Series-Parallel Multiple Integrated Modular Multilevel DC-DC Converter for All-DC Offshore Wind Power System

All-dc offshore wind power system is a promising solution for long distance and high power transmission. This paper proposes a series-parallel multiple integrated modular multilevel dc-dc converter (SPMI-MMC) for all-dc offshore wind power systems to meet its demand for high voltage, high power, and large-step ratio dc voltage transmission. Different from the traditional isolated dc-dc converter, the submodules (SMs) of the SPMI-MMC adopt the input-parallel-output-series (IPOS) arrangement to achieve a large step ratio. The symmetrical parallel multiple submodule (SPM-SM) is proposed to realize parallel multiple of SMs through coupling reactors (CRs), which not only can enhance the large current operation ability but also can cancel out the nonzero-sequence high frequency harmonics. Considering the cost factor, the simplified full-bridge double submodule (S-FBDSM) is first introduced in dc-dc converter. It is economical and has good bidirectional dc-fault blocking ability. Digital simulation and real time hard-in-loop (HIL) simulation tests are conducted to verify the effectiveness and superiority of the proposed SPMI-MMC.