The application of the back-to-back AC-AC converter consisting of a diode rectifier unit (DRU) and a modular multilevel converter (MMC) in low-frequency alternating current (LFAC) transmission systems can realize low-cost grid integration of offshore wind farms. To mitigate the harmonic content in this system

a low-cost harmonic suppression strategy is proposed. Firstly

the harmonic characteristics of the DRU-LFAC system are analyzed

based on which a novel AC-side current harmonic suppression strategy is proposed. The proposed strategy injects specific current harmonics into the positive and negative DC poles to suppress AC-side current harmonics

while the return path of the DC current harmonics is provided by the neutral line of the bipolar DRU-MMC. Then

the additional power losses during the proposed strategy are calculated

and a comparative economic analysis is conducted against conventional suppression methods. Finally

the effectiveness of the proposed strategy is verified by PSCAD/EMTDC simulations and StarSim controller hardware-in-the-loop experiments. The results demonstrate that the proposed strategy can be adopted to effectively reduce the total harmonic distortion (THD) of AC-side currents and to achieve approximately 70% cost reduction over the full lifecycle of offshore wind power projects

compared with traditional suppression methods.