DC Fault Currents for FB-MMC HVDC with Bipolar Configuration

Inhalt:
Full bridge modular multilevel converter HVDC schemes with bipolar configuration represent a promising approach to meet the requirements introduced by the integration of HVDC systems in highly meshed AC grids mainly utilizing overhead lines. As this type of HVDC scheme has not been implemented in any project worldwide, this paper intends to give an overview of the behavior of the system during DC faults with a focus on fault currents. A classification in four different fault categories is introduced to cover the full range of possible DC side faults and the expected behavior of the system during the respective faults is explained. EMT-type simulations are conducted to show that the requirements – namely fast DC fault clearance and recovery as well as redundancy during pole-to-ground faults – can be met by this scheme. It is shown that pole-to-ground and pole-to-line faults can be detected and cleared easily and that the desired behavior of the system can be achieved. Faults involving the dedicated metallic return conductor (pole-to-DMR and DMR-to-ground) pose additional challenges as a proper fault behavior of the converters cannot be achieved by simple overcurrent fault detection.