Isolated Bipolar Modular Multilevel DC-DC Converter with Self-balancing Capability for Interconnection of MVDC and LVDC Grids

Bipolar medium-voltage DC (MVDC) and low-voltage DC (LVDC) grids have the advantages of flexible integration of distributed renewable-energy generation and reliable power supply. In order to achieve voltage conversion, power transfer, and electrical isolation for bipolar MVDC and LVDC grids, a high-power DC-DC converter is essential. Therefore, this paper proposes an isolated bipolar modular multilevel DC-DC converter (BiMMDC) with power self-balancing capability for interconnection of MVDC and LVDC grids. The proposed BiMMDC consists of two series connected MMCs in the MV stage to configure a bipolar MVDC interface, and interleaved converters combined with a dual-transformer are designed in the LV stage to configure the bipolar LVDC interface and to provide a self-balancing capability. Equivalent circuits of two series-connected MMCs and a dual-transformer with interleaved converters are derived. After that, operation principles of the proposed BiMMDC are introduced, considering balanced/ unbalanced power transfer of bipolar LVDC grid and monopolar short-circuit or open-circuit faults at MVDC grid. The control scheme is also presented for the proposed BiMMDC under different operating conditions. Finally, a Matlab simulation and controller hardware-in-the-loop (CHIL) evaluation results are provided to validate the feasibility and effectiveness of the proposed typology and its operating performance.

Automated summary

This paper proposes an isolated bipolar modular multilevel DC-DC converter (BiMMDC) with power self-balancing capability for interconnection of MVDC and LVDC grids. The BiMMDC consists of two series connected MMCs in the MV stage to configure a bipolar MVDC interface, and interleaved converters combined with a dual-transformer in the LV stage to configure the bipolar LVDC interface and provide a self-balancing capability. The operation principles and control scheme of the proposed BiMMDC are discussed, and simulation and CHIL evaluation results are provided to validate its feasibility and effectiveness.