A Modified Discontinuous PWM Method for Three-Level Inverters With the Improved <i>LCL</i> Filter

The three-level inverter with improved LCL filter is employed in photovoltaic system to improve the power quality and suppress the leakage current. Nevertheless, this type of topology has the challenge of neutral point (NP) voltage balance, common mode (CM) resonance current suppression, and switching loss reduction. Furthermore, when a conventional discontinuous pulsewidth modulation (DPWM) method is adopted, the coupled relationship between these three control objectives becomes more complex. Due to the mutually coupled problems, the existing DPWM methods cannot achieve all control objectives at the same time. To address these issues, a modified DPWM method with NP voltage balance and CM resonance current suppression is proposed in this article. First, the conventional 60° clamping period of DPWM1 is divided into three intervals, which reduces the NP voltage ripple and the complexity of NP voltage balance control by shorting the clamping period. Then, an NP voltage limit is set to determine the control priority of NP voltage balancing and CM resonance current suppression. Finally, the clamping states and three-phase switching states are adjusted by an offset voltage according to the control priority, which can realize NP voltage balance and CM resonance current suppression simultaneously with reduced switching losses. The effectiveness of the proposed method is verified by simulation and experimental results.