中文摘要：

高效、电网友好型等特征是光伏逆变器的发展方向,多电平光伏并网逆变器较两电平逆变器谐波更小、效率更高,因而受到了广泛的关注和应用。然而,其直流母线中点电压的不均衡是二极管箝位式多电平逆变器的固有问题,其根本原因在于中点电流的存在,因此,中点电压平衡的关键在于消除中点电流的影响。介绍了三电平逆变器的空间矢量控制方法,分析了中点电压不平衡的原理,并研究了几种常用的中点电压平衡控制策略,包括滞环中点电压控制、精确中点电压控制和虚拟空间矢量控制（VSVPWM）等,最后在MATLAB/simulink平台上对这几种控制策略进行了仿真验证,仿真结果表明控制策略能够很好地平衡中点电压。

英文摘要：

PV inverters tend to be developed for high-efficiency and power grid-friendly type.Compared to two-level inverters,multi-level grid-connected PV inverters can produce smaller harmonic and higher efficiency,so are widely concerned and applied.However,the imbalance of the neutral point voltage of the DC bus is an inherent problem of the multi-level inverter.The basic factor of the imbalance is the existence of the neutral point current.Thus,it is critical to eliminate the influence of the neutral point current.This paper summarizes the control strategy of balancing the neutral point voltage.Firstly,the SVPWM（space vector pulse width modulation） control strategy of the three-level inverter is introduced and the principle of the neutral point voltage imbalance is analyzed.Then several neutral point balancing strategies are brought out,including hysteresis neutral point control,accurate neutral point control and VSVPWM（virtual SVPWM） control.At last,the control strategies are simulated on MATLAB/Simulink platform.The simulation results show that the control methods are effective to balance the neutral point voltage.