Solid State Transformer Control Aspects For Various Smart Grid Scenarios

The power distribution networks can be classified as active/passive and stiff/weak based on the nature of loads and network impedances. As an alternative to the conventional iron and -copper based passive transformer, a solid state transformer (SST) can be used for interfacing such distribution networks to the medium voltage AC grids. Although the power architectures of SST for distribution grids are studied in detail, the control schemes for SST considering the nature of network is yet to be explored. In this paper, the control aspects for the popular three-stage cascaded multilevel solid state transformer (CMSST) are studied for the following grid scenarios: (i) SST interfacing two stiff grids, (ii) SST interfacing a stiff grid and a weak grid, and (iii) SST interfacing a stiff grid with a passive network. In this context, an average model of the CMSST is developed by replacing the basic switching cell (H-bridge) with its equivalent average model. It is shown that the control aspects are network specific and are not interchangeable.