Protection scheme for DC lines in AC/DC hybrid distribution grids with MMCs

The reliable identification of the fault zone, as well as the coordination between downstream and upstream relays and AC and DC protections, is one of the difficulties in DC line protection. Existing methods for identifying faulty zones, which rely on the physical boundary formed by smoothing and filtering reactors, are unsuitable for AC/DC hybrid grids with modular multilevel converters (MMCs). In view of this, after analyzing the fault characteristics of DC lines, the protection zones in AC / DC hybrid systems are divided, and the DC line protection scheme, consisting of main protection, local and remote backup relays, is designed. Considering the possibility of the AC-side protection failure after faults, especially single-phase grounded faults at the AC outlet of the converter, the AC outlet of the converter is designed to be included in the remote backup protection range of upstream lines of the converter. Moreover, two-terminal current direction information of the protected line is used to determine the fault location selectively. Finally, in the simulation model of a PV grid-connection system, the reliability, selectivity and quickness of the proposed protection scheme are verified.