부산대학교 도서관

This article proposes a single-phase transformerless ac–dc–ac multilevel converter composed of a three-leg module series-connected to an H-bridge at the load side to achieve a higher and multilevel voltage at this side. The proposed structure is appropriated for applications with the same input and output frequencies, such as uninterrupted power supply and unified power quality conditioner, and for electric power conversion systems in which the output voltage required is higher than the input voltage (voltage step-up applications). System model, converter power analysis, dc-link voltage specifications, and a space vector PWM strategy are developed. The proposed system control ensures fixed load voltage with constant amplitude and frequency, maximizes the grid power factor, and regulates the dc-link voltages. Compared with conventional solutions, the proposed configuration has presented lower dc-link voltage ratings and lower mean switching frequency on the semiconductor devices, lower harmonic contents, and lower switching losses. The comparative analysis shows that the mean weighted total harmonic distortion (WTHD) of the generated voltages of the proposed topology are reduced about 27% and 16%, respectively, compared to the topologies 4L and 3L, and 4L-LLC; and the total semiconductor losses are reduced about 26%, 10%, and 49%, respectively, compared to 4L, 3L, and 4L-LLC, when all topologies operate with the same mean WTHD. Operating with the same switching frequency, the proposed converter presented total semiconductor losses similar to conventional 3L, and about 17% and 45% lower than 4L and 4L-LLC. Experimental results demonstrate the operation of the proposed converter under load transients and grid voltage variations.