An improved hybrid valve based on diode rectifier units (DRUs) and
full-bridge MMC (FB-MMC) to transmit large scale offshore wind power to
onshore is proposed to balance the performance and cost of pure DRU- or
MMC-based schemes. Approximate one third of DC voltage is constructed by
the FB-MMC and the remaining by DRUs. Active power-DC voltage droop
control is proposed to regulate the DC voltage of FB-MMC to adaptively
transmit the offshore wind farm active power to onshore. Meanwhile, the
reactive power consumed by DRUs and generated by wind turbine converters
(WTCs) is dynamically balanced by the FB-MMC. During start-up of
offshore wind farm, the DC link capacitors of WTCs and auxiliary systems
can be energized from onshore. And most importantly, DC pole-to-pole
short circuit fault can be ridden-through as quickly as possible by the
reverse and double DC voltage of FB-MMC with no severe submodule (SM)
capacitor overvoltage after fault occurrence. The system performance is
investigated by time-domain simulations in PSCAD/EMTDC. Thus, the
feasibility of the proposed scheme is verified.
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