中文摘要：

中点箝位型（neutralpointclamped，NPC）三电平逆变器已被广泛应用，中点电位的平衡是其关键技术之一。首先建立基于零序电压注入的中点电位的数学模型，然后根据模型分析采用三种不同零序电压注入的计算方法。在实验室搭建三电平并网逆变器实验样机，比较在不同功率因数下，3种不同零序电压注入方法的中点电位动稳态平衡能力。结果表明，3种方法均能很好地平衡中点电位。此外，在实际条件下，上下电容值常会不相等，因此有必要补充在上下电容值误差为12％条件下的实验。实验验证了3种不同零序电压注入方法在上下电容值误差较大时仍能很好地控制中点电位的平衡。

英文摘要：

The neutral point clamped（NPC）three-level inverter has been widely used in many different cases, and the balance of the neutral point voltage is one of the key technologies. The neutral point voltage model based on different zero-sequence voltage injection was established in this paper, and then the calculation methods using three different zero-sequence voltages injection were analyzed according to the model. A three-level inverter experimental prototype was built in the lab, and the steady state and dynamic balance ability of the neutral point voltage based on three different zero-sequence voltage injection methods were compared with each other in different power factors. The experimental results showed that three different zero-sequence voltage injecting methods can keep the balance of the neutral point voltage well. The upper and lower capacitance values are often unequal in actual conditions, thus it is necessary to do experiments under the condition that the error of upper and lower capacitance value is 12%. The experiment verified that the three different zero-sequence voltage injection methods can still hold the balance of the neutral point voltage well even though the error of upper and lower capacitance value is so large.