Voltage Balance Control of Five-Level Cascaded H-Bridge Rectifier-Based Smart Transformer

The Smart Distribution Grid (SDG) applications have a high potential to improve the power system performance under the high penetration of distributed energy resources. Nevertheless, the conventional transformer (CT), which is the most important component of the distribution system, cannot fulfill the SDG requirement. In this regard, the modular smart transformer (ST) has emerged as a promising alternative to CT in the SDG. The structure of this transformer includes power conversion stages and multiple DC buses on the Medium Voltage (MV) and Low Voltage (LV) sides. This paper proposes an ST model based on a five-level Cascaded H-Bridge (CHB) rectifier in the MV AC-DC power conversion stage, six dual active bridge converters in the high-frequency DC-DC power conversion stage, a three-phase four-leg inverter in the LV DC-AC power conversion stage, and six DC buses on the MV and LV side, for SDG applications. The use of the four-leg inverter in the ST model enables to improve the quality of the grid voltage, minimize the neutral current and provide power supply for a three-phase or single-phase load. Moreover, a voltage balance control for the CHB rectifier is presented in this paper to eliminate the voltage unbalance problems in DC buses and improve the ST performance. Finally, the simulation results verify the good performance of the proposed control system.