Operation of a Modular DC-DC Converter for Hybrid Interconnection of Monopolar and Bipolar HVDC Links

Multiterminal high-voltage dc (HVdc) grid has huge potential in long-distance bulk power transmission. Such HVdc structures have reached greater heights due to the concept of hybrid interconnections in HVdc by promoting power-sharing between different technologies of HVdc terminals. Modular multilevel converter (MMC) technology has further paved its way to act as a high-power HVdc transformer, which has encouraged the invention of many dc-dc MMC topologies. This article furthers the operation of an existing modular dc-dc converter for hybrid interconnection by proposing philosophies that establish its operation when one of the poles of an HVdc terminal is removed while the other terminal is bipolar. Moreover, hybrid arms, comprising of both full-bridge and half-bridge submodules are used in the converter, instead of only the full-bridge, to optimize the number of devices. The effectiveness of the proposed philosophies is demonstrated in multiterminal operation by simulation in the MATLAB/Simulink platform. The performance of these philosophies is also verified in a down-scaled laboratory prototype.