Enhanced Carrier-Based Discontinuous PWM for Three-Level Inverters with Unbalanced Neutral-Point Voltage

Unbalanced neutral-point (NP) voltage leads to severe alternating output current distortion in three-phase three-level inverters with the conventional discontinuous PWM (DPWM) scheme. Existing improved DPWM schemes are based on space-vector algorithms, which bring about a huge calculation burden under unbalanced NP voltage conditions. Therefore, this article proposes an enhanced carrier-based DPWM (ECB-DPWM) scheme to achieve low total harmonic distortion (THD) and simple digital implementations. Firstly, the basic mechanism that NP voltage imbalance distorts the alternating output current is revealed. Then, modulation signals of the conventional carrier-based DPWM (CB-DPWM) scheme are further modified according to the NP voltage. Consequently, the harmonics in the alternating output current no longer significantly increase with the unbalanced NP voltage. Finally, a 3-kVA grid-tied T-type inverter prototype is built, and experiments demonstrate that the proposed ECB-DPWM scheme can effectively improve the alternating output current distortion caused by the NP voltage imbalance. Compared with the existing DPWM scheme which improves the alternating output current distortion under unbalanced NP voltage, the proposed ECB-DPWM scheme features a lower computation burden.