A voltage source converter, for power conversion between AC and DC in a HVDC power transmission network, comprising: first and second DC terminals (12, 14) for connection to a DC network and having at least one converter limb (16A, 16B, 16C) extending therebetween, the or each converter limb (16A, 16B, 16C) including first and second limb portions (18A, 18B, 18C, 20A, 20B, 20C) separated by a corresponding AC terminal (22A, 22B, 22C) for connection to a respective phase (A, B, C) of an AC network, each limb portion (18A, 18B, 18C, 20A, 20B, 20C) including a chain-link converter extending between the associated AC terminal (22A, 22B, 22C) and a corresponding one of the first or the second DC terminal (12, 14), and each chain-link converter including a chain-link converter controller programmed to control a plurality of series connected chain-link modules each having a plurality of switching elements connected in parallel with an energy storage device, whereby each chain-link converter is controllable to provide a stepped variable voltage source; and a voltage source converter controller arranged in operative communication with each chain-link converter controller to coordinate operation of the chain-link converters, at least one chain-link converter controller being programmed to establish at a first time instant preceding a second time instant, wherein the second time instant corresponds to the sum of the first time instant and the signal transmission delay, a total voltage sum of all energies stored by the energy storage devices in each chain-link converter within the voltage source converter, and to send the established total voltage sum to the voltage source converter controller, the voltage source converter controller being programmed to establish a modulation index demand at the second time instant for each chain-link converter, each said modulation index demand corresponding to a chain-link converter voltage reference at the second time instant, that the corresponding chain-link converter is required to produce, divided by the received total voltage sum, and further being programmed to send the established modulation index demand to each corresponding chain-link converter controller, and each chain-link converter controller being further programmed to reconstruct from the received modulation index demand the chain-link converter voltage reference, the chain-link converter it is controlling is required to produce, by multiplying the received modulation index demand by the total voltage sum established at the first time instant. first and second DC terminals (12, 14) for connection to a DC network and having at least one converter limb (16A, 16B, 16C) extending therebetween, the or each converter limb (16A, 16B, 16C) including first and second limb portions (18A, 18B, 18C, 20A, 20B, 20C) separated by a corresponding AC terminal (22A, 22B, 22C) for connection to a respective phase (A, B, C) of an AC network, each limb portion (18A, 18B, 18C, 20A, 20B, 20C) including a chain-link converter extending between the associated AC terminal (22A, 22B, 22C) and a corresponding one of the first or the second DC terminal (12, 14), and each chain-link converter including a chain-link converter controller programmed to control a plurality of series connected chain-link modules each having a plurality of switching elements connected in parallel with an energy storage device, whereby each chain-link converter is controllable to provide a stepped variable voltage source; and a voltage source converter controller arranged in operative communication with each chain-link converter controller to coordinate operation of the chain-link converters, at least one chain-link converter controller being programmed to establish at a first time instant preceding a second time instant, wherein the second time instant corresponds to the sum of the first time instant and the signal transmission delay, a total voltage sum of all energies stored by the energy storage devices in each chain-link converter within the voltage source converter, and to send the established total voltage sum to the voltage source converter controller, the voltage source converter controller being programmed to establish a modulation index demand at the second time instant for each chain-link converter, each said modulation index demand corresponding to a chain-link converter voltage reference at the second time instant, that the corresponding chain-link converter is required to produce, divided by the received total voltage sum, and further being programmed to send the established modulation index demand to each corresponding chain-link converter controller, and each chain-link converter controller being further programmed to reconstruct from the received modulation index demand the chain-link converter voltage reference, the chain-link converter it is controlling is required to produce, by multiplying the received modulation index demand by the total voltage sum established at the first time instant.