A current flow controller comprising: first and second terminals (36, 38) being operatively connectable, in use, to first and second DC power transmission media (30, 32) respectively; a third terminal being operatively connectable, in use, to an electrical element, the third terminal being switchably electrically connected to each of the first and second terminals (36, 38); a current flow control module extending between the first and second terminals (36, 38), the current flow control module including at least one auxiliary switching element (Q 1, Q 2, Q 3, Q 4, Q 5, Q 6) and at least one energy storage device, the or each auxiliary switching element (Q 1, Q 2, Q 3, Q 4, Q 5, Q 6) and the or each energy storage device combining to selectively provide a voltage source, the or each auxiliary switching element (Q 1, Q 2, Q 3, Q 4, Q 5, Q 6) being switchable to selectively switch the or each energy storage device into and out of circuit between the first and second terminals (36, 38); and characterized in that the current flow controller further comprises: a first main switching element (Q s1), the main switching element (Q s1) being operatively connected between the first and third terminals (36, 40), the main switching element (Q s1) being switchable to turn on to permit flow of current between the third terminal and the first terminal and to turn off to inhibit flow of current between the third terminal and the first terminal, and a control unit configured to control switching of the first main switching element (Q s1) and of the auxiliary switching elements (Q 1, Q 2, Q 3, Q 4, Q 5, Q 6) in an energy transfer mode, wherein the control unit in the energy transfer mode turns off the main switching element (Q s1) and switches the or each energy storage device into circuit between the first and second terminals (36, 38) to inject a voltage drop, in use, in the first DC power transmission medium so as to divert energy from one of the DC power transmission media (30, 32) into the other of the DC power transmission media (30, 32) via the current flow control module and thereby carry out regulation of current flow in each DC power transmission medium (30, 32). first and second terminals (36, 38) being operatively connectable, in use, to first and second DC power transmission media (30, 32) respectively; a third terminal being operatively connectable, in use, to an electrical element, the third terminal being switchably electrically connected to each of the first and second terminals (36, 38); a current flow control module extending between the first and second terminals (36, 38), the current flow control module including at least one auxiliary switching element (Q 1, Q 2, Q 3, Q 4, Q 5, Q 6) and at least one energy storage device, the or each auxiliary switching element (Q 1, Q 2, Q 3, Q 4, Q 5, Q 6) and the or each energy storage device combining to selectively provide a voltage source, the or each auxiliary switching element (Q 1, Q 2, Q 3, Q 4, Q 5, Q 6) being switchable to selectively switch the or each energy storage device into and out of circuit between the first and second terminals (36, 38); and characterized in that the current flow controller further comprises: a first main switching element (Q s1), the main switching element (Q s1) being operatively connected between the first and third terminals (36, 40), the main switching element (Q s1) being switchable to turn on to permit flow of current between the third terminal and the first terminal and to turn off to inhibit flow of current between the third terminal and the first terminal, and a control unit configured to control switching of the first main switching element (Q s1) and of the auxiliary switching elements (Q 1, Q 2, Q 3, Q 4, Q 5, Q 6) in an energy transfer mode, wherein the control unit in the energy transfer mode turns off the main switching element (Q s1) and switches the or each energy storage device into circuit between the first and second terminals (36, 38) to inject a voltage drop, in use, in the first DC power transmission medium so as to divert energy from one of the DC power transmission media (30, 32) into the other of the DC power transmission media (30, 32) via the current flow control module and thereby carry out regulation of current flow in each DC power transmission medium (30, 32). a first main switching element (Q s1), the main switching element (Q s1) being operatively connected between the first and third terminals (36, 40), the main switching element (Q s1) being switchable to turn on to permit flow of current between the third terminal and the first terminal and to turn off to inhibit flow of current between the third terminal and the first terminal, and a control unit configured to control switching of the first main switching element (Q s1) and of the auxiliary switching elements (Q 1, Q 2, Q 3, Q 4, Q 5, Q 6) in an energy transfer mode, wherein the control unit in the energy transfer mode turns off the main switching element (Q s1) and switches the or each energy storage device into circuit between the first and second terminals (36, 38) to inject a voltage drop, in use, in the first DC power transmission medium so as to divert energy from one of the DC power transmission media (30, 32) into the other of the DC power transmission media (30, 32) via the current flow control module and thereby carry out regulation of current flow in each DC power transmission medium (30, 32).