VOLTAGE SOURCE CONVERTER

In the field of high voltage direct current (HVDC) transmission, a voltage source converter (10) comprises first and second DC terminals (12, 14) that are for connection to a DC network (16). The voltage source converter (10) also includes at least one converter limb (18). The or each converter limb (18) extends between the first and second DC terminals (12, 14) and includes first and second limb portions (20, 22) that are separated by a respective AC terminal (24) for connection to a corresponding phase of an AC network (26). Each first limb portion (20) extends between the first DC terminal (12) and a corresponding AC terminal (24), and each second limb portion (22) extends between the second DC terminal (14) and the corresponding AC terminal (24). Each limb portion (20, 22) includes a respective chain-link converter (28) which is defined by a plurality of series-connected switching modules (30) which are operable in combination to provide a stepped variable voltage source. In addition the voltage source converter (10) also includes a control apparatus (32) that is configured to coordinate operation of the chain-link converters (28) to cause an exchange of power between the DC and AC networks (16, 26). The control apparatus (32) evaluates the combined performance of the switching modules (30) within at least one chain-link converter (28) and acts to modify the coordinated operation of the chain-link converters (28) when the combined performance of the said switching modules (30) becomes degraded to a predetermined extent.