DC Fault Tolerant Hybrid Multilevel Converter Topologies for High Power Applications

Modular multilevel converter (MMC) based high voltage direct current (HVDC) systems are quite popular in high power transmission systems as it offers low losses, high quality output voltage, modular and scalable structure as compared to the two level voltage source converter based HVDC systems. However, inability to block dc fault current is a major concern of conventional MMC-HVDC system, especially for the overhead HVDC system. In this paper, the performance of conventional MMC with half bridge submodules is analyzed for dc side fault. In the event of a dc side fault, the conventional MMC acts as an uncontrolled diode rectifier and feeds very high current to dc fault. To tackle this problem, different techniques have been discussed in the literature. This paper gives a brief overview of dc fault tolerant techniques discussed in the literature. Moreover, in this paper, a detailed comparison between different hybrid topologies with dc fault tolerant capability is performed on the basis of required number of switches and capacitors. Furthermore, the fault tolerant capability of MMC with bipolar (full bridge) submodule is experimentally validated.