EXPLAINER | Why a 'major incident' in Zambia affected SA's power supply

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Government will amend the embedded generation threshold to 100 MW.
Government will amend the embedded generation threshold to 100 MW.
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  • Over the weekend, a "disturbance" led to the loss of significant generation to Zambia's power system, which in turn contributed to power supply challenges in South Africa.
  • According to Eskom the issue in Zambia affected the Southern African Power Pool, which is interconnected.
  • Eskom meanwhile has escalated load shedding from stage 2 to stage 4 this week, as seven units have not returned to service as anticipated and unplanned breakdowns persist.

Over the weekend Zambia experienced a national blackout due to a loss of "significant generation" to the country's power system. This however had a knock-on effect on South Africa's own power supply challenges, eventually leading to the implementation of Stage 4 load shedding for most of this week. 

According to a statement issued on Saturday by Zambian power utility - Zesco Limited - their system experienced a "disturbance", following the loss of generation capacity, the cause of which is still being investigated.

"Preliminary investigations indicate that the cause of the disturbance was external to the Zambian interconnected power system," the statement issued by Hazel Zulu, Zesco's public relations manager read.

Zesco has not yet responded to questions from Fin24 on updates of the investigations.

On Saturday, Eskom also issued a statement citing the "major incident" in Zambia, which affected the entire Southern African Power Pool (SAPP).

The SAPP was established in August 1995 at the Southern African Development Community (SADC) summit held in Kempton Park, South Africa. Member nations and their power utilities, apart from Mauritius, signed a Memorandum of Understanding to form an electricity power pool for the region.

According to its website, the SAPP seeks to be a fully integrated, competitive energy market that provides sustainable energy solutions and services for the SADC region.

The grid map for the Southern African Development
The grid map for the Southern African Development Community.


According to Eskom, the incident in Zambia affected power imported from Cahora Bassa hydroelectric scheme which is located in Mozambique.

The Cahora Bassa project was first initiated in 1969, according to Eskom's website. Portugal and South Africa signed contracts to finalise plans for the construction of the scheme - which includes a hydroelectric generating station at the dam and a  over 1400 km high-voltage direct current transmission system transporting electricity to South Africa's converter station Apollo near Pretoria.

At the time, the view was that the scheme would reduce the need to build more coal fired power stations in South Africa, and this would free up water supplies that otherwise would have been used at the stations.

The dam is along the Zambezi river. Cahora Bassa h
The dam is along the Zambezi river. Cahora Bassa has the capacity to generate over 2 000 MW of power.


Eskom's power network is also linked to Lesotho, Botswana, Swaziland, Namibia and Mozambique. The energy imported from Cahora Bassa can be distributed to any point of the network, information on Eskom's website showed.

But why would an incident in Zambia, the Cahora Bassa transmission and South Africa's load shedding?

According to Eskom, the SAPP is an interconnected system. "Whatever goes into the system will impact the flow within the system overall," Eskom said in response to questions from Fin24.

Information to Chris Yelland of EE Business Intelligence from the Eskom System Operator, indicated that subsequent to the loss of generation capacity in Zambia, the frequency of the power system dropped to 49.31 Hertz (Hz).

The system operator targets a frequency band of between 49.5 Hz and 50.5 Hz. According to Tobias Bischof-Niemz and Terence Creamer, authors of South Africa’s Energy Transition: A Roadmap to a Decarbonised, Low-cost and Job-rich Future, a frequency of 50 Hz indicates energy generation matches demand. Usually generation output is adjusted to meet demand. But when there is not enough generation, customer demand must be adjusted and that is why load shedding is introduced, they explain. This ensures the power system remains stable.

When the frequency dropped - it caused a loss of 1 000 MW from Cahora Bassa, Eskom said. It also tripped a unit at Tutuka, Yelland was informed.

During a briefing on the state of the system on Tuesday, Eskom's group executive for transmission Segomotso Scheppers explained that the whole region operates at a frequency of 50 Hz and that if the frequency is too high or too low it could lead to the tripping of equipment or plants, and then protective measures may automatically kick in to stabilise the system.

He explained there are interconnections between South Africa, Botswana and into Zimbabwe. In turn, Zimbabwe has interconnections with Zambia. There is alsolinks between Mozambique and Zimbabwe as well as Zimbabwe and South Africa, and links between Mozambique and South Africa.

Prelimenary information indicates the incident in Zambia had a cascading effect across the network, leading to trips into Zimbabwe. He highlighted power flows from South Africa through Zimbabwe to Zambia.

At the time the incident occurred, South Africa was already on stage 2 load shedding in order to replenish reserves - but the the pump storage and open cycle gas turbines automatically kicked in to maintain the frequency of 50 Hz to protect the system. Scheppers explained this is a function of system design and occurred automatically and there was no manual intervention to have the emergency reserves start up.

Eskom managed to run these sources for two hours until the Cahorra Bassa supply was restored. But this impacted the level of security in the South African network, Scheppers said.

Scheppers added that so far it has been determiend that the trip at Tutuka was not linked to the incident.

He noted however that the apollo network was affected, as there was a deviation of power in the alternating current link between Mozambique and Zimbabwe - which ultimately led to the loss of the 1 000 MW. A more detailed investigation is being conducted on the matter. All utilities part of the SAPP are furnishing data to the coordinating centre in Harare, in order to confirm exactly what happened. 

*This article was updated at 20:00 on Tuesday 9 November, 2021 to include additional comment from Eskom's Segomotso Scheppers.


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