Background Harmonic Transfer and Mitigation in a Two-Terminal MMC-HVDC System

Recently, the modular multilevel converter (MMC) topology has been increasingly applied in power systems such as high voltage direct current (HVDC) transmission systems, for the interconnection of offshore wind power plants into onshore AC power systems, interconnection of asynchronous AC power systems, etc. However, in some cases such as the HVDC system connected with wind power plants or hybrid HVDC systems, background harmonics may arise at the PCC of systems, these harmonics may transfer to HVDC systems and linked AC power systems, causing adverse effects or unexpected faults to the systems. The harmonic analysis of MMC phase-leg voltage under the influence of background harmonics is depicted in this work, from which a harmonic compensation method is proposed to mitigate the harmonic propagation in the system. Simulation results based on the model built in PSCAD/EMTDC platform approve the effectualness of the recommended method. The suggested method not only mitigates the harmonic transfer from one AC source to another in the HVDC transmission system, but also benefits in avoiding possible harmonic resonances in the system as well as the reduction of costly harmonic filters.