cahora bassa was one of the first hvdc schemes built with thyristor valves from its inception. unusually, the thyristor valves are outdoor mounted. in the original installation they were oil filled for both cooling and electrical insulation. the only other hvdc scheme in the world equipped in this way from the outset was the first phase – now decommissioned – of the shin shinano frequency converter in japan. each valve tank contains two valves, forming a "double-valve" connecting the two dc terminals to one single-phase, two-winding converter transformer. each six-pulse bridge contains three such tanks and hence each station contains 24 double-valves.