A power converter, for use in a high voltage power transmission network, comprising: first and second DC terminals (12, 18) between which the power converter is operable to generate a voltage difference; and a control unit operatively connected with the power converter and configured to operate in a normal mode during normal operation of the power converter and in a fault mode when a fault occurs in a respective DC power transmission medium (14, 20) operatively connected in use to one of the first or second DC terminals (12, 18), wherein the control unit in the normal mode is configured to generate a normal operating voltage difference between the first and second DC terminals (12, 18) with each of the first and second DC terminals (12, 18) having a respective positive or negative voltage potential with respect to earth, wherein the control unit is configured to operate in the fault mode when a short-circuit to earth fault occurs in a respective DC power transmission medium (14, 20) operatively connected in use to one of the first or second DC terminals (12, 18); and wherein the control unit in the fault mode is configured to generate a modified operating voltage difference between the first and second DC terminals (12, 18), while maintaining the respective voltage potential with respect to earth of the other of the first and second DC terminals (12, 18), wherein the other of the first and second DC terminals (12, 18) corresponds to a DC terminal that is not short-circuited to earth, the modified operating voltage difference being lower than the normal operating voltage difference to control the level of voltage experienced by a respective DC power transmission medium (14, 20) operatively connected in use to the other of the first and second DC terminals (12, 18); and characterized in that the power converter includes at least one converter limb, the or each converter limb including series-connected first and second modules (38, 40), each of the modules (38, 40) including at least one switching element and at least one energy storage device, the or each switching element and the or each energy storage device in each module (38, 40) combining to selectively provide a voltage source, and being able to conduct current in two directions, in order to generate the modified operating voltage difference. first and second DC terminals (12, 18) between which the power converter is operable to generate a voltage difference; and a control unit operatively connected with the power converter and configured to operate in a normal mode during normal operation of the power converter and in a fault mode when a fault occurs in a respective DC power transmission medium (14, 20) operatively connected in use to one of the first or second DC terminals (12, 18), wherein the control unit in the normal mode is configured to generate a normal operating voltage difference between the first and second DC terminals (12, 18) with each of the first and second DC terminals (12, 18) having a respective positive or negative voltage potential with respect to earth, wherein the control unit is configured to operate in the fault mode when a short-circuit to earth fault occurs in a respective DC power transmission medium (14, 20) operatively connected in use to one of the first or second DC terminals (12, 18); and wherein the control unit in the fault mode is configured to generate a modified operating voltage difference between the first and second DC terminals (12, 18), while maintaining the respective voltage potential with respect to earth of the other of the first and second DC terminals (12, 18), wherein the other of the first and second DC terminals (12, 18) corresponds to a DC terminal that is not short-circuited to earth, the modified operating voltage difference being lower than the normal operating voltage difference to control the level of voltage experienced by a respective DC power transmission medium (14, 20) operatively connected in use to the other of the first and second DC terminals (12, 18); and characterized in that the power converter includes at least one converter limb, the or each converter limb including series-connected first and second modules (38, 40), each of the modules (38, 40) including at least one switching element and at least one energy storage device, the or each switching element and the or each energy storage device in each module (38, 40) combining to selectively provide a voltage source, and being able to conduct current in two directions, in order to generate the modified operating voltage difference.