulation Region With Neutral-Point Voltage Balancing for Full Power-Factor Range

This paper presents a carrier-based pulsewidth modulation (PWM) strategy for a three-level neutral-point-clamped (NPC) inverter that exhibits good control over dc-link capacitor voltages and mitigates low-frequency oscillations in the neutral point (NP) for operation extending into overmodulation region. The overmodulation signals are derived in a generalized way by first determining a bias signal from the zero-sequence injected signals, and then adding the bias with the modulating signals. Such an approach enables seamless transition across different modulation modes covering full modulation depth. It is shown that the incorporation of a signal compression factor generates a nonzero duty ratio for NP connectivity in every PWM cycle in overmodulation region, and thereby creates a room for addition of unbalance compensation signal which can be used to generate neutral current to mitigate existing unbalance in capacitor voltages in the overmodulation region. The compensating signals are in phase with the load currents exhibiting linearity in compensation for full power-factor range. A thorough investigation of the PWM algorithm for operation in both undermodulation and overmodulation with NP voltage regulation is carried out. The performances of the proposed scheme are evaluated through extensive simulation and validated by experiments using a three-level NPC inverter prototype with induction motor load.