Start-Up Strategy of DR-MMC Based on Thyristors in Parallel for Offshore Wind Farms

Modular multilevel converters (MMCs) based HVDC is the primary technological pathway for far offshore wind power delivery. MMC contains numerous sub-modules, leading to high costs for the offshore converter platform, which is sensitive to both volume and weight. To reduce the cost of offshore platforms, the diode rectifier (DR) is configured in series with MMC. However, the unidirectional current flow characteristics of DR make the start-up difficult. To realize the start-up, onshore converter encompasses two MMCs with their voltage levels corresponding to offshore MMC and DR respectively. What's more, thyristors are utilized to provide a power route from onshore to offshore. The detailed start-up process consists of three steps. Firstly, two onshore MMCs are charged and triggered to control the DC voltage. Next, offshore MMC is charged through thyristors in parallel with offshore MMC and activated to control the offshore AC voltage. With this AC voltage, wind turbines (WTs) start up and DR is put in. After the normal power achieve normal power output, the thyristors are naturally switched off. Wind power is then transferred through the MMC and DR, completing the start-up. The start-up strategy is validated by simulation results in PSCAD.