Integration of PV Panels and EV Chargers on the Modular Multilevel Converter Based SST

Abstract

This article develops a solid-state transformer (SST), which integrates PV panels and EV chargers together through the open-dc modular multilevel converter (MMC). Different from the well-known MMC, which has a common dc link, the discussed MMC is open-circuited at the dc link so that it should be analyzed from the three-phase viewpoint. The developed SST performs like two parallel-connected cascaded converters, and they are operated with opposite power flow directions. Compared with the conventional SST solution using two separate cascaded H-bridge (CHB) converters, the developed SST has relatively low component count so that it is expected to be of superiority in cost, footprint, and even power density. In addition, the usage of zero sequence voltage in the CHB converter can be replaced by circulating currents in the open-dc MMC so as to greatly reduce the risk of overmodulation. After the thorough power flow analysis on the open-dc MMC and the corresponding controller design, the developed SST is capable of freely working at any operating points with PV panels and EV chargers. Moreover, the developed SST takes the unbalanced grid conditions into account, which is another key contribution of this article. Finally, the balanced and unbalanced operations of the developed SST are both confirmed by the laboratory test results, and furthermore, the dynamic performance of the developed SST considering the time-variant characteristics of PV panels and EV chargers is also demonstrated through the PLECS simulation platform.