A converter scheme (30,130) comprises a plurality of poles and a plurality of converters (32, 34), the plurality of poles including at least one positive pole (60,60a,60b), at least one negative pole and a neutral pole, said plurality of converters (32, 34) including at least one first AC to DC converter and at least one second AC to DC converter and DC to AC converters, the or each first AC to DC converter connected to the neutral pole and the or the respective positive pole (60,60a,60b), the or each first AC to DC converter operable to control a converter voltage across the neutral pole and the corresponding positive pole (60,60a,60b), the or each second AC to DC converter connected to the neutral pole and the or the respective negative pole, the or each second AC to DC converter operable to control a converter voltage across the neutral pole and the corresponding negative pole, the converter scheme further comprising a conductor path, said conductor path having a first end operatively connected to said neutral pole while a second end of said conductor path is connected to a terminal of said DC to AC converters, wherein the converter scheme (30,130) further includes a controller programmed to perform a voltage control mode when there is an imbalance between power or current levels of the positive and negative poles (60,60a,60b,62) such that a current flows through said conductor path and that the neutral pole is at a non-zero potential (V n), the controller being 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 (60,60a,60b,62) is equal to or lower than a voltage rating of the corresponding positive or negative pole (60,60a,60b,62), the converter scheme further comprising means configured to determine the non-zero potential (V n) of the neutral pole, the controller being further configured to, in the voltage control mode, operate an identified AC to DC converter of the first and second AC to DC converters (32a,32b) to control the corresponding converter voltage to be the lower of: (i) an operator-defined converter voltage and (ii) a difference between a pole-to-ground voltage limit (Vdc_limit) and the determined non-zero potential (V n) of the neutral pole. wherein the converter scheme (30,130) further includes a controller programmed to perform a voltage control mode when there is an imbalance between power or current levels of the positive and negative poles (60,60a,60b,62) such that a current flows through said conductor path and that the neutral pole is at a non-zero potential (V n), the controller being 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 (60,60a,60b,62) is equal to or lower than a voltage rating of the corresponding positive or negative pole (60,60a,60b,62), the converter scheme further comprising means configured to determine the non-zero potential (V n) of the neutral pole, the controller being further configured to, in the voltage control mode, operate an identified AC to DC converter of the first and second AC to DC converters (32a,32b) to control the corresponding converter voltage to be the lower of: (i) an operator-defined converter voltage and (ii) a difference between a pole-to-ground voltage limit (Vdc_limit) and the determined non-zero potential (V n) of the neutral pole.