Hierarchical Control for dc-Link Voltage and Power Sharing of CHB-based Solid State Transformer

This paper proposes a hierarchical control strategy for dc-link voltage and power sharing among power modules in a cascaded H-bridge based solid state transformer (CHB-SST). By analyzing the sharing mechanisms, it is revealed that having the dc-link voltage balance controlled by the dual active bridges (DABs) is an effective scheme for voltage and power sharing. A hierarchical control strategy with two control layers is then developed. The bottom layer implements distributed sub-controllers for the DABs to balance the dc-link voltage and regulate the low-voltage dc (LVdc) bus voltage without current sensors and high-speed communication links. At the top layer, a main controller handles two control targets, i.e., the total dc-link voltage is controlled by the CHB; and the LVdc voltage offset caused by the distributed voltage balance control is eliminated by a secondary voltage adjusting control of the DABs. The proposed method is a low-cost, low-communication-requirement, scalable and reliable solution for voltage and power sharing control of CHB-SST. The LVdc voltage performance under the proposed control is improved comparing to existing distributed control based strategies. Experimental results are provided to verify the proposed strategy.