Multicarrier PWM With DC-Link Ripple Feedforward for Multilevel Inverters

Multilevel inverters have become an attractive alternative for medium voltage and power quality demanding applications. However, like most power converter families, multilevel inverters are controlled with modulation techniques that are conceptually based on nonlinear waveform synthesis assuming constant dc-link voltages. Nevertheless, real applications have load and supply dependent dc-links that usually present important low frequency ripple. Due to the high switching frequency operation of the converter and the low-pass nature of the loads, the dc-link ripple is also modulated and transmitted to the load, generating undesired low frequency voltage and current distortion. This can be critical in high power applications due to the amount of energy involved in these harmonics. This paper introduces a simple but effective dc-link ripple feedforward strategy into traditional carrier-based modulation techniques, which eliminates the undesired harmonics. Experimental results show the accuracy of the proposed method, which is specially useful for open loop applications.