基于两电平线性输出的级联型逆变器过调制算法

Two-level and Fundamental Voltage Amplitude Linear Output Algorithm for Cascaded Multilevel Converter in Overmodulation Range

应用于级联型逆变器的传统多电平空间矢量调制技术往往只考虑线性调制区的实现方法,在功率单元直流电压下降的情况下不能调整为过调制算法,然而在基波电压幅值不变的要求下,仍采用线性调制方法会出现零矢量作用时间为负数的不利情况,对现场运行不利。采用的错时相差两电平空间矢量脉宽过调制技术(space vector pulse width modulation,SVPWM),利用错时相差技术将多电平SVPWM变为在每个采样点仅需控制某层功率单元左右桥臂矢量的两电平SVPWM,简化了矢量区域的判断时间;同时把相关的多电平过调制问题简化为两电平过调制算法。仿真和实验表明,该过调制算法在保证谐波含量比较小的前提下,可以使系统从线性工作状态平稳过渡到六阶梯波状态,且能保证基波电压的幅值随调制比线性输出,直至六阶梯波时的最大输出值,很适合于级联型逆变器的过调制运行。

Linear modulation is a traditional method, frequently utilized in cascaded multilevel converter, and cannot jump to over-modulation mode when the DC voltage is dropping. Meanwhile, the action time of zero vectors appears negatively and the fundamental wave amplitude cannot be maintained which is harmful to field operation. This paper applied phase-difference sample time staggered space vector pulse width modulation （SVPWM） to change multilevel SVPWM into two-level SVPWM by controlling the three-phases H-bridges of any certain layer, and the multilevel over-modulation problem is simplified into two-level over-modulation, hence it needs less time to distinguish the space vector regions. The simulation and experimental result demonstrated that the over-modulation method can smoothly transform the system from linear condition to six pulse condition and can keep fundamental wave amplitude with the modulation ratio in a linear condition, and the output harmonics is low, quite suitable for the over-modulation method in cascaded multilevel converter.