A DC Fault Protection Scheme for Half-bridge Sub-module Based MMC-HVDC

The widely used half-bridge sub-module (HB-SM) topology in modular multi-level converter based HVDC systems (MMC-HVDC) is vulnerable to DC-side faults. The dual thyristor switch scheme (DTSS) eliminates the freewheeling effect and allows dc-link fault current decaying to zero freely, but it has the disadvantages of slow isolation speed and high overcurrent level. This paper proposes a new fault isolation method based on improved redundant sub-module topology and DTSS. With the new added thyristors and IGBT, the capacitor in the redundant sub-module can be quickly fed into the DC fault circuit to reduce the fault current peak and accelerate the fault current decay rate. Finally, simulation tests in PSCAD/EMTDC verifies that this approach realizes a substantial increase in the fault isolation speed and a significant decline of the DC overcurrent level without tripping AC circuit breakers, which provides the prerequisites of rebuilding the dc-link voltage and restarting power transmission.