Fault protection for voltage source converters

Fault protection of a voltage source converter (VSO) comprising a chain-link circuit 103 in series with a director switch 104. The chain-link circuit comprises a plurality of cells 106, each comprising an energy storage element, a plurality of cell switching elements 109 such that the energy storage element can be selectively connected in series between terminals of the cell or bypassed, and a cell switching element controller 501, 502. The director switch comprises a plurality of director switch units 202, each having a director switching element 110 and a director switch unit controller 503. Each cell switching element controller monitors for a fault current and, if detected, turns-off its associated cell switching element and reports a fault event to a fault controller 504. Each director switch unit controller also monitors for a fault current and, if detected, reports a fault event to the fault controller, but the director switch unit control keeps its associated director switching element turned-on. The fault controller monitors for any reports of fault events and, in the event of a fault occurring, orders all of the cell switching elements of at least some of the plurality of cells to turn-off.