A current flow controller (10, 110, 210) comprising: a plurality of terminals (12a, 12b, 14a, 14b, 50, 52, 54) for connection, in use, to a plurality of DC power transmission media (16, 18, 44) such that each DC power transmission medium (16, 18, 44) is connected to at least one of the plurality of terminals (12a, 12b, 14a, 14b, 50, 52, 54); a current flow control unit interconnecting the plurality of terminals (12a, 12b, 14a, 14b, 50, 52, 54), the current flow control unit including at least one switching element (24, 28, 58a, 58b, 60a, 60b, 62a, 62b), the or each switching element (24, 28) of the current flow control unit being operable to selectively establish a current path through the current flow controller (10, 110, 210) between at least one DC power transmission medium (16, 18, 44) and at least one other DC power transmission medium (16, 18, 44), and the or each switching element (24, 28) further being connected to at least one energy storage device (26, 30, 56) to selectively provide at least one voltage source; and a switching control unit to control switching of the or each switching element (24, 28) of the current flow control unit to selectively establish the said current path through the current flow controller (10, 110, 210) and to selectively connect the or each energy storage device (26, 30, 56) between the said at least one DC power transmission medium (16, 18, 44) and the said at least one other DC power transmission medium (16, 18, 44) in order to inject a voltage drop (V 1, V 2), in use, into each DC power transmission medium (16, 18, 44) and thereby divert current through the current path from the said at least one DC power transmission medium (16, 18, 44) to the said at least one other DC power transmission medium (16, 18, 44) so as to simultaneously regulate current flow (I 1, I 2, I 3) in each DC power transmission medium (16, 18, 44) and divert energy from the said at least one DC power transmission medium (16, 18, 44) into the said at least one other DC power transmission medium (16, 18, 44) via the current flow control unit. a plurality of terminals (12a, 12b, 14a, 14b, 50, 52, 54) for connection, in use, to a plurality of DC power transmission media (16, 18, 44) such that each DC power transmission medium (16, 18, 44) is connected to at least one of the plurality of terminals (12a, 12b, 14a, 14b, 50, 52, 54); a current flow control unit interconnecting the plurality of terminals (12a, 12b, 14a, 14b, 50, 52, 54), the current flow control unit including at least one switching element (24, 28, 58a, 58b, 60a, 60b, 62a, 62b), the or each switching element (24, 28) of the current flow control unit being operable to selectively establish a current path through the current flow controller (10, 110, 210) between at least one DC power transmission medium (16, 18, 44) and at least one other DC power transmission medium (16, 18, 44), and the or each switching element (24, 28) further being connected to at least one energy storage device (26, 30, 56) to selectively provide at least one voltage source; and a switching control unit to control switching of the or each switching element (24, 28) of the current flow control unit to selectively establish the said current path through the current flow controller (10, 110, 210) and to selectively connect the or each energy storage device (26, 30, 56) between the said at least one DC power transmission medium (16, 18, 44) and the said at least one other DC power transmission medium (16, 18, 44) in order to inject a voltage drop (V 1, V 2), in use, into each DC power transmission medium (16, 18, 44) and thereby divert current through the current path from the said at least one DC power transmission medium (16, 18, 44) to the said at least one other DC power transmission medium (16, 18, 44) so as to simultaneously regulate current flow (I 1, I 2, I 3) in each DC power transmission medium (16, 18, 44) and divert energy from the said at least one DC power transmission medium (16, 18, 44) into the said at least one other DC power transmission medium (16, 18, 44) via the current flow control unit.