Neutral Voltage Modulation for Maximizing the Linear Modulation Region and Limp-Home Mode Operation of Multilevel Cascaded Inverters Under DC-Link Imbalance Conditions

This paper proposes an improved neutral voltage modulation (NVM) method for symmetrical and asymmetrical three-phase multilevel cascaded inverters (MLCIs). The maximum synthesizable space vector region is analyzed using the number of healthy inverter modules and the amplitudes of the dc-link voltages in each phase. After that, the NVM strategy is proposed to maximize the balanced three-phase output voltages in the given space vector region. The proposed method matches the polarities of the neutral and the phase voltage references as well as minimizing the neutral voltage. By using the proposed method, the linear modulation region of the three-phase MLCI are fully utilized allowing no overmodulations even under severe dc-link voltage imbalances. Moreover, the proposed NVM can synthesizes rotating voltage vectors even all modules of one phase are not available. To verify the effectiveness of the proposed method, a two-by-three MLCI prototype is built and tested. The experimental results show that the proposed method reduces the total harmonic distortion (THD) and the ripple component of the phase current by 28% and 32.8%, respectively, compared to traditional modulation strategies under severe dc-link imbalance conditions.