Impedance Modeling and DC Bus Voltage Stability Assessment of a Solid-State-Transformer-Enabled Hybrid AC–DC Grid Considering Bidirectional Power Flow

The solid-state-transformer (SST) is a promising technology in the future smart grid. Compared to the line-frequency transformer, the SST has more control flexibilities and the plug and play ability, which allow it to interconnect a hybrid ac-dc multiterminal grid for reliable and flexible power distribution. The dc bus voltage stability of an SST-enabled hybrid ac-dc multiterminal grid is a critical issue due to the dynamic interactions of power converters, which has been investigated in this article. The output impedance modeling method of the dual active bridge (DAB) dc/dc converters considering both the control schemes and bidirectional power flow effect is proposed. Depending on the control strategy and power flow direction, the DAB converters have three different output impedance characteristics at the low-frequency range, which are capacitive, inductive, and resistive. It is found that the dynamic interaction between the inductive impedance and the capacitive impedance of DAB converters could cause instability issues when the system damping ratio is low. A hardware-in-the-loop-based impedance measurement testbed is used to extract the DAB converters' output impedance, which validates the theoretical derivation. The real-time simulation results are also provided to demonstrate the dc bus voltage instability.