Eskom blackout warning

Eskom warned that South Africa’s power system is critically short of operating reserves, which poses a risk to the system’s ability to arrest frequency deviations and prevent a blackout.

To understand the risk of a total blackout, it is important to understand South Africa’s electricity grid.

Eskom operates power stations across South Africa linked to the national electricity grid by transmission lines.

The transmission lines, typically thick aluminium and copper wires, carry electricity over long distances at high voltage and low current.

If the current is too high, the transmission lines would heat up too much and potentially melt. If the voltage were too low, hardly any energy would be carried.

Power stations generate electricity at around 20,000 volts (20 kV). Transformers then raise the voltage before it is sent out.

Eskom generates and transmits alternating current (AC). Its generators are synchronised to the national grid at a frequency of 50 Hertz (Hz).

The grid’s frequency indicates what the electricity demand is at any given moment. For a balanced frequency, supply must always meet demand.

• When demand drops, the frequency will increase. In this case, the power station control systems must reduce supply to bring the frequency back to 50 Hz.
• When demand increases, the frequency will decrease. In this case, the generation output must be increased. If that is not possible, demand should be reduced through load-shedding.

Should the frequency drop below a certain level, automatic load shedding will occur. To prevent this, Eskom implements controlled load-shedding.

Unless Eskom reduces demand, the grid’s frequency will continue to drop and cause damage to some electrical appliances because of “under-frequency”.

Kela Securities explained that power plants and household appliances can accommodate a slight variation of frequency changes without any reaction.

However, these changes are limited to a 1% frequency deviation in most cases. This equates to 0.5 Hz in South Africa – 49.5 Hz to 50.5 Hz.

This is known as the dead band, where frequency changes can be accommodated without any change in plant output.

Any changes to the frequency of more than 1% will cause the plant to produce more or less power to bring the frequency back.

If the frequency cannot be returned to within the dead band, it will switch itself off. This is a safety feature that is built into the design of the plant.

Eskom, therefore, closely monitors the frequency of the grid to ensure it does not fall outside of the dead band.

Eskom’s “Medium-Term System Adequacy Outlook 2024 – 2028” report showed that the frequency regularly fell outside the dead band in 2022.

In 47 cases, the frequency dropped below 49.5 Hz, and in 12 cases, it exceeded 50.5 Hz. In December 2022 alone, it fell outside the dead band twelve times.

Eskom explained that ancillary services, also known as reserves, play a crucial role in ensuring the system is within the frequency band.

These reserves are also necessary to support renewable energy integration, particularly the integration of intermittent resources.

“However, actual reserve provision is underperforming, indicating a power system critically short of operating reserves, which poses a risk to the system’s ability to arrest frequency deviations,” it said.

The table below from Eskom’s “Medium-Term System Adequacy Outlook 2024 – 2028” shows the “frequency incidents” outside the 49.7 Hz to 50.3 Hz frequency band last year.

The frequency incidents that fell outside the dead band and posed a risk to the system are shown in red.