IMPROVEMENTS IN OR RELATING TO VOLTAGE SOURCE CONVERTERS

In the field of voltage source converters, for power conversion between alternating current (AC) and direct current (DC) in high voltage direct current (HVDC) power transmission networks, there is a need for an improved voltage source converter.A voltage source converter (10) comprises first and second DC terminals (12, 14), which are for connection to a DC network, and which have at least one converter limb (16A, 16B, 16C) extending therebetween. The or each converter limb (16A, 16B, 16C) includes first and second limb portions (18A, 18B, 18C, 20A, 20B, 20C) that are separated by a corresponding AC terminal (22A, 22B, 22C), which are for connection to a respective phase (A, B, C) of an AC network. Each limb portion (18A, 18B, 18C, 20A, 20B, 20C) includes a chain-link converter (24) that extends between the associated AC terminal (22A, 22B, 22C) and a corresponding one of the first or the second DC terminal (12, 14). Each chain-link converter (24) also includes a chain-link converter controller (26) which is programmed to control a plurality of series connected chain-link modules (28), each of which has a plurality of switching elements that are connected in parallel with an energy storage device, and whereby each chain-link converter (24) is controllable to provide a stepped variable voltage source. The voltage source converter (10) additionally includes a voltage source converter controller (30) which is arranged in operative communication with each chain-link converter controller (26) to coordinate operation of the chain-link converters (24). At least one chain-link converter controller (26) is programmed to establish at a first time instant a total voltage sum of all energies stored by the energy storage devices in each chain-link converter (24) within the voltage source converter (10), and to send the established total voltage sum to the voltage source converter controller (30). The voltage source converter controller (30) is programmed to establish a modulation index demand for each chain-link converter (24). Each said modulation index demand corresponds to a chain-link converter voltage reference, that the corresponding chain-link converter (24) is required to produce, divided by the received total voltage sum. In addition, the voltage source converter controller (30) is further programmed to send the established modulation index demand to each corresponding chain-link converter controller (26). In turn, each chain-link converter controller (26) is