in 2006 a contract was awarded to abb to replace the thyristor valves at the apollo station. the outdoor mounting concept was retained, but each of the new housings contains a complete six-pulse bridge instead of only two valves, and the replacement thyristor valves are of a more conventional air insulated, water-cooled design using 125mm, 8.5 kv thyristors. 36 such thyristors are connected in series in each valve, without parallel connection, and the new valves are capable of a subsequent upgrade to 600 kv, 3300 a. at the same time, the ac filters were replaced.