Control strategies for Cascaded Modular Solid-State Transformer Operating in PV Power Generation Mode and SVG Mode

The three-phase photovoltaic (PV) solid-state transformer (SST) based on cascaded H-bridge (CHB) topology has the characteristics of expandable modular structure and no need for bulky line-frequency transformer to connect directly with medium voltage grid, so it has great advantages in high-voltage and high-power applications. To further improve system utilization and reduce the construction cost of PV power plant, the converter is able to operate in PV power generation mode during the day, and in static var generation (SVG) mode at night to compensate the reactive power consumption of the whole PV plant. For this purpose, this paper proposes the control strategies suitable for PV power generation mode and SVG mode, respectively. The strategy of PV power generation mode can not only achieve the controls of grid currents, dc-bus voltages and power balance among modules, but also simplify the communication design. The control method of SVG mode takes inter-phase and inter-module power balance into account and has a simple calculation process. Simulation and experimental results validate the effectiveness and feasibility of the proposed methods.