A Multiport DC Solid-State Transformer for MVDC Integration Interface of Multiple Distributed Energy Sources and DC Loads in Distribution Network

In the traditional dc distribution networks with both a low-voltage dc (LVdc) and a medium-voltage dc (MVdc) bus, dc units, such as photovoltaics solar, storage devices, and dc loads, are connected to the LVdc bus through LVdc converters, such as dc/dc boost converters and bidirectional buck-boost dc/dc converters. In this approach, no galvanic isolation is provided and two power conversion stages are needed between the dc units and the MVdc grid, hence leading to a high number of converters and higher costs. To address these shortcomings, this article proposes a novel multiport dc solid-state transformer (MDCSST) to interface these dc units directly to the MVdc bus. Multiple modules are connected in series on the medium-voltage side to the MVdc bus, whereas dc units are independently connected on the low-voltage side of the MDCSST. Compared with a traditional dc distribution network, the proposed scheme connects dc units to the MVdc bus without an extra converter or an LVdc bus, therefore, saving cost and reducing the number of converters. In addition, dc units are galvanically isolated by high-frequency transformers. The main challenge of the MDCSST is to resolve the voltage-imbalance problem on the MVdc side, which is caused by the dc units' power differences. An LC branch is used to balance the voltages among the modules and to transfer their differential powers. Simulations and experiments were carried out to validate the proposed approach and verify the theoretical analysis.