Control Design and Fault Handling Performance of MMC for MMC-Based DC Distribution System

Abstract

Modular multilevel converters (MMCs) have emerged as a viable choice in future DC grid architectures due to their scalability to meet voltage level requirements. However, MMC-based DC distribution systems are at risk of short-term outages during the faults in either the DC or AC networks feeding the MMC, so it remains a challenge to accomplish AC and DC fault ride-through (FRT) capability in such applications. To ensure stable operations of the DC terminals, FRT strategies are required for the faults on both the AC and DC sides of the converter. This paper proposes a FRT strategy for the AC and DC side of the converter to ensure stable and economically viable operation of the DC distribution network. The asymmetrical faults in the upstream AC grids are managed by using the integrated energy of the MMC. Whereas, the DC FRT capability of the MMC is accomplished by changing the redundant submodules of the MMC to full-bridge submodules (FBSMs), thus allowing a DC FRT to be achieved by using DC circuit breakers that are low cost and reduced in size. Applying the proposed DC FRT strategy, which makes possible the use of low-cost and reduced in size DC circuit breakers in DC distribution, results in a reduction in the overall initial investments.INDEX TERMS Modular multilevel converter, DC FRT, Asymmetrical fault in AC grid, DC faults.

An Enhanced AC Fault Ride through Scheme for Offshore Wind-Based MMC-HVDC System

“…To allow WGs to operate at a variable speed, the B2B converter injects active power into the grid from the rotor when the speed of rotor is over the synchronous speed and absorbs reactive power from the grid and feeds it back to the grid when the rotor speed is below the synchronous speed. The rotor-side converter (RSC) connected to the rotor helps with speed control, while the grid-side converter (GSC) compensates for the DFIG's reactive power demand at the grid connection point [42]. Type 4 WG employs either a squirrel-cage induction generator (SCIG), a WRIG, or a permanent magnet synchronous generator (PMSG) in offshore wind applications.…”