Seven-level cascaded inverters for Uninterruptible Power Supply (UPS) applications

This paper considers single phase seven-level cascaded H-bridge (CHB) inverters for Uninterruptible Power Supplies (UPS) applications and proposes a fundamental switching frequency procedure, based on graphic analysis, to improve the output waveform quality. It fixes the first switching angle and gets the other two by intersection of harmonics functions set to zero. The performances of the proposed procedure, in terms of total harmonic distortion (THD), are compared with those obtained by a procedure that identifies the two remain optimal switching angles through a THD minimization algorithm. Experimental results are obtained by measurements performed on a prototype built at DigiPower laboratory. The agreement between computed and experimental results confirms the accuracy of the presented procedure.