D10.14 - Carrier-Based Modulation Scheme Plus Neutral-Point Current Control for Balancing Neutral-Point Voltage of Three-Phase Four-Leg Three-Level Inverter Over Entire Power Factor

The carrier-based pulse-width-modulation (CB-PWM) plus neutral-point (NP) voltage control is simple and effective for modulating the three-phase four-leg three-level (3P4L 3L) inverter. It is found in this paper, however, that the neutral-point (NP) voltage of the 3P4L 3L inverter becomes unbalanced when the output power factor (PF) is 0, even if the NP voltage control loop has been applied to control the NP voltage. The root cause is analyzed, indicating that the gain sign of the NP voltage control loop should not be constant, when the output voltages’ phase is subject to different polarities of perturbations in the case of PF=0. To address this issue, the gain sign of the NP voltage loop is proposed to be regulated as per the NP current direction, for balancing the NP voltage over the entire power factor. Furthermore, the CB-PWM plus NP current control is proposed to replace the CB-PWM plus NP voltage control. The doing so not only balances the NP voltage but also minimizes the NP voltage ripple throughout all the power factor. Finally, preliminary results are presented to verify the feasibility of the proposed work.