A Sharing-Branch Modular Multilevel DC Transformer With Wide Voltage Range Regulation for DC Distribution Grids

As the key equipment in dc distribution grids, dc transformer (DCT) suffers from low power density caused by a large number of submodules (SMs). Hence, this article proposes a novel DCT for the interconnection of medium-voltage dc (MVdc) and low-voltage dc (LVdc) distribution grids, which adopts a series-connected half-bridge SM branch (SSB) on MV-side to achieve low voltage stress of devices and reduce the number of SMs. Quasi square wave modulation strategy and phase-shifted control are employed to realize bidirectional power transmission control and zero-voltage switching (ZVS) for all switches. Furthermore, in order to ensure low current stress and ZVS within wide voltage range, a certain number of SMs are constantly inserted or bypassed to achieve voltage matching. Besides, smooth transition control strategy is employed to avoid current spikes caused by abruptly bypassing or inserting SMs. Based on the theoretical analysis, design consideration and control strategy are elaborated. Finally, a 500 kW/6-12 kV/750 V simulation model and a 2 kW prototype are built to verify the proposed DCT.

Automated summary

This article proposes a novel direct current transformer (DCT) for the interconnection of medium-voltage DC (MVdc) and low-voltage DC (LVdc) distribution grids. The DCT uses a series-connected half-bridge submodule branch (SSB) on the MV-side to achieve low voltage stress of devices and reduce the number of submodules. Quasi square wave modulation strategy and phase-shifted control are employed for bidirectional power transmission control and zero-voltage switching (ZVS) for all switches. The effectiveness of the proposed DCT is verified through theoretical analysis and simulation.