An Improved Series-Connected Hybrid Valve Based on Diode Rectifier Units and Full-Bridge MMC for Large-Scale Offshore Wind Power Transmission

Abstract

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.