Qualla: Cahora Bassa (HVDC)

Cahora Bassa (HVDC): Introduction

Thu, 23 Apr 2026 00:00:00 GMT

Photo credit Herbert Hönigsperger, CC BY-SA 4.0. Twenty-two thousand, six hundred and fifty-six thyristors. That is how many semiconductor switches had to fire in synchronized, millisecond precision at each end of the line just to make the electricity flow from Cahora Bassa to Johannesburg. When the project was designed in the late 1960s, the thyristors available were rated at a mere 1.6 kilovolts each, which meant each valve needed 280 of them wired in series, with two more in parallel - the largest count ever used in a single HVDC valve before or since. Two converter stations, outdoor-mounted thyristors oil-cooled for insulation, 1,420 kilometers of wire, and two parallel pylons named Zeus and Apollo: this was the frontier of power electronics in 1974.

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Image: Herbert Hönigsperger | CC BY-SA 4.0

Cahora Bassa (HVDC): First of Its Kind

Thu, 23 Apr 2026 00:00:00 GMT

Photo credit NJR ZA, CC BY-SA 3.0. Cahora Bassa was a collection of firsts. First HVDC scheme in Africa. First anywhere in the world to operate above 500 kilovolts - its working voltage of 533 kV DC with 1,800 amperes gave it a transmission capacity of 1,920 megawatts. First HVDC scheme ordered with thyristor valv...

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Image: NJR ZA | CC BY-SA 3.0

Cahora Bassa (HVDC): Zeus and Apollo

Thu, 23 Apr 2026 00:00:00 GMT

Photo credit Herbert Hönigsperger, CC BY-SA 4.0. The two power lines, both named for gods, run from the Songo converter station near the dam to the Apollo converter station near Johannesburg. They travel one kilometer apart, monopolar rather than bipolar along most of their length, because the terrain is so inaccessible that if...

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Image: Herbert Hönigsperger | CC BY-SA 4.0

Cahora Bassa (HVDC): Out of Service for a Decade

Thu, 23 Apr 2026 00:00:00 GMT

Photo credit NJR ZA, CC BY-SA 3.0. From 1985 to 1997, the HVDC system did not transmit power. Mozambique's civil war had arrived in the transmission corridor. Sabotage dropped towers. Long stretches of the line became unusable. Nearly all 4,200 transmission-line towers on the 893 kilometers in Mozambican territory...

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Image: NJR ZA | CC BY-SA 3.0

Cahora Bassa (HVDC): The Apollo Upgrade

Thu, 23 Apr 2026 00:00:00 GMT

Photo credit Herbert Hönigsperger, CC BY-SA 4.0. By the mid-2000s, the original oil-filled thyristor valves at the Apollo station were showing their age. In 2006, ABB was awarded the contract to replace them. The outdoor mounting concept was kept - it had worked for thirty years - but each new housing now contained a complete s...

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Image: Herbert Hönigsperger | CC BY-SA 4.0