An alternate arm converter

In the field of alternate arm voltage source converters there is a need for a means of reestablishing a substantially equal balance in the energy stored in respective first and second limb portions of a given converter limb within an alternate arm converter. An alternate arm converter (50) comprises at least one converter limb (52) that includes first and second DC terminals (54, 56) for connection in use to a DC network (58) and an AC terminal (60) for connection in use to an AC network (62). The or each converter limb (52) defines first and second limb portions (64, 66). Each limb portion (64, 66) includes at least one director switch (68, 72) which is connected in series with a chain-link converter (70, 74) between a respective one of the first and second DC terminals (54, 56) and the AC terminal (60). The chain-link converters (70, 74) are operable to generate a voltage waveform at the AC terminal (60) and the director switches (68, 72) of the first and second limb portions (64, 66) are operable to switch the respective chain-link converters (70, 74) in and out of circuit between the respective DC terminal (54, 56) and the AC terminal (60). The alternate arm converter (50) also includes a controller configured to selectively control the switching of each director switch (68, 72) and thereby switch both limb portions (64, 66) into circuit concurrently so as to form a current circulation path (80) which includes each limb portion (64, 66) and the DC network (58). The controller, during formation of the current circulation path (80), forcing an alternating current (I mod ) to flow through the current circulation path (80) to transfer energy (E Mod ) between the chain-link converter (70, 74) of one limb portion (64, 66) and the chain-link converter (70, 74) of the other limb portion (64, 66).