Eskom recently explained that the reason it implemented rotational load shedding was to avoid a potential national blackout in South Africa.
Such a blackout would be no small matter, the power utility said, as it would take weeks to restart the grid if South Africa went completely dark.
Why would excessive demand in some parts of South Africa’s electricity grid cause a national blackout and not just a blackout in that area, though?
Asked about this, director of EE Publishers and electricity expert Chris Yelland explained that it is the very safety features built into electricity equipment to prevent catastrophic failure that would lead to a blackout.
“When demand exceeds supply, what happens is you have overloading of generators, transformers, cables, and switch gear,” Yelland said.
To protect against the massive damage such an overload would cause, trip mechanisms are installed in such equipment.
If there is an overload and equipment starts to trip, the overall electricity supply decreases, but the demand stays the same.
This then places even greater strain on the parts of the grid that are still on, causing them to overload and trip if you do not act quickly – once again reducing supply while demand remains the same.
Yelland said that if you had an aerial view of the country you would see these cascading trip-outs spread as a wave of darkness sweeping the country.
“You cannot operate an electrical system where demand exceeds supply,” Yelland said, adding that load shedding was a process of switching off demand to prevent such a blackout from happening.
The idea behind rotational load shedding is to prevent certain communities from being unduly disadvantaged. “So the pain is felt by everyone,” Yelland said.
Besides leaving the country without power, a national blackout would also leave us in a situation where Eskom would have to restart power plants without any electricity — or “black start”.
This could take 2–3 weeks, Yelland said, and would be very costly to South Africa’s economy.
Power plants use some of the electricity they generate to operate equipment such as conveyor belts that feed coal into furnaces, so to start them up you need to start with a small generator.
To bootstrap a power plant, a small diesel generator is typically used to start up a larger generator, which in turn is used to start parts of the plant.
“There are procedures and protocols in place to do a black start,” Yelland said. “This matter has been carefully thought out and procedures put in place.”
More importantly, there are many protocols in place to prevent a national blackout.
A blackout on such a scale is an unlikely event associated with being unprepared, Yelland said.
“Fortunately the 2008 blackouts did prepare South Africa for this eventuality,” he added. “If we had these problems back then we would have had a really serious situation on our hands.”