A control scheme of three phase solid state transformer for PV generation based on improved voltage-tracking method of DC links

A three-phase solid state transformer (SST) based on serial H-bridge multilevel converter, with advantages as expandable modular structure, controlled power quality and low dependence on copper/steel, is applied to replace the traditional line frequency step-up transformer on PV generation. Because of the input-parallel output-serial dual active bridge (DAB) based submodules with inner dc links, the voltage balance of medium voltage (MV) dc links and the control of low voltage side input dc link are concerned. An overall control scheme is proposed based on the voltage tracking control scheme of dc links, within which the voltage balance between submodules is realized through average voltage control and dc link voltage tracking control. Improvements are also proposed for shaping DAB current and increasing the speed of dynamic response. Simulation results are given in the end.