Grid operators: Managing more renewables increasingly challenging

How Eskom plans to mitigate this.

Ensuring the secure and reliable operation of power transmission systems is becoming increasingly challenging as the share of renewables grows. This makes the role of system operators ever more crucial, says the GO15, an association of operators of the largest grids in the world.

In a recently published position paper, the organisation says a balance must be struck between security of supply, cost-efficiency and environmental concerns.

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“Several countries have already witnessed the negative effects of the rapid growth of solar and wind generators connected to the grid, underlining the importance of defining and implementing corrective actions to improve grid flexibility and mitigate their impact on power system stability and security,” the organisation says. 

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Eskom is a GO15 member and supports this position, says Isabel Fick, head of the system operator’s office since 2020.

She emphasises the enormity of the South African grid. “It compares well with the total European grid.”

The European grid, however, consists of smaller grids in several countries, and one country can rely on another.

That is not the case in South Africa, where our neighbours are too small to provide any significant assistance. To illustrate this, South Africa’s peak demand is around 36 000MW, compared to Lesotho’s 150MW.

Balancing act

It is the system operator’s job to keep the grid frequency at 50Hz by balancing supply and demand. There is little room for variance. 

In managing this, inertia plays an important role. That refers to the energy stored in large rotating generators, typically part of coal-fired power stations, which gives them the tendency to remain rotating. This stored energy is particularly valuable when a large power plant fails, as it can temporarily make up for the power lost from the failed generator.

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It helps to keep the frequency stable and prevent blackouts.

Solar and wind farms do not have such generators, and the power system’s inertia reduces as the share of coal in the generation mix shrinks and more renewables are added. 

Weather impact 

A total of 6 430MW of renewables has already been connected as a result of the government’s Renewable Energy Independent Power Producer Procurement Programme (Reipppp). Fick estimates that rooftop solar represents almost as much, mostly situated in Gauteng. 

There is insufficient formal data about the amount of rooftop generation, but Fick says Eskom can see the effect when clouds move in over Gauteng. 

The load profile has also changed. During the day, when the sun shines, the demand for Eskom generation is lower than before, but as the sun sets towards the evening peak, Eskom needs to ramp up by 4 000MW to 5 000MW. “That is a massive ramp rate,” she says. 

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Coal-fired power stations are not built for that. In fact, it takes up to 72 hours for them to go from cold to full operation.

One of the big benefits of South African wind resources is that they follow the load demand profile in summer, generating power when it is most needed. “It is the only place in the world where that happens,” says Fick.

But there is a double whammy in winter when the wind drops behind the cold front, and demand increases as temperatures drop in Gauteng.

Eskom does have some of the best weather forecasters, with a fault factor of only 1.4%, but managing the grid is increasingly challenging. “The system updates every four seconds and, when there is a crisis, every 20 milliseconds.”

Eskom’s plan

To mitigate against the increasing instability, Eskom is in the process of buying 13 synchronous generators to increase inertia. 

Fick says another tool to manage instability is curtailment for frequency control, which is provided for in Eskom’s power purchase agreements with renewable IPPs.

Eskom has also applied to energy regulator Nersa to extend the conditions under which it will be allowed to curtail IPP output. The tariff to be paid to generators to compensate them for the curtailment must be set.

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Fick says nuclear is a clean, reliable technology, as has been proven at Koeberg. More nuclear can be deployed to replace coal as the baseload. The country can also benefit from more pumped storage schemes, and two sites have already been identified for that purpose. 

Batteries are also very useful to the system operator because they can be dispatched quickly, but large-scale battery deployment has not yet occurred in South Africa.

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