An Improved Sub-Module Topology for Protecting MMC Power Devices under DC-side Short Circuit Fault

The modular multilevel converter (MMC) is a promising topology for high-voltage direct current (HVDC) system. However, typical half bridge sub-module (HBSM) based MMC is vulnerable to dc-side short circuit fault. To protect power-electronic devices from the damage of MMC dc-side short circuit current, the protective thyristors with small on-resistance connected in anti-parallel with the lower insulated gate bipolar transistor (IGBT) of HBSM are applied. However, the use of a large number of thyristors will bring higher costs and losses to the MMC system. In allusion to this problem, this paper presents an improved sub-module (SM) topology, which not only has the capability of protection, but also uses fewer and cheaper high-power thyristors and has lower power losses than the conventional HBSM-MMC. The effectiveness and performance of the suggested SM topology are verified by the PSCAD/ EMTDC simulations.