Research On Modular Cascaded Three-phase Solid-State Transformer And Its Control Strategy

The three-phase solid state transformer (SST) is one of the key elements applied in the smart microgrid system in the future. It is an emerging power electronic device which could realize the functions of the traditional power transformer by power electronic technology. Compared with the traditional power transformer, SST has such merits of high transform efficiency, small volume, light weight and so on. The ac input currents and ac output voltages can be regulated flexible by SST. The SST system also provide a dc bus for the distributed energy used for grid-connection.The three-phase solid state transformer mainly consists of three stages: the three-phase input rectifier stage, the dual active bridge (DAB) stage and the output inverter stage. The module cascaded topology applied in the SST system can adapt to different voltage rate occasion flexibly by increasing or decreasing its modules. But the module cascaded topology also has several disadvantages such as voltages and power unbalance between modules which often results in IGBTs or MOSFETs damage of the system.In order to resolve the voltage and power unbalance issues of the SST system, the topology has been detail analyzed and modeling. At last an efficient control method has been proposed for the SST system. A common-duty-ratio based three-phase dqO control strategy is proposed for the rectifier stage to balance power transferred by each phase and rectifier module, and a voltage-track based control method is presented for the Dual active bridge(DAB) stage to ensure the input voltages of DAB modules identical and regulate the output voltage of each DAB module. Both the simulation and experimental results have verified the effective of the proposed control method.