The Smart Transformer (ST) is a solid-state transformer with control and communication functionalities, interfacing medium voltage and low voltage (LV) grids. The ST can work in both ways: it can operate in a radial network independently, and it also provides meshed operation. In particular, a meshed ST-fed grid is able to optimize the power flow of the network, therefore largely avoiding the reinforcement of utilities. Though the ST can address system-level issues, challenges in terms of control and stability are encountered. The modeling as well as stability analysis of ST converter and its control for meshed operation have rarely been studied. More importantly, the interactions between ST and grid-interfaced converters during meshed operation have not yet been investigated. To ensure reliable ST-fed grid, this paper develops a complete model of ST LV converter considering all the key elements for the meshed operation. System stability is assessed based on the developed model for different scenarios. The interactions between the ST and local grid converters are examined to provide comprehensive design guidelines for the meshed ST-fed grid. The control strategy and the theoretical analysis are validated by the simulation and experimental results.

The Smart Transformer (ST) is a solid-state transformer with control and communication functionalities, interfacing medium voltage and low voltage grids. The ST can independently operate in a radial network configuration, and also in a meshed grid operation. The meshed operation has high potential to optimize the power flow in the network, avoiding or postponing the reinforcement of distribution grids. Nevertheless, the control and synchronization during the meshed operation are not trivial. The perturbation of the voltage in transition between the two operations is inevitable. This could lead to uncontrolled power flow and endanger the meshed operation. Moreover, the stability of the meshed grid has seldom been studied. To address these issues, this paper proposes a voltage control with power-based synchronization for the ST. This allows the universal operation of both radial and meshed grids, while ensuring smooth transition. Modeling and stability analysis of such a system are investigated to make sure of safe operation. Simulation and experimental results are provided to validate the effectiveness of the proposed control and theoretical analysis.