IMPROVEMENTS IN OR RELATING TO LINE COMMUTATED CONVERTERS

In the field of HVDC power transmission networks, there is a need for an improved line commutated converter. [0002] A line commutated converter (10) comprises a plurality of converter limbs (12A, 12B, 12C) which extend between first and second DC terminals (14, 16) that are connected, in-use, to a DC network (18). Each converter limb (12A, 12B, 12C) includes first and second limb portions (20A, 20B, 20C, 22A, 22B, 22C) that are separated by an AC terminal (24A, 24B, 24C), which is connected, in-use, to a respective phase (A, B, C) of an AC network (26). Each limb portion (20A, 20B, 20C, 22A, 22B, 22C) has a switching element (28) which is connected between a respective AC terminal (24A, 24B, 24C) and a corresponding one of the first or the second DC terminal (14, 16). The converter (10) also includes a controller (30) that is programmed to, in-use, control operation of the converter (10) and to monitor during operation of the converter (10) a DC converter current (I) flowing between the first and second DC terminals (14, 16) and an AC converter current (I) flowing between respective pairs of AC terminals (24A, 24B, 24C). The controller (30), while the DC converter and AC converter currents (I, I) are equal to one another, operating the converter (10) in a normal mode during which the controller (10) sets a first firing angle for each switching element (28). The first firing angle is greater than ninety degrees and so operates the converter (10) as an inverter. The controller (30), when a fault occurs temporarily causing the DC converter current (I) to exceed the AC converter current (I), operates the converter (10) in a fault detection and mitigation mode. [0003] When operating the converter (10) in the fault detection and mitigation mode, the controller (30): sets a second firing angle for each switching element (28) which is less than ninety degrees, and thereby operates the converter (10) as a rectifier; checks whether operating the converter (10) as a rectifier causes the AC converter current (I) to exceed the DC converter current (I); and (i) bypasses the converter (10) if the AC converter current (I) exceeds the DC converter current (I), or (ii) sets a third firing angle for each switching element (28) which is greater than ninety degrees but reduced relative to the first firing angle, and thereby operates the converter (10) as an inverter with a reduced power transfer capability, if the AC converter current (I) does not exceed the DC converter current (I).