Harmonic suppression and fault ride-through method of diode-rectifier-based hybrid high-voltage direct current system for offshore wind farms integration

The diode-rectifier (DR)-based high-voltage direct current (HVDC) transmission is regarded as a low-cost alternative to the HVDC transmission for offshore wind farms (OWFs) integration. However, the DR would generate harmonic voltages in the offshore grid, which greatly affects the power quality. To solve this problem, this paper utilizes a parallel-connected small-capacity auxiliary converter to suppress the harmonic voltages and provide black-start energy for the OWFs. In addition, the submodule branches (SMBs) of the auxiliary converter can be used as part of the HVDC chopper. On this basis, a coordinated grid fault ride-through (FRT) strategy is proposed in this paper. After a grid fault occurs, the HVDC chopper constructed by the auxiliary converter is able to constrain the DC-link voltage. At the same time, the OWFs are controlled to reduce the output active power. As a result, the cost of the FRT can be significantly reduced. The proposed harmonic voltage suppression and the FRT method are verified through the simulation model in Simulink.