A converter arm fault protection system for a voltage source converter wherein the system comprises: a chain-link circuit in series with a director switch ; and a fault controller ; wherein the chain-link circuit comprises a plurality of cells , each cell comprising an energy storage element and a plurality of cell switching elements (109-1 - 109-4) connected with antiparallel diodes (201-1 - 201-4), the cell switching elements (109-1 - 109-4) being configured such that the energy storage element can be selectively connected in series between terminals of the cell or bypassed; wherein each cell switching element (109-1 - 109-4) has an associated cell switching element controller ; characterised in that the director switch comprises a plurality of director switch units (202-1 - 202-n), each director switch unit (202-1 - 202-n) comprising a director switching element and a director switch unit controller ; wherein each cell switching element controller is configured to monitor for a fault current and, in the event a fault current is detected, to turn-off its associated cell switching element and report a fault event to the fault controller ; wherein each director switch unit controller is configured to monitor for a fault current and, in the event a fault current is detected, to report a fault event to the fault controller and to keep its associated director switching element turned-on; and wherein the fault controller is configured to monitor for any reports of fault events to determine whether a fault is occurring and, in the event that a fault is occurring, to order all of the cell switching elements of at least some of the plurality of cells to turn-off. a chain-link circuit in series with a director switch ; and a fault controller ; wherein the chain-link circuit comprises a plurality of cells , each cell comprising an energy storage element and a plurality of cell switching elements (109-1 - 109-4) connected with antiparallel diodes (201-1 - 201-4), the cell switching elements (109-1 - 109-4) being configured such that the energy storage element can be selectively connected in series between terminals of the cell or bypassed; wherein each cell switching element (109-1 - 109-4) has an associated cell switching element controller ; characterised in that the director switch comprises a plurality of director switch units (202-1 - 202-n), each director switch unit (202-1 - 202-n) comprising a director switching element and a director switch unit controller ; wherein each cell switching element controller is configured to monitor for a fault current and, in the event a fault current is detected, to turn-off its associated cell switching element and report a fault event to the fault controller ; wherein each director switch unit controller is configured to monitor for a fault current and, in the event a fault current is detected, to report a fault event to the fault controller and to keep its associated director switching element turned-on; and wherein the fault controller is configured to monitor for any reports of fault events to determine whether a fault is occurring and, in the event that a fault is occurring, to order all of the cell switching elements of at least some of the plurality of cells to turn-off. <EOS>