Four-channel control of hybrid MMC with pole-to-ground DC fault ride through capability

Compared with half bridge MMC, the hybrid MMC based on full bridge sub-modules (FBSM) and half bridge sub-modules (HBSM) has more control freedoms. It has been reported that the hybrid MMC can regulate the cell capacitor voltages independent of the DC-link voltage. Thus it is able to ride through pole-to-pole DC fault without tripping IGBTs. But few literatures studied the fault ride through control of pole-to-ground (PTG) DC fault which happens more commonly and frequently. To ride through PTG DC fault, this paper presents a four-channel control for symmetrical monopole hybrid MMC. Firstly, system dynamics of hybrid MMC under PTG DC fault is analysed. Based on the analysis, a four-channel control scheme is proposed for hybrid MMC. This controller is mainly composed of a DC current control loop, an AC current control loop and outer control loops. Fault ride through capability under permanent and temporary PTG DC faults is verified by simulations using PSCAD/EMTDC.