Analysis and protection of valve-side faults in offshore bipolar MMC-HVDC systems based on active neutral voltage shift

This paper provides a practical solution for valve-side faults in offshore high-voltage direct current (HVDC) systems, which can guarantee the stable operation of offshore AC systems during fault ride-through. Considering that the faulty modular multilevel converter (MMC) will be blocked as a harmonic source, the fault interactions in offshore AC systems occur not only under fundamental frequency, i.e., between wind power plants and MMCs, but also under harmonic frequencies, i.e., between the faulty MMC and other converters, thereby greatly complicating the fault analysis and protection design. In this scenario, this paper proposes a novel protection scheme based on neutral voltage shift, which couples the commutation of the lower arms of the faulty MMC to that of the upper arms through the grounding impedance, this is proved not only to enable the fast fault detection but also to suppress the overvoltage of the sub-module capacitors. Moreover, the AC chopper with parallel capacitors is suggested to replace the DC chopper, which is more beneficial for improving the existence of equilibrium points and the transient performance of the system. Finally, the feasibility of the analysis is demonstrated by Matlab/Simulink results.