this scheme, equipped from the outset with thyristor valves provided by swedish subcontractor asea, was designed to transmit 560 megawatts, in the first phase, at a symmetrical bipolar voltage of ±500 kilovolts. although not the first 500 kv hvdc project in operation (the ±533 kv cahora bassa scheme having been completed in 1977) it was the first such scheme to use only a single twelve-pulse bridge per pole. consequently, its thyristor valves were the highest-rated in the world (in terms of voltage) at the time and remained so until the completion, in 2007, of the ±450 kv, single-converter norned scheme. the thyristor valves for inga shaba had 258 thyristors in series in each valve and were air-insulated and air-cooled.