Construction and ownership

The Cahora-Bassa transmission project was a joint venture of the two electrical utilities, Electricity Supply Commission (ESCOM, as it was known prior to 1987), latterly Eskom, Johannesburg, South Africa and Hidroelectrica de Cahora Bassa (HCB), a firm owned 15% by the government of Portugal and 85% by Mozambique. Equipment was constructed and supplied by ZAMCO, which was a consortium of AEG-Telefunken JV, Brown Boveri Company, and Siemens AG of Germany. Brown Boveri subsequently became part of ABB and AEG subsequently became part of Alstom.

The commercial arrangements also included Electricidade de Moçambique (EDM) which took supply from Cahora Bassa through a wheeling arrangement with Eskom. Effectively, Eskom supplied southern Mozambique (Maputo) from the then Eastern Transvaal at 132 kV with the 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 426 metres (466 yd).

The maximum span is 700 metres (770 yd) 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 two 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.^[2] In the first phase of the project (4 bridges at each end) each valve contained 280 such thyristors in series with two in parallel^[1] – the largest number ever used in a single HVDC valve.

Phases 2 and 3 used improved thyristors with a rating of 2.4 kV each and only required 192 in series per valve – still a large number by modern standards – with two in parallel. As a result, each converter station contained a total of 22,656 thyristors.

Other equipment

The thyristors also had poor transient overcurrent capability, so another unusual feature of the scheme was the existence of overcurrent diverters between the valves and transformers, although these were later decommissioned at the Apollo station.^[2]

AC filters tuned to the 5th, 7th, 11th and 13th harmonics of the 50 Hz power supply are installed at each station, approximately 195 MVAr at Apollo and 210 MVAr at Songo.

There are two PLC repeater stations: one at Gamaboi in South Africa and one at Catope in Mozambique.