Cascaded multilevel SST medium voltage converter for solar applications

Most of the large-scale PV power plants (solar farms) are based on central inverters, in which the PV modules are concentrated in few maximum power point trackers and connected to the grid through three phase inverters. Although this topology has typically high efficiency, it has limited performance in terms of capturing all the power available from the PV modules. This paper proposes a cascaded multilevel converter based on solid state transformers (SST) technology, in which multiple high frequency transformers replace the bulky medium-voltage line-frequency function. In the proposed alternative, the converter grid front-end consists of a series connection of H-bridge inverters supplied by independent PV module arrays through solid state transformers. Among the many advantages of the proposed solution, it is important to point out: multiple MPP trackers, reduced size and weight, improved reliability and conversion efficiency and easier maintenance. The proposed SST based converter topology for PV power plants and its main design and control issues is addressed.