A line commutated converter, for use in high voltage DC power transmission, comprising: a plurality of converter limbs (12A, 12B, 12C) extending between first and second DC terminals (16, 18), each converter limb (12A, 12B, 12C) including first and second limb portions (22, 24) separated by an AC terminal (26A, 26B, 26C), the first limb portions together defining a first limb portion group and the second limb portions together defining a second limb portion group, each limb portion (22, 24) including at least one switching element (36 1, 36 2, 36 3, 36 4, 36 5, 36 6) configured to turn on and conduct current when it is forward biased and it receives a turn on signal and to naturally turn off and no longer conduct current when it is reverse biased and the current flowing through it falls to zero; and a control unit programmed to, in use, control successive switching of the switching elements (36 1, 36 2, 36 3, 36 4, 36 5, 36 6) whereby a first switching element (36 1) in the first limb portion group and a second switching element (36 2) in the second limb portion group and a different converter limb (12B, 12C) to the first switching element (36 1) connect two corresponding AC terminals (26A, 26C in series between the first and second DC terminals (16, 18), the control unit being further programmed to send a third switching element (36 3) in the first limb portion group a turn on signal whereby the third switching element (36 3) turns on and begins to conduct current while the current flowing through the first switching element (36 1) begins to fall to zero and the first switching element (36 1) prepares to naturally turn off, and to subsequently send a fourth switching element (36 4) in the second limb portion group and a different converter limb (12A, 12C) to the third switching element (36 3) a turn on signal whereby the fourth switching element turns (36 4) on and begins to conduct current while the current flowing through the second switching element (36 2) begins to fall to zero and the second switching element (36 2) prepares to naturally turn off, and current detectors (40A, 40B, 40C), wherein each current detector is arranged to detect a phase current of a corresponding AC terminal (26A, 26B, 26C), characterised in the control unit being configured to check, based on signals from the current detectors, for an abnormal current flow associated with the first switching element (36 1) during a finite monitoring period (42', 42"; 62', 62") to establish whether correct natural turn off of the first switching element (36 1) occurs, wherein the finite monitoring period (42', 42"; 62', 62") is applied as and when the first switching element is expected to naturally turn off during the continued and ongoing operation of the converter and commences (44; 64) when the first switching element (36 1) becomes forward biased. a plurality of converter limbs (12A, 12B, 12C) extending between first and second DC terminals (16, 18), each converter limb (12A, 12B, 12C) including first and second limb portions (22, 24) separated by an AC terminal (26A, 26B, 26C), the first limb portions together defining a first limb portion group and the second limb portions together defining a second limb portion group, each limb portion (22, 24) including at least one switching element (36 1, 36 2, 36 3, 36 4, 36 5, 36 6) configured to turn on and conduct current when it is forward biased and it receives a turn on signal and to naturally turn off and no longer conduct current when it is reverse biased and the current flowing through it falls to zero; and a control unit programmed to, in use, control successive switching of the switching elements (36 1, 36 2, 36 3, 36 4, 36 5, 36 6) whereby a first switching element (36 1) in the first limb portion group and a second switching element (36 2) in the second limb portion group and a different converter limb (12B, 12C) to the first switching element (36 1) connect two corresponding AC terminals (26A, 26C in series between the first and second DC terminals (16, 18), the control unit being further programmed to send a third switching element (36 3) in the first limb portion group a turn on signal whereby the third switching element (36 3) turns on and begins to conduct current while the current flowing through the first switching element (36 1) begins to fall to zero and the first switching element (36 1) prepares to naturally turn off, and to subsequently send a fourth switching element (36 4) in the second limb portion group and a different converter limb (12A, 12C) to the third switching element (36 3) a turn on signal whereby the fourth switching element turns (36 4) on and begins to conduct current while the current flowing through the second switching element (36 2) begins to fall to zero and the second switching element (36 2) prepares to naturally turn off, and current detectors (40A, 40B, 40C), wherein each current detector is arranged to detect a phase current of a corresponding AC terminal (26A, 26B, 26C), characterised in the control unit being configured to check, based on signals from the current detectors, for an abnormal current flow associated with the first switching element (36 1) during a finite monitoring period (42', 42"; 62', 62") to establish whether correct natural turn off of the first switching element (36 1) occurs, wherein the finite monitoring period (42', 42"; 62', 62") is applied as and when the first switching element is expected to naturally turn off during the continued and ongoing operation of the converter and commences (44; 64) when the first switching element (36 1) becomes forward biased.