An Improved Hybrid MMC with Thyristor-Switched Supercapacitors for Fault Ride-Through Support

The hybrid MMC (HMMC) based on half fullbridge submodules (FBSMs) and half half-bridge submodules (HBSMs) has become a promising topology in HVDC for its dc fault ride-through capability. However, without extra energy storage units (ESUs), a huge power shortage of ac-side during dc fault will inevitably occur. The sudden power loss may cause a significant frequency drop and load shedding, which reduces the stabilty and economy of ac grid. Therefore, an improved HMMC with thyristor-switched supercapacitors (SCs) is discussed in this paper. Compared with the general HMMC, no extra IGBTs are required and FBSMs are ultilized not only to generate the MMC output, but also to transfer the energy between SM capacitors and SCs. By appropriate control, voltage deviation of SM capacitors can be avoided in various conditions, which is proved by quantitative analysis. Simulation results validate the effectiveness of the proposed scheme.