A converter having at least one converter apparatus (101a, 101b) defining a current path within the converter, the converter comprising: a plurality of sub-modules connectable in series to form at least part of the converter apparatus such that current flowing along the current path flows through each of those sub-modules, each sub-module comprising an energy storage device and switching circuitry (201, 203, 204, 205) configured to direct the flowing current so that it either flows through or bypasses the energy storage device, the switching circuitry (201, 203, 204, 205) controllable to cause the sub-module to transition between at least one pair of different configurations via a corresponding intermediate configuration applied in order to avoid switches (201, 203) of the switching circuitry being on at the same time and shorting out the energy storage device, that transition affecting a potential difference such that it exhibits an associated change either when the sub-module is in the intermediate configuration concerned or after it has exited that intermediate configuration dependent on a direction of current flow along the current path for that transition; and characterized in that it comprises a controller (104, 103a, 103b, 206) configured to carry out a direction-determination operation in respect of at least one said sub-module, the direction-determination operation determining the direction of current flow along the current path for a said transition of that sub-module by measuring the time taken for the associated change in potential difference to occur and by measuring the affected potential difference concerned at least once over that transition to determine whether the associated change occurs when that sub-module is in the intermediate configuration concerned or after it has exited that intermediate configuration. a plurality of sub-modules connectable in series to form at least part of the converter apparatus such that current flowing along the current path flows through each of those sub-modules, each sub-module comprising an energy storage device and switching circuitry (201, 203, 204, 205) configured to direct the flowing current so that it either flows through or bypasses the energy storage device, the switching circuitry (201, 203, 204, 205) controllable to cause the sub-module to transition between at least one pair of different configurations via a corresponding intermediate configuration applied in order to avoid switches (201, 203) of the switching circuitry being on at the same time and shorting out the energy storage device, that transition affecting a potential difference such that it exhibits an associated change either when the sub-module is in the intermediate configuration concerned or after it has exited that intermediate configuration dependent on a direction of current flow along the current path for that transition; and characterized in that it comprises a controller (104, 103a, 103b, 206) configured to carry out a direction-determination operation in respect of at least one said sub-module, the direction-determination operation determining the direction of current flow along the current path for a said transition of that sub-module by measuring the time taken for the associated change in potential difference to occur and by measuring the affected potential difference concerned at least once over that transition to determine whether the associated change occurs when that sub-module is in the intermediate configuration concerned or after it has exited that intermediate configuration.