Alternate arm converter

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 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, each limb portion including at least one director switch connected in series with a chain-link converter between a respective one of the first and second DC terminals and the AC terminal, the chain-link converters being operable to generate a voltage waveform at the AC terminal, and the director switches of the first and second limb portions being operable to switch the respective chain-link converters in and out of circuit between the respective DC terminal and the AC terminal;
• and
• a controller programmed to selectively control the switching of each director switch and thereby switch both limb portions into circuit concurrently during an overlap period of an operation cycle of the alternate arm converter so as to form a current circulation path including each limb portion and the DC network, the controller during formation of the current circulation path being further programmed to force an alternating current to flow through the current circulation path so that energy is transferred between the chain-link converter of one limb portion and the chain-link converter of the other limb portion, and at the end of the overlap period the chain-link converter of one limb portion has lost energy and the chain-link converter of the other limb portion has gained energy.