Isolated Bipolar Modular Multilevel DC-DC Converter with Self-balancing Capability for Interconnection of MVDC and LVDC Grids
Jinmu Lai, Xin Yin, Yaoqiang Wang, Lin Jiang, Zia Ullah, Xianggen Yin
Abstract
Bipolar medium-voltage DC (MVDC) and lowvoltage 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 shortcircuit 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.