Study on pre-charging process and pre-insertion resistors selection of VSC-HVDC system with different MMC topologies

Here, the uncontrolled rectification pre-charging characteristics of the VSC-HVDC system with different modular multilevel converter (MMC) topologies such as full-bridge, half-bridge, and hybrid topologies are proposed based on theoretical analysis and PSCAD/EMTDC simulations. Different MMC topologies have a great impact on the selection of pre-insertion resistors. The surge energy of pre-insertion resistors is the lowest with the full-bridge sub-module (FBSM) MMC, while much higher with the half-bridge sub-module (HBSM) MMC. In order to prevent start-up failure caused by extremely non-uniform voltage between FBSM and HBSM of hybrid MMC, the FBSM should be turned to half-bridge state by valve control system as soon as possible so as to carry out the second charge, and then balance the sub-modules voltage by bypass certain high-voltage sub-modules. The half-controlled charging process has a great influence on the absorbed energy of the pre-insertion resistors of hybrid MMC. The research results can be used to guide the selection of VSC-HVDC pre-insertion resistors and start-up strategy making.