Converter scheme

A converter scheme (30) comprises a plurality of poles and a plurality of converters (32), the plurality of poles (60,62,64) including at least one positive pole (60), at least one negative pole (62) and a neutral pole (64), the plurality of converters (32) including at least one first converter (32a) and at least one second converter (32b), the or each first converter (32a) connected to the neutral pole (64) and the or the respective positive pole (60), the or each first converter (32a) operable to control a converter voltage across the neutral pole (64) and the corresponding positive pole (60), the or each second converter (32b) connected to the neutral pole (64) and the or the respective negative pole (62), the or each second converter (32b) operable to control a converter voltage across the neutral pole (64) and the corresponding negative pole (62), wherein the converter scheme (30) includes a controller (36) programmed to perform a voltage control mode when there is an imbalance between power or current levels of the positive and negative poles (60,62) and when the neutral pole (64) is at a non-zero potential, the controller (36) programmed to perform the voltage control mode to operate each converter (32a,32b) to control the corresponding converter voltage so that a pole-to-ground voltage of the corresponding positive or negative pole (60,62) is equal to or lower than a voltage rating of the corresponding positive or negative pole (60,62).