A chain-link module, for connection in series with other chain-link modules to form a chain-link converter selectively operable to provide a stepped variable voltage source within a voltage source converter, the chain-link module comprising: a first pair of series-connected switching elements (14A, 14B) separated by a first connection terminal and connected in parallel with first and second series-connected energy storage devices (18, 20); and a second pair of series-connected switching elements (14C, 14D) separated by a second connection terminal and connected in parallel with one or other of the first and second energy storage devices (18, 20); a module controller configured to control switching of the switching elements 14A, 14B, 14C, 14D, in use, to selectively: (i) directing current through the first and second energy storage devices (18, 20), whereby the chain-link module provides a positive voltage across the first and second connection terminals (16, 24) that can oppose a fault current flowing from the first connection terminal to the second connection terminal; (ii) causing current to bypass the first and second energy storage devices (18, 20) whereby the chain-link module provides zero voltage; and (iii) directing current through the one of the first and second energy storage devices (18, 20) with which the second pair of switching elements (14C, 14D) is connected in parallel, whereby the chain-link module provides a negative voltage across the first and second connection terminals (16, 24) that can oppose a fault current flowing from the second connection terminal to the first connection terminal, wherein the switching elements (14A, 14B, 14C, 14D) are configured to be selectively switched by the module controller which is programmed to control such switching, in order to direct current through the chain-link module along different current flow paths in order to cause the chain-link module to provide the positive, zero and negative voltage, and the chain-link module being characterised by further having the module controller programmed to cause the one of the first and second energy storage devices (18, 20) that is connected in parallel to the second pair of series-connected switching elements (14C, 14D) to store more energy than the other one of the first and second energy storage devices (18, 20). a first pair of series-connected switching elements (14A, 14B) separated by a first connection terminal and connected in parallel with first and second series-connected energy storage devices (18, 20); and a second pair of series-connected switching elements (14C, 14D) separated by a second connection terminal and connected in parallel with one or other of the first and second energy storage devices (18, 20); a module controller configured to control switching of the switching elements 14A, 14B, 14C, 14D, in use, to selectively: (i) directing current through the first and second energy storage devices (18, 20), whereby the chain-link module provides a positive voltage across the first and second connection terminals (16, 24) that can oppose a fault current flowing from the first connection terminal to the second connection terminal; (ii) causing current to bypass the first and second energy storage devices (18, 20) whereby the chain-link module provides zero voltage; and (iii) directing current through the one of the first and second energy storage devices (18, 20) with which the second pair of switching elements (14C, 14D) is connected in parallel, whereby the chain-link module provides a negative voltage across the first and second connection terminals (16, 24) that can oppose a fault current flowing from the second connection terminal to the first connection terminal, (i) directing current through the first and second energy storage devices (18, 20), whereby the chain-link module provides a positive voltage across the first and second connection terminals (16, 24) that can oppose a fault current flowing from the first connection terminal to the second connection terminal; (ii) causing current to bypass the first and second energy storage devices (18, 20) whereby the chain-link module provides zero voltage; and (iii) directing current through the one of the first and second energy storage devices (18, 20) with which the second pair of switching elements (14C, 14D) is connected in parallel, whereby the chain-link module provides a negative voltage across the first and second connection terminals (16, 24) that can oppose a fault current flowing from the second connection terminal to the first connection terminal, wherein the switching elements (14A, 14B, 14C, 14D) are configured to be selectively switched by the module controller which is programmed to control such switching, in order to direct current through the chain-link module along different current flow paths in order to cause the chain-link module to provide the positive, zero and negative voltage, and the chain-link module being characterised by further having the module controller programmed to cause the one of the first and second energy storage devices (18, 20) that is connected in parallel to the second pair of series-connected switching elements (14C, 14D) to store more energy than the other one of the first and second energy storage devices (18, 20).