由自身闭环控制系统控制并稳定直流电压的三电平变流器,其直流母线电压相对该变流器是电流控制电压源,因此不能以直流电压恒定为前提分析此类变流器的中点电压平衡问题.以直流电压为受控电压源为前提,分析并推导出了二极管箝位(NPC)三电平变流器应用标准SVPWM与虚拟矢量合成SVPWM技术时的电容电流表达式,并得到了2种方法的中点电压平衡控制能力随调制比与功率因数变化的规律.理论分析表明,2种SVPWM的中点电压平衡控制能力随调制比的增大而减小,随功率因数绝对值的增大而增大,且在同样条件下,虚拟矢量合成SVPWM技术具有更好的中点电压平衡控制能力.仿真结果证明了理论分析的正确性.%As the DC-bus voltage of three-level converter,which is stabilized by its close-loop control system is a current-controlled voltage source,the premise of constant DC-bus voltage for analyzing the neutral-point voltage fluctuation does not exist. With the DC voltage as a controlled voltage source,the expressions of capacitor current is analyzed and deduced for the NPC three-level converter based on the standard SVPWM and virtual vector SVPWM technologies and the laws of the neutral-point voltage variation along with the modulation ratio and power factor are obtained for two schemes,which indicates that,the balancing ability of neutral-point voltage decreases along with the increase of modulation ratio and the decrease of absolute power factor. Under same conditions,the scheme based on virtual vector SVPWM technology has better balancing ability. Simulative results are consistent with the theoretical analysis.
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