Rectifier stage operation and controller design for a medium voltage solid state transformer with LCL filter

This paper presents the operation and control of the rectifier stage of a medium voltage solid state transformer (SST). As an alternative scheme to integrate DG at distribution grid, the grid-connected operation of SST needs to be analysed. By defining the P-Q diagram of SST´s rectifier stage, component selection such as dc link capacitor can be evaluated in respect to allowable dc ripple voltage. The rectifier´s controller regulates (1) active power by preserving dc link energy and (2) PCC voltage restoration by var compensation. This paper outlines the controller components and explains how parameters are selected. Last, with simulation and experimental results, the laboratory MV SST prototype is tested at various MV levels up to 3.6kV. The controller performance is verified with different scenarios including start-up, bi-directional power flow and PCC voltage restoration.