Suggesting Hybrid HB and Three-Quarter-Bridge MMC-Based HVDC Systems: Protection and Synchronous Stability Under DC Faults

The HVDC systems have been employing modular multilevel converters (MMC) due to their unique high voltage high power potential. However, the lack of fault ride-through capability imposes restrictions on MMCs that are based on half-bridge (HB) modules. Since it is necessary to limit the fault currents to protect MMCs from serious short circuit faults against pole-to-pole DC fault, several protection schemes have been introduced to deal with the DC short circuit faults. This paper proposes a new hybrid MMC protective topology consisting of a combination of the HB and three-quarter-bridge (TQB) modules. The TQB behaves like an HB under normal operation and has low power losses compared to other protective topologies. Modulation and control of this configuration are like HB-based MMC. In the proposed hybrid protective topology, during a pole to pole DC fault, the fault current can be either blocked or be limited by TQB-SMs. Both simulations and experiments were performed to verify the operation of the suggested hybrid MMC in different modes of operation.