By Dominique Preus
Blackout, load shedding stage infinity, and total grid collapse across the whole country is the scariest situation that Eskom, South Africa’s power utility, can put the country in.
Like all complex systems, multiple factors are a concern when considering grid collapse.
First, a triggering event must occur, like a power plant breakdown in combination with a surge in demand from consumers. That can initiate a cascading effect throughout the grid.
The failure of a critical component can overload other parts of the system, causing them to fail as well. This can lead to a domino effect, where failures propagate rapidly across the network, potentially impacting a large area.
The mechanism that Eskom uses to mitigate the effect of such failures on the system is to implement load shedding. This keeps failures limited to local areas.
With load shedding, the balance between supply and demand is always being monitored and modified to prevent local system failures from propagating to the entire country.
Control monitors are watching grid frequency and phase. This refers to the frequency at which alternating current (AC) electricity is generated and maintained in an electrical grid system. In South Africa, that is a sweet 50 Hz.
When supply exceeds demand, the frequency increases, and when demand exceeds supply, the frequency decreases.
Power plants connected to the grid generate electricity at the same frequency to ensure synchronization. This synchronization is essential for the proper functioning of electrical equipment and devices connected to the grid.
If the frequency deviates significantly from the nominal value (e.g., due to sudden imbalances between supply and demand), it can lead to grid instability and, potentially, a collapse.
A severe drop in frequency, known as under-frequency, can cause protective relays to trip, resulting in cascading failures and a widespread blackout. On the other hand, a significant increase in frequency, known as over-frequency, can damage electrical equipment and disrupt the operation of the grid.
Grid inertia refers to the inherent resistance of an electrical grid system to changes in frequency. The inertia here refers to large rotating masses of synchronous generators providing physical inertia to the grid system, creating stability and acting as a buffer against sudden imbalances.
Grids with low levels of inertia, such as those incorporating a higher share of renewable energy sources (e.g., wind or solar), may experience more rapid frequency changes. Lulls and gusts in wind or clouds obscuring panels, in these cases, the loss or sudden changes in power generation, can lead to more significant frequency deviations.
Renewable energy needs to be coupled with battery technology so that in the event of power fluctuations, the stored energy in the batteries can be released, stabilizing the grid. This requirement for renewables adds a hidden cost to the construction and operation of the technology. Solar panels capture around 20% of the 1,3 kilowatts of solar energy per square meter. This means for utility-sized power plants you need a lot of space.
Whereas coal-powered plants have been well studied and modelled in the last century. The mass and rotational inertia of the generator rotor provides grid stability. Coal itself is cheap per ton, with modern power plants getting up to 40% thermal efficiency if run correctly and well maintained.
Coal is ground up into a powder. This is the first chink in Eskom’s logistics supply chain, as poor-quality coal that is delivered to the power plants is fed into grinders designed to crush coal, not rocks, rubble or concrete.
The coal powder is burnt in a boiler to heat up water and turn it into steam. A pump is used to pump the water through the boiler to the steam turbine. The pumps need power, and in a total grid collapse, you need to first have power to the pumps in order to pump the steam to the turbine.
One efficiency measure to get extra power is to add extra pressure to the water/steam via the pump to have extra pressure to spin the turbine. However, the more pressure water is under, the higher the boiling point of the water, the hotter the boiler needs to be. The materials of the boiler must be able to resist the extra heat, and the turbine must also be designed not to melt from the super-hot steam. Once the steam has passed the turbines and exchanged its kinetic energy, turning into electricity, the water is condensed, cooled and pumped back into the boiler.
Eskom’s new power plant, Kusile, uses giant fans to condense the water rather than a water-cooled system. Therefore, because the fans take lots of power from the power plant and pumps need power, most power plants need power to produce power.
That is why restoring total grid collapse will take a while. It will involve identifying the cause of the collapse, repairing damaged equipment, and gradually reconnecting sections of the grid. Restoration efforts prioritise critical infrastructure, such as other power plants, before gradually restoring power to residential and commercial areas.
What are the other solutions? Renewable energy is variable and unstable; coal is dirty and unattractive. How about hydro?
Hydro also has large rotating masses of synchronous generators creating high grid inertia. It is also clean and can be designed as a battery. Hydro pumped storage is a scheme whereby water is pumped up to a top dam either when the sun is shining, and solar plants are overproducing or at night when demand is low. Then releasing the water downhill into turbines to generate extra power during peak demand.
Hydro pumped storage is the main contributing factor as to why Cape Town has one stage less load shedding than the rest of the country. The City of Cape Town invested approximately R1 billion for the refurbishment and extension of the Steenbras Hydro Pumped Storage Scheme, and approximately R200 million is earmarked for the maintenance of Steenbras.
* Preuss is an engineering technologist at Amore Hydro.
PERSONAL FINANCE