Comparison of improved asymmetric space vector modulation and sinusoidal pulse width modulation for voltage source converters with low carrier ratio

This paper presents the comparison between improved asymmetric space vector modulation (ASVM) and sinusoidal pulse width modulation (SPWM) for voltage source converters (VSCs) when the switching frequency is as low as 9 times of line frequency. By adding two pulses in each line cycle when the fundamental voltage crosses zero, the total harmonic distortion (THD) of output current can be reduced significantly. The penalty of additional switching loss is very limited for high power factor operation. The applications of the improved ASVM in a single VSC is shown. With the optimization of the duration and position of the additional pulses, the ac current THD can be reduced to as low as 25% for single VSC. Such THD reduction has close relationship with space vectors´ position and modulation index. Improved ASVM can also reduce the amplitude of circulating current, leading to smaller inter-phase inductors. The analysis results are verified by experiments on a demo system.