Output voltage balancing methods of 3-level NPC rectifier for low voltage DC distribution

In this paper, in order to improve the neutralpoint voltage balancing of 3-level NPC rectifier for a bipolar ± 750 Vdc DC distribution system, a simple and practical method was proposed. The proposed method composed of the digital balancing control which is an offset voltage control using SVPWM and the analog control method with an additional voltage balancing circuit. During asymmetrical light load condition, the offset voltage control using SVPWM is only operated. When the output loads in positive and negative pole ware seriously different, the proposed voltage balancing circuit can reduce the voltage difference. In addition, the proposed voltage balancing circuit can limit the short circuit current of the output side. The detail analysis and additional experiment data will be presented with various experimental results.