多电平变换器简化虚拟矢量PWM及应用

Implementation of Simplified Virtual Vector PWM for Multilevel Converter

二极管钳位型变换器存在着直流母线电容电压不平衡的问题,对于多电平(>3)拓扑,传统空间矢量PWM在单位功率因数且调制比高于0.55时无法实现中点电压平衡,已有的虚拟矢量PWM随着电平数的增多计算负担急剧增加。针对这一问题,该文提出一种简化虚拟矢量PWM算法,通过组合2个或多个相邻的开关状态来定义新的虚拟开关状态,使得每个虚拟开关状态下流过所有中性点的电流相等,从而保证理想稳态条件下中点电压的自平衡。另外,简化虚拟PWM将任意电平的空间矢量图转换为三电平空间矢量图,简化了子扇区的辨识和占空比的计算。对五电平二极管钳位型变换器的仿真和实验结果表明,简化虚拟矢量PWM可以在全调制比和全功率因数范围内实现直流母线中点电压平衡,验证了该算法的有效性。

Diode-clamped converter(DCC) suffers from an inherent problem of dc-link capacitor voltage imbalance.For the multilevel topology(>3),by using conventional space vector pulse width modulation(CSVPWM),the balance of capacitor voltages cannot be attained for higher modulation index(>0.55) while operating under unity power factor.Virtual vector pulse width modulation(VVPWM) ensures the balance of capacitor voltages for all operational conditions;however,compared to CSVPWM,the computational burden increases with the rapid increase of triangular regions for higher levels.To overcome this problem,a simplified virtual vector pulse width modulation(SVVPWM) algorithm for multilevel DCC was proposed in this paper.New virtual switching states were defined by combining two or more adjacent switching states,so that each virtual state had similar neutral point currents flowing through all neutral points,ensuring the natural voltage balance of DC-link capacitors under ideal and steady-state conditions.The proposed SVVPWM method simplified the sub-sector identification and duty ratio computation by converting the space vector diagram of any level converter to three-level space vector diagram.The simulation and experimental results of five-level DCC prove that the voltage balancing of DC-link capacitors can be achieved by using the proposed SVVPWM algorithm for the whole range of modulation index and load power factor.