Harmonic Analysis of Medium-Frequency DRU Under Non-ideal Conditions

DC transmission scheme for medium-frequency grid-forming offshore wind farms is the highly anticipated offshore wind power project. Its rectifier is diode rectifier unit (DRU), which is involved in the issue of harmonics. This paper uses time-domain simulation to investigate the harmonic of DRU based a 12-pulse rectifier configuration under non-ideal operating conditions. The analysis establishes the harmonic profile of DRU with an ideal condition as the reference baseline, against which various non-ideal scenarios are simulated, encompassing both asymmetric commutation reactance and AC supply background harmonics in simulation software PSCAD/EMTDC. The investigation systematically evaluates seven distinct cases: (1) commutation reactance variations (both above and below nominal values), and (2) background harmonic contamination comprising negative-sequence fundamental component, positive- and negative-sequence second harmonics, and positive- and negative-sequence third harmonics. The simulation results quantitatively assess how 1% deviations in asymmetry factors influence electrical parameters at the point of common coupling (PCC). Comparative evaluations of total harmonic distortion (THD) and commutation overlap angles are performed across all test conditions. The findings reveal that background harmonic distortion exerts a more pronounced effect on system performance compared to other non-ideal operational factors.