An alternate arm converter for use in high voltage DC power transmission and reactive power compensation, the alternate arm converter comprising: at least one converter limb including first and second DC terminals (54, 56) for connection in use to a DC network and an AC terminal for connection in use to an AC network, the or each converter limb defining first and second limb portions (64, 66), each limb portion (64, 66) including at least one director switch (68, 72) 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, the chain-link converters (70, 74) being operable to generate a voltage waveform at the AC terminal, and the director switches (68, 72) of the first and second limb portions (64, 66) being 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; and 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 during an overlap period of an operating cycle of the alternate arm converter so as to form a current circulation path including each limb portion (64, 66) and the DC network, the alternate arm converter being characterised in that the controller during formation of the current circulation path forces an alternating current (I MOD) to flow through the current circulation path 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), whereby at the end of the overlap period the chain-link converter (70, 74) of one limb portion (64, 66) has lost energy and the chain-link converter (70, 74) of the other limb portion (64, 66) has gained energy. at least one converter limb including first and second DC terminals (54, 56) for connection in use to a DC network and an AC terminal for connection in use to an AC network, the or each converter limb defining first and second limb portions (64, 66), each limb portion (64, 66) including at least one director switch (68, 72) 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, the chain-link converters (70, 74) being operable to generate a voltage waveform at the AC terminal, and the director switches (68, 72) of the first and second limb portions (64, 66) being 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; and 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 during an overlap period of an operating cycle of the alternate arm converter so as to form a current circulation path including each limb portion (64, 66) and the DC network, the alternate arm converter being characterised in that the controller during formation of the current circulation path forces an alternating current (I MOD) to flow through the current circulation path 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), whereby at the end of the overlap period the chain-link converter (70, 74) of one limb portion (64, 66) has lost energy and the chain-link converter (70, 74) of the other limb portion (64, 66) has gained energy.