Alternate arm converter

A converter (30) comprises first and second DC terminals (32,34) connectable in use to a DC network (40), the converter (30) including a converter limb (36) extending between the DC terminals (32,34), the converter limb (36) including first and second limb portions separated by an AC terminal (38), the AC terminal (38) connectable in use to an AC voltage, each limb portion including at least one director switch (44) connected in series with a waveform synthesizer between the AC terminal (44) and a respective one of the first and second DC terminals (32,34), the waveform synthesizers operable to control the modulation of an AC voltage waveform at the AC terminal (38), each director switch (44) operable to switch the respective waveform synthesizer into and out of circuit between the respective DC terminal (32,34) and the AC terminal (38); and a controller (56) programmed to selectively control the switching of the director switches (44) to switch both of the limb portions into circuit concurrently to form a current conduction path between the DC terminals (32,34), the current conduction path configured to carry a current for presentation to the DC network (40), wherein the controller (56) is programmed to selectively operate the waveform synthesizers to inject at least one harmonic component to modulate the AC voltage waveform at the AC terminal (38) so that during the formation of the current conduction path the magnitude of the modulated AC voltage waveform is lower than the magnitude of the fundamental component of the AC voltage waveform, and wherein the at least one harmonic component is a non-triplen harmonic component.