Zero DC voltage ride through of a hybrid Modular Multilevel Converter — Part II: Analysis and suppression of electromagnetic transients

DC short circuit fault is a serious problem modular multilevel converter (MMC)-HVDC has to deal with, in particular for the applications using overhead transmission lines typically in China. This two series papers present a zero DC voltage ride through method during bipolar DC short circuit fault for HVDC system based on hybrid modular multilevel converter, which is composed of half-bridge sub-modules (HBSMs) and full-bridge sub-modules (FBSMs). As the first part, this paper illustrates the principle and control implementation using the negative level output capability of FBSMs to rapidly reduce the converter's output DC voltage and speed up the fault current attenuation. Moreover, it is found out that there are plenty of coordinated schemes between FBSMs and HBSMs during DC fault ride through period, which are dependent on the distributed relationships of fundamental components in converter arm's output voltage between FBSMs and HBSMs. As a result, in order to describe this relationship, a variable, viz., voltage distributing factor, is defined. Finally, comparison studies among the proposed zero DC voltage ride through method of hybrid MMC with different voltage distributing factors, and the conventional fault blocking method of FBSM-based MMC were conducted by simulations in PSCAD.