Harmonic Analysis of Transmission Scheme for Offshore Wind Farms Based on Diode Rectifier Unit

The diode rectifier unit (DRU) based transmission scheme is the technology hotspot in the field of transmission scheme for offshore wind farms. DRU not only realizes the rectification but also brings harmonic problems to the system. Firstly, in this paper, a general mathematical model is built for the multiple rectifier circuit commonly used in DRU, and the frequency and the current harmonics on the AC side are derived theoretically, and the harmonic characteristics of the 12‐pulse and 24‐pulse rectifier circuits are compared and analyzed. Secondly, the equivalent circuit is constructed for the wind turbine, submarine cable, DRU and AC filters. Thirdly, taking an offshore wind farm integration project as a case, DRU adopts the 12‐pulse and 24‐pulse rectifier circuits to build a 257‐node offshore power system model, respectively. The voltages of harmonics at the point of common coupling (PCC) are calculate under different orders of harmonics, and the corresponding AC filters are designed reasonably. The offshore wind power system with different DRU rectifier without AC filters at PCC is simulated by PSCAD/EMTDC software to verify the accuracy of the harmonic calculation method proposed in this paper. Finally, the harmonics of DRU using different rectifier circuits before and after the installation of AC filters are compared respectively, and the harmonic characteristics and filtering schemes of transmission scheme for offshore wind farms are summarized.