sales deducted from the HCB supply to Eskom. The tripartite agreement was suspended due to force majeure when the line from Cahora Bassa was unavailable in the 1980s. The system was commissioned in three stages starting in March 1977 with four six-pulse bridges, and in full operation of eight bridges on 15 March 1979. Transmission line The power line runs from the Songo converter station, which is near the hydroelectric station and normally operates as a rectifier, to the Apollo converter station near Johannesburg, which normally operates as an inverter. Each of the self-supporting steel towers along the route carries two bundles of four 565 square millimetre (1120 kcmil) cables, and a single 117 square millimetre (231 kcmil) grounding conductor. There are approximately 7,000 towers with an average span of . The maximum span is using reinforced towers. Earth return for unipolar operation is provided by buried graphite electrodes at each station. The DC line has smoothing reactors and surge arrester capacitors at each station. Northeast of Apollo Converter Station the poles of HVDC Cahora Bassa undercross several 400 kV AC lines at 25°54'58"S 28°16'46"E respectively 25°54'57"S 28°16'51"E in such a low altitude that the area under the line may not be walked on and is fenced in . The 2 lines are named as Zeus and Apollo respectively. Thyristor valves 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. The development work for the thyristor valves began in the late 1960s when the only thyristors available at the time were, by today’s standards, small, and were rated only 1.6 kV each. In the first phase of the project (4 bridges at each end) each valve contained 280 such thyristors in series with two in parallel – the largest number ever used in a single HVDC valve. Phases 2 and 3 used improved thyristors with a rating