A cost-effective converter system for HVDC links integrated with offshore wind farms

This paper proposes a cost-effective configuration of high-voltage direct current (HVDC) links for integrating the offshore wind farms with the grid, which consists of a hybrid of a twelve-pulse diode rectifier (12P-Rec) and a modular multilevel converter (MMC) for the sending-end converter on the offshore side and a fully-rated MMC for the receiving-end converter on the onshore side. For this scheme, the receiving-end MMC (REMMC) controls the DC voltage of the HVDC link, whereas the sending-end MMC (SEMMC) regulates the offshore AC voltage at a constant magnitude and frequency. Also, the SEMMC operates as if it is an active power filter to compensate for the high order harmonic currents such as 11^th-, 13^th-, 23^rd-, and 25^th-ones produced by the 12P-Rec. By utilizing the diode rectifier, the cost and power loss of the HVDC converter are lower than those of the fully-rated voltage-source converter (VSC), whereas the performance is kept almost the same. The validity of the proposed scheme is verified by the PSCAD/EMTDC simulation results for a 600-MW HVDC transmission system integrated with an offshore wind farm.