An AC-AC Modular Multilevel Converter-Based Partially Rated Solid-State Transformer

Increasing penetration rate of the renewable energy resources in conjunction with emergence of new dynamic loads such as electric vehicles, necessitate more flexible, efficient, and economical operation of the power grid. To maximize the utilization of the power system infrastructure in an efficient and economical way, partially rated solid-state transformer (PSST) has been proposed to control the power flow and to improve the power quality. In this paper, based on a newly developed ac-ac modular multilevel converter (MMC), new PSSTs along with their supporting control strategies are proposed. The proposed PSSTs borrow the salient features of the MMC and are capable of controlling power flow, compensating voltage sag/swell, filtering the current harmonics, and removing unbalanced current components of the grid. Simulation studies in the PSCAD/EMTDC software environment are carried out to validate the performance and effectiveness of the proposed MMC-based PSSTs and the supporting control methods for different applications.