Research on Low-Frequency Fault Ride-Through Control for Offshore Wind Delivery System Based on M3C

This paper systematically analyses the fault characteristics and investigates fault ride-through (FRT) control strategies for a low-frequency (LF) transmission system in offshore wind power based on Modular Multilevel Matrix Converter (M3C). The study addresses transient issues of power imbalance, submodule capacitor overvoltage, and bridge-arm overcurrent arising from three-phase ground faults on both the industrial-frequency (IF) and LF sides. The underlying mechanisms of power surplus and submodule capacitor overvoltage, induced by decoupling control and current-limiting protection during IF-side faults, are examined in detail, along with the transient characteristics of bridge-arm currents under voltage sags on the LF side. Two innovative control strategies are proposed to enhance system resilience: (1) For IF-side faults, a controllable energy dissipation device on the LF side achieves precise dissipation of surplus power via real-time monitoring of the average submodule capacitor voltage. (2) For LF-side faults, the FRT strategy based on dynamic adjustment of the LF modulation voltage rapidly reduces the reference to 0.1 p.u. and restores it linearly at a predefined rate, thereby enabling fault information transmission and wind turbine derating. The effectiveness and feasibility of the proposed scheme are verified through simulations on a 1000 MW system model.