An earthing switch is installed in one phase (AC side) of the MMC converter between the upper and lower arm. In normal operation the switch is open. In case of pole to ground fault, after opening of the AC breaker the earthing switch can be closed. It is then possible to control separately the positive and negative pole to ground voltages. The healthy pole to ground voltage can be rapidly discharged to 1pu, by transferring the energy stored in the line stray capacitance into the SM capacitors. After the de-ionization time is elapsed, the faulty pole to ground voltage can be controlled and restored to 1pu. This operation can be repeated several time in case the fault is still present. Once the voltage is reestablished the AC breaker can be reclosed and power restored. This solution could speed up the restoring time and reduce it up to 0.3-0.5s.
Patent appl. FR1871143 (+ FR1753433)