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Let’s talk about the elephant in the room – system inertia

An Eskom coal-burning power station near Sasolburg. Picture: File

An Eskom coal-burning power station near Sasolburg. Picture: File

Published Apr 20, 2022


Matshela Koko

Pretoria - Speaking during an energy transition breakaway at the South African Investment Conference of 2022, Eskom CEO Andre de Ruyter was quoted by the Engineering News online magazine as saying: “The utility estimated that some 68 000MW of mostly new variable renewable-energy capacity would have to be added to cater for the 22  000MW of coal (sic) that the utility would be retiring by about 2035.”

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He was further quoted as having said: “The state-owned utility was hoping to build about 8 000MW of that new capacity, leaving 60 000MW that would have to be built by private and/or municipal generators.”

In the same breakaway session, Brian Dames, who is the former CEO of Eskom, is quoted by the same Engineering News online magazine to have warned that the current Integrated Resource Plan is significantly underestimating what new capacity should be built to cater for the decommissioning of the coal-fired power stations. According to Dames, the plan “should be a lot more aggressive”.

Dames was supported by Dr Crispian Olver, who is the Presidential Climate Change Commission’s executive director. He argued that South Africa should be procuring new renewables at a yearly rate of at least 3 000MW to address existing supply constraints and ensure supply security as Eskom begins decommissioning its coal plants.

De Ruyter, Dames and Olver are not engineers. For reasons unbeknown to me, engineers have surrendered the thought leadership on topics as critical as energy security and power quality to industry leaders who are not engineers. I have a serious problem with that. Black engineers, in particular, must show up and provide thought leadership on these topics.

Let me take this opportunity to introduce a well-known factor, called system inertia, to de Ruyter, Dames, Olver, and others. In its press release of April 7, General Electric succinctly described system inertia as a critical factor because it keeps the grid running when a synchronous generator suddenly disconnects from the grid, and thus buys time for other generators to ramp up.

It is trite that the national grid must have sufficient system inertia at all material times to keep the lights on.

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Secil Varbak Nese et al, in a paper published in 2020, highlighted that “a power system’s stability is a key factor for secure and uninterrupted system operation.

“The stability of the power system is defined as the ability to restore the operating balance after being subjected to a physical disturbance. One of the most important parameters in the simultaneous operation of power systems is system inertia.

“The lower the inertia of the system is, the more the system is sensitive to frequency deviations.”

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Matshela Koko is the Managing Director of Matshela Energy. Picture: Bheki Radebe/African News Agency (ANA)

System inertia comes mainly from Eskom’s 15 coal power stations with an installed capacity of 44 013MW, Koeberg power plant with an installed capacity of 1 934MW, emergency gas generators with an installed base of 2 426MW, pump storage schemes with an installed capacity of 2 426MW, 1 500MW from Cahora Bassa, 1 000MW from Avon and Didisa peaking power plants and 2 125MW from municipal and private generators.

According to the South African Grid Code version 9, these conventional generators must remain connected to the grid between 47 and 51.5Hz. Renewable energy power plants do not contribute to system inertia because they are connected to the network by power electronics, and they are electrically isolated from the grid.

We learnt from Balint Hartmann et al, in a paper published in 2019, that renewable energy power plants operate between 49.5 and 50.5Hz.

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If the frequency decreases or increases outside this range, the renewable energy power stations disconnect from the grid.

What I find thought-provoking is that in the Grid Connection Code for renewable energy power plants version 2.8 of 2014, the minimum frequency operating range of renewable energy power plants during frequency disturbances is identical to the minimum frequency range of conventional power stations published in figure 2 of the South African Grid Code version 9.

I am extremely surprised by this. It may well be that the assumption was made that the systems operator would ensure that a minimum level of system inertia would continue to be supplied by conventional power plants, be they coal or nuclear, in the absence of gas.

The efforts to decommission 22 000MW of coal power stations and to replace them with 68 000MW of intermittent generation technologies by 2035, while well intended, could inadvertently hamper the creation of a robust and reliable smart grid of the future.

In a very rudimentary way, De Ruyter took the 22 000MW of the coal power stations that he intends to decommission by 2035 and divided it by the average capacity factor of renewable power stations of 32% to get to 68 000MW of intermittent generation capacity by 2035. It equates to a renewable energy build programme of 5 230MW per annum compared to the 2 600MW per annum of the renewable energy generation that is in the current Integrated Resource Plan.

This is the reason Dames wants a much more aggressive plan. If this is how the integrated supply electricity plan is implemented, then South Africans can rest assured that rotational load-shedding will still be with us in 2035. Surely, South Africans deserve better than this.

The real problem of the change in the dynamic behaviour of the power system due to low system inertia is swept under the carpet. The assumption is made that the decline in system inertia will not pose a significant technical barrier to the rolling out of a renewable energy build programme.

South Africans are advised to be mindful of the fact that renewable technologies plus battery storage are useful to the grid, but they are not the grid. Flexible base-load capacity will continue to supply the minimum system inertia that is required to make the national grid reliable now and into the foreseeable future.

This flexible base-load capacity will most likely come from high-temperature modular reactors and imported gas power plants.

For reasons of national security, the power generated from imported gas should not exceed the National Energy Regulator of South Africa’s (Nersa) reserve margin of 19%. To be precise, all the electricity imports should not exceed Nersa’s approved reserve margin of 19%. We would have lost the plot if the total electricity that is imported exceeds the approved reserve margin of 19%.

I agree with Meredith Angwin, who, in her book titled Shorting the Grid, wrote that, “as usual, the problem is not with renewable technologies or batteries. The problem is that people are not planning for their use or how they might be most useful.”

Pretoria News