Research on the Control of Unidirectional Power Flow DC Transformer for Photovoltaic DC High-Voltage Convergence Based on Soft Switching

In the centralized photovoltaic grid-connected applications, the high-power unidirectional power flow DC transformer is the key equipment to realize the convergence of DC high-voltage. This paper proposes a DC transformer topology with multi-module whose outputs connected in series, and each module connected with an independent PV battery pack. A single module consists of a boost converter and a LLC resonant converter. The boost converter is used to realize the MPPT of the input photovoltaic battery pack, while LLC resonant converter is used to realizes voltage and power conversion and isolation. IN the paper the soft switching conditions of LLC converter is studied, and the relationship curve between voltage gain and frequency is obtained. On this basis, based on the premise of realizing soft switching, a sub-module LLC transformer control strategy based on the voltage gain-frequency characteristic curve is proposed. Finally, the validity of the proposed strategy is verified by simulation and experiments, and the fault ride through characteristics of the proposed solid-state transformer are studied by simulation.