An Enhanced Fault Ride-Through Capability for MTDC Systems Using Vector Current Control and Inherent Energy of Submodules of MMC

Modular multi-level converter (MMC) has been recognized as the most prominent topology in building the multi-terminal direct current (MTDC systems) due to its numerous advantages over other converter topologies. The primary concern for the operation of such advanced MTDC grids is to ensure the reliability and safety of the electrical equipment during dynamic and transient conditions following standard grid codes. This research work proposes a combination of different control strategies developed for MMC based MTDC systems to promote fault ride-through capability during three-phase transients and voltage dips. The proposed control architecture contains inner and outer control loops for MMC stations. Along with the different control strategies, the inherent energy available in the capacitors of submodules MMC has also been utilized, which ensures constant power delivery to the load system during three-phase transients & voltage dips and provides power oscillation damping (POD) with quick post fault recovery operation. The energy dimensions required for capacitors of each MMC station are calculated. The proposed control strategies have been validated with an experimental setup of a four-terminal MMC based MTDC system developed in the simulation environment. The proposed control strategies envision a robust fault ride-through capability with reduced oscillations and total harmonic distortion (THD). Such a method is more reliable for an MTDC system than the classical solutions, which are based on drawing increased current from the grid or using additional circuitries. Moreover, the proposed control strategies can work efficiently for any MTDC system as compared to previously presented solutions, which are restricted to two-terminal HVDC systems.