说明：  三相三线和三相四线 NPC 三电平逆变器的并网控制策略仿真，包括恒有功/恒直流电压、系统无功闭环、系统负序电流及零序电流闭环控制策略，以及中点电位平衡控制策略。
(The simulation of three-phase three wire and three-phase four wire NPC three-level inverter grid connected control strategy includes constant active / constant DC voltage, system reactive closed-loop, system negative sequence current and zero sequence current closed-loop control strategy, and neutral point potential balance control strategy.)